Relationship between Changes over Time in Factors, Including the Impact of Meteorology on Photochemical Oxidant Concentration and Causative Atmospheric Pollutants in Kawasaki

: To clarify the relationship between changes in photochemical oxidants’ (Ox) concentrations and their precursors in Kawasaki, a series of analyses were conducted using data on Ox, their precursors, nitrogen oxides (NOx) and volatile organic compounds (VOCs), and meteorology that had been monitored throughout the city of Kawasaki for 30 years from 1990 to 2019. The trend in air temperature was upward, wind speed was downward, and solar radiation was upward, indicating an increasing trend in meteorological factors in which Ox concentrations tend to be higher. Between 1990 and 2013, the annual average Ox increased throughout Kawasaki and remained ﬂat after that. The three-year moving average of the daily peak increased until 2015, and after that, it exhibited a slight decline. The amount of generated Ox is another important indicator. To evaluate this, a new indicator, the daytime production of photochemical oxidant (DPOx), was proposed. DPOx is deﬁned by daytime averaged Ox concentrations less the previous day’s nighttime averaged Ox concentrations. The trend in DPOx from April to October has been decreasing since around 2006, and it was found that this indicator reﬂects the impact of reducing emissions of NOx and VOCs in Kawasaki.


Introduction
Kawasaki is part of the Keihin (Tokyo-Yokohama, Japan) industrial area, and therefore industries such as steel, chemical, and petrochemical are concentrated in the city. At one time, the city experienced serious pollution problems, such as atmospheric pollution and water contamination, but in 1972, the Kawasaki city government implemented local regulations for total pollution loads (Appendix A) first in Japan, and in 1974, the Environment Agency, government of Japan (later Ministry of the Environment, Tokyo, Japan), implemented similar regulations. As such, it is a city that has been a pioneer in Japan of advanced measures to control air pollution. As a result, in fiscal year (FY)2019, the average concentration of SO 2 had dropped by 99% from its peak in FY1965, while the average concentration of CO had fallen by 92% compared with its FY1972 peak. Data like these illustrate that there has been a huge improvement in the concentrations of primary pollutants. In this environment, air quality is well maintained, and with the aim of protecting the health of residents, atmospheric pollution is monitored continuously in Kawasaki. Figure 2 shows the air monitoring system in operation in Kawasaki. For this paper, we employed constant-observation data from all 18 stations that were used, as of October 2020, to take measurements in Kawasaki. These comprise nine general stations (General sta., explained in the description of administrative criteria) and nine roadside stations (Roadside sta., explained in the description of administrative criteria).
The period of analysis was 30 years from FY1990 to FY2019, and the variables surveyed were Ox, NOx (total of NO and NO2), NMHC, and climate (air temperature, solar radiation, and wind speed). In this paper, fiscal year (FY) denotes a period from April of a year until March of the following year, while year denotes a period from January to December.
Ox is monitored at all 9 General sta.; NOx, which is the total of NO2 and NO, is monitored at all 18 General sta. and Roadway sta.; NMHC are monitored at 7 General sta., excluding Miyamae and Asao; air temperature and wind speed are monitored at all 9 General sta.; and solar radiation is monitored at just 2 stations, Tajima and Saiwai.
Even as part of the TMA, Kawasaki covers only a limited area and is regarded as a single, self-contained area, so we based our analysis not on the measurements from each station but on the average values from all the stations. In this environment, air quality is well maintained, and with the aim of protecting the health of residents, atmospheric pollution is monitored continuously in Kawasaki. Figure 2 shows the air monitoring system in operation in Kawasaki. For this paper, we employed constant-observation data from all 18 stations that were used, as of October 2020, to take measurements in Kawasaki. These comprise nine general stations (General sta., explained in the description of administrative criteria) and nine roadside stations (Roadside sta., explained in the description of administrative criteria).
The period of analysis was 30 years from FY1990 to FY2019, and the variables surveyed were Ox, NOx (total of NO and NO 2 ), NMHC, and climate (air temperature, solar radiation, and wind speed). In this paper, fiscal year (FY) denotes a period from April of a year until March of the following year, while year denotes a period from January to December.
Ox is monitored at all 9 General sta.; NOx, which is the total of NO 2 and NO, is monitored at all 18 General sta. and Roadway sta.; NMHC are monitored at 7 General sta., excluding Miyamae and Asao; air temperature and wind speed are monitored at all 9 General sta.; and solar radiation is monitored at just 2 stations, Tajima and Saiwai.
Even as part of the TMA, Kawasaki covers only a limited area and is regarded as a single, self-contained area, so we based our analysis not on the measurements from each station but on the average values from all the stations.  Figure 3 shows the trend in the annual average Ox calculated as the average of hourly values for all nine general air monitoring/measurement stations installed in Kawasaki. The three lines (All the time, Daytime, and Night) refer, respectively, to the 24 h average, the average for 6 a.m.-8 p.m., and the average for 12 a.m.-5 a.m. and 9 p.m.-12 a.m.

Annual Average
The three annual averages generally all exhibited an upward trend from FY1990, and the tendency for daytime Ox concentrations to increase was similar to that seen with the nationwide average [17]. The averages also remained flat from around FY2013 onward.   The three annual averages generally all exhibited an upward trend from FY1990, and the tendency for daytime Ox concentrations to increase was similar to that seen with the nationwide average [17]. The averages also remained flat from around FY2013 onward.  Figure 3 shows the trend in the annual average Ox calculated as the average of hourly values for all nine general air monitoring/measurement stations installed in Kawasaki. The three lines (All the time, Daytime, and Night) refer, respectively, to the 24 h average, the average for 6 a.m.-8 p.m., and the average for 12 a.m.-5 a.m. and 9 p.m.-12 a.m.

Annual Average
The three annual averages generally all exhibited an upward trend from FY1990, and the tendency for daytime Ox concentrations to increase was similar to that seen with the nationwide average [17]. The averages also remained flat from around FY2013 onward.    The trend over time in the monthly average Ox for each month was investigated to obtain a detailed breakdown of the annual average, and the results are shown in Figure 4. It became clear that Ox concentrations in spring, which is in April and May, are higher than those in other seasons and these concentrations have markedly increased over the years. Transboundary transport from the continent is likely to be the reason for the high Ox concentrations seen in April and May [18,19].
Atmosphere 2021, 12, x FOR PEER REVIEW 5 of 26 The trend over time in the monthly average Ox for each month was investigated to obtain a detailed breakdown of the annual average, and the results are shown in Figure 4. It became clear that Ox concentrations in spring, which is in April and May, are higher than those in other seasons and these concentrations have markedly increased over the years. Transboundary transport from the continent is likely to be the reason for the high Ox concentrations seen in April and May [18,19]. To quantitatively grasp the rise in Ox over the years, the year-on-year rate of increase in the monthly average Ox for each decade is shown in Figure 5. In the 1990s, there was an upward trend in all months except July. In the 2000s, January and December exhibited a slight downward trend, but the trend was upward for the other months and during the period from April to October, during which photochemical smog warnings (PSW) are issued, the upward trend was comparatively higher in May and September. In the 2010s, the trend was upward for all months with the exception of July.

The Number of Days on Which Photochemical Smog Warnings Issued
In Japan, if the hourly value of Ox is 0.12 ppm or higher and the meteorological condition average of this situation is expected to continue, the prefectural governor issues, pursuant to the Air Pollution Control Act, photochemical smog warnings (PSW) to raise awareness among citizens and urge factories to take steps such as reducing their fuel usage (Appendix C). In the TMA, PSW are issued every year, mainly during summer, and smog has become a serious issue relating to the air environment [20].
As an indicator of high Ox concentrations, the trend over time in the number of days on which PSW were issued in Kawasaki is shown in Figure 6. Regarding the number of days on which PSW were issued, in the 1990s, the number did not exceed 10 days in any fiscal year, so the number was comparatively small. In the 2000s, however, the number of PSW topped 10 in FY2001, FY2004, FY2006, and FY2007, with the peak being in FY2007,  Dec   1990  1991  1992  1993  1994  1995  1996  1997  1998  1999  2000  2001  2002  2003  2004   2005  2006  2007  2008  2009  2010  2011  2012  2013  2014  2015  2016  2017  2018   To quantitatively grasp the rise in Ox over the years, the year-on-year rate of increase in the monthly average Ox for each decade is shown in Figure 5. In the 1990s, there was an upward trend in all months except July. In the 2000s, January and December exhibited a slight downward trend, but the trend was upward for the other months and during the period from April to October, during which photochemical smog warnings (PSW) are issued, the upward trend was comparatively higher in May and September. In the 2010s, the trend was upward for all months with the exception of July. The trend over time in the monthly average Ox for each month was investigated to obtain a detailed breakdown of the annual average, and the results are shown in Figure 4. It became clear that Ox concentrations in spring, which is in April and May, are higher than those in other seasons and these concentrations have markedly increased over the years. Transboundary transport from the continent is likely to be the reason for the high Ox concentrations seen in April and May [18,19]. To quantitatively grasp the rise in Ox over the years, the year-on-year rate of increase in the monthly average Ox for each decade is shown in Figure 5. In the 1990s, there was an upward trend in all months except July. In the 2000s, January and December exhibited a slight downward trend, but the trend was upward for the other months and during the period from April to October, during which photochemical smog warnings (PSW) are issued, the upward trend was comparatively higher in May and September. In the 2010s, the trend was upward for all months with the exception of July.

The Number of Days on Which Photochemical Smog Warnings Issued
In Japan, if the hourly value of Ox is 0.12 ppm or higher and the meteorological condition average of this situation is expected to continue, the prefectural governor issues, pursuant to the Air Pollution Control Act, photochemical smog warnings (PSW) to raise awareness among citizens and urge factories to take steps such as reducing their fuel usage (Appendix C). In the TMA, PSW are issued every year, mainly during summer, and smog has become a serious issue relating to the air environment [20].
As an indicator of high Ox concentrations, the trend over time in the number of days on which PSW were issued in Kawasaki is shown in Figure 6. Regarding the number of days on which PSW were issued, in the 1990s, the number did not exceed 10 days in any fiscal year, so the number was comparatively small. In the 2000s, however, the number of PSW topped 10 in FY2001, FY2004, FY2006, and FY2007, with the peak being in FY2007,

The Number of Days on Which Photochemical Smog Warnings Issued
In Japan, if the hourly value of Ox is 0.12 ppm or higher and the meteorological condition average of this situation is expected to continue, the prefectural governor issues, pursuant to the Air Pollution Control Act, photochemical smog warnings (PSW) to raise awareness among citizens and urge factories to take steps such as reducing their fuel usage (Appendix C). In the TMA, PSW are issued every year, mainly during summer, and smog has become a serious issue relating to the air environment [20].
As an indicator of high Ox concentrations, the trend over time in the number of days on which PSW were issued in Kawasaki is shown in Figure 6. Regarding the number of days on which PSW were issued, in the 1990s, the number did not exceed 10 days in any fiscal year, so the number was comparatively small. In the 2000s, however, the number of PSW topped 10 in FY2001, FY2004, FY2006, and FY2007, with the peak being in FY2007, when the number of such days was 13. In the 2010s, the number of PSW surpassed 10 only in FY2013, so a decline was seen relative to the 2000s. Over the entire 30-year period, the level of fiscal-year-to-fiscal-year variation in the number of PSW was large.
Atmosphere 2021, 12, x FOR PEER REVIEW 6 of 26 when the number of such days was 13. In the 2010s, the number of PSW surpassed 10 only in FY2013, so a decline was seen relative to the 2000s. Over the entire 30-year period, the level of fiscal-year-to-fiscal-year variation in the number of PSW was large. As an indicator of high Ox concentrations throughout the year, the annual averages of daytime daily maximums are shown in Figure 7. Because the daily maximum is prone to influence by meteorological conditions, it is shown as a three-year moving average. Although an upward trend was seen until around FY2015, it has remained flat in recent years.

A New Indicator from the MOE
In 2016, the MOE presented the three-year moving average of the annual 99th percentile of the daily maximum for the eight-hour value as an indicator for properly assessing the effectiveness of Ox-related environmental improvements (Appendix B). The 99th percentile of the daily maximum for the eight-hour value involves calculating the maximum from among the eight-hour moving averages on a single day. A year's worth of these maximum values is arranged, and the 99% counting from the lowest day is the 99th percentile of the daily maximum for the eight-hour value. It is suitable for determining the situation in a high-concentration zone. This value has been on a downward trend since around FY2008 in the TMA, Nagoya area, Osaka/Kyoto area, and Kyushu area [21]. Figure 8 shows the application of this indicator to the average from all nine General sta. that perform measurements in Kawasaki. Peaks were also seen in Kawasaki around FY2008; after that, there is variation over time, depending on the fiscal year and the value As an indicator of high Ox concentrations throughout the year, the annual averages of daytime daily maximums are shown in Figure 7. Because the daily maximum is prone to influence by meteorological conditions, it is shown as a three-year moving average. Although an upward trend was seen until around FY2015, it has remained flat in recent years.
Atmosphere 2021, 12, x FOR PEER REVIEW 6 of 26 when the number of such days was 13. In the 2010s, the number of PSW surpassed 10 only in FY2013, so a decline was seen relative to the 2000s. Over the entire 30-year period, the level of fiscal-year-to-fiscal-year variation in the number of PSW was large. As an indicator of high Ox concentrations throughout the year, the annual averages of daytime daily maximums are shown in Figure 7. Because the daily maximum is prone to influence by meteorological conditions, it is shown as a three-year moving average. Although an upward trend was seen until around FY2015, it has remained flat in recent years.

A New Indicator from the MOE
In 2016, the MOE presented the three-year moving average of the annual 99th percentile of the daily maximum for the eight-hour value as an indicator for properly assessing the effectiveness of Ox-related environmental improvements (Appendix B). The 99th percentile of the daily maximum for the eight-hour value involves calculating the maximum from among the eight-hour moving averages on a single day. A year's worth of these maximum values is arranged, and the 99% counting from the lowest day is the 99th percentile of the daily maximum for the eight-hour value. It is suitable for determining the situation in a high-concentration zone. This value has been on a downward trend since around FY2008 in the TMA, Nagoya area, Osaka/Kyoto area, and Kyushu area [21]. Figure 8 shows the application of this indicator to the average from all nine General sta. that perform measurements in Kawasaki. Peaks were also seen in Kawasaki around FY2008; after that, there is variation over time, depending on the fiscal year and the value

A New Indicator from the MOE
In 2016, the MOE presented the three-year moving average of the annual 99th percentile of the daily maximum for the eight-hour value as an indicator for properly assessing the effectiveness of Ox-related environmental improvements (Appendix B). The 99th percentile of the daily maximum for the eight-hour value involves calculating the maximum from among the eight-hour moving averages on a single day. A year's worth of these maximum values is arranged, and the 99% counting from the lowest day is the 99th percentile of the daily maximum for the eight-hour value. It is suitable for determining the situation in a high-concentration zone. This value has been on a downward trend since around FY2008 in the TMA, Nagoya area, Osaka/Kyoto area, and Kyushu area [21]. Figure 8 shows the application of this indicator to the average from all nine General sta. that perform measurements in Kawasaki. Peaks were also seen in Kawasaki around FY2008; after that, there is variation over time, depending on the fiscal year and the value is flat. FY2013 was an unusual fiscal year, as during summer, high pressure over the Pacific was combined with high pressure over Tibet, and these high-pressure systems covered the whole of Japan. As a result, air temperatures were high, solar radiation was high, and wind speed was slow [22]. In FY2017-FY2019, meanwhile, we believe that the new indicator increased due to the impact of the nationwide transboundary pollution that occurred in May 2019 [23].
Atmosphere 2021, 12, x FOR PEER REVIEW 7 of 26 is flat. FY2013 was an unusual fiscal year, as during summer, high pressure over the Pacific was combined with high pressure over Tibet, and these high-pressure systems covered the whole of Japan. As a result, air temperatures were high, solar radiation was high, and wind speed was slow [22]. In FY2017-FY2019, meanwhile, we believe that the new indicator increased due to the impact of the nationwide transboundary pollution that occurred in May 2019 [23].

Meteorological Conditions
Because Ox is a secondary pollutant generated by ultraviolet (UV) radiation, the development of high concentrations is closely related to meteorological factors such as air temperature, wind speed, and solar radiation [2]. Because of this, long-term trends in factors such as air temperature, wind speed, and solar radiation in Kawasaki were analyzed over 30 years from 1990 to 2019.

Air Temperature
The trends in the annual averages of the daily maximum temperature, daily average temperature, and daily minimum temperature are shown in Figure 9. All of them exhibit a rising trend, and the rate of increase in the minimum temperature is the greatest (0.00338 °C/year), which is followed by the rate of increase in the average temperature (0.0274 °C/year) and the rate of increase in the maximum temperature (0.0234 °C/year), as seen from the slopes of the regression lines.

Meteorological Conditions
Because Ox is a secondary pollutant generated by ultraviolet (UV) radiation, the development of high concentrations is closely related to meteorological factors such as air temperature, wind speed, and solar radiation [2]. Because of this, long-term trends in factors such as air temperature, wind speed, and solar radiation in Kawasaki were analyzed over 30 years from 1990 to 2019.

Air Temperature
The trends in the annual averages of the daily maximum temperature, daily average temperature, and daily minimum temperature are shown in Figure 9. All of them exhibit a rising trend, and the rate of increase in the minimum temperature is the greatest (0.00338 • C/year), which is followed by the rate of increase in the average temperature (0.0274 • C/year) and the rate of increase in the maximum temperature (0.0234 • C/year), as seen from the slopes of the regression lines. The rate of increase over time in the monthly average temperature is shown in Figure  10. The daily minimum temperature and the daily average temperature trend upward for all months, and the daily maximum temperature exhibits a rising trend in every month except February, November, and December. The rates of increase from March to October are greater than those for January, February, November, and December, which indicates The rate of increase over time in the monthly average temperature is shown in Figure 10. The daily minimum temperature and the daily average temperature trend upward for all months, and the daily maximum temperature exhibits a rising trend in every month except February, November, and December. The rates of increase from March to October are greater than those for January, February, November, and December, which indicates that winters are shorter and summers are longer. From April to October, the period during which PSW are issued, the rising trend is particularly marked. Figure 9. Trend in annual averages of air temperature. Air temperature: annual averages of daily maximum, daily average, and daily minimum of air temperature at all nine General sta.
The rate of increase over time in the monthly average temperature is shown in Figure  10. The daily minimum temperature and the daily average temperature trend upward for all months, and the daily maximum temperature exhibits a rising trend in every month except February, November, and December. The rates of increase from March to October are greater than those for January, February, November, and December, which indicates that winters are shorter and summers are longer. From April to October, the period during which PSW are issued, the rising trend is particularly marked.

Wind Speed
The trend in the annual average wind speed is shown in Figure 11. Wind speed data of six stations continuously operated were used for analyzing. Although there was yearto-year variation between 1990 and 2019, a gradual declining trend was observed.

Wind Speed
The trend in the annual average wind speed is shown in Figure 11. Wind speed data of six stations continuously operated were used for analyzing. Although there was year-to-year variation between 1990 and 2019, a gradual declining trend was observed. The rate of increase over time in the monthly average wind speed for each month is shown in Figure 12. A declining trend is seen in every month. During the period April-October, when PSW are issued, the downward trend is greater than the annual average in May, June, August, and September and smaller than the annual average in July.  The rate of increase over time in the monthly average wind speed for each month is shown in Figure 12. A declining trend is seen in every month. During the period April-October, when PSW are issued, the downward trend is greater than the annual average in May, June, August, and September and smaller than the annual average in July. The rate of increase over time in the monthly average wind speed for each month is shown in Figure 12. A declining trend is seen in every month. During the period April-October, when PSW are issued, the downward trend is greater than the annual average in May, June, August, and September and smaller than the annual average in July.  Figure 11. Rate of increase in wind speed: slope of linear regression line for the monthly average temperature for each month over 30 years.

Solar Radiation
The trends in the annual averages of solar radiation compared with those of air temperature are shown in Figure 13. Solar radiation exhibited a rising trend between 1990 and 2019, and it could be said that there has been an increase in the tendency of photochemical reactions to occur. Trends of an increase/decrease in solar radiation and air temperature approximately matched. The rate of increase in solar radiation for each month is shown in Figure 14. An increasing trend is observed in all the months, with a particularly large rate of increase in

Solar Radiation
The trends in the annual averages of solar radiation compared with those of air temperature are shown in Figure 13. Solar radiation exhibited a rising trend between 1990 and 2019, and it could be said that there has been an increase in the tendency of photochemical reactions to occur. Trends of an increase/decrease in solar radiation and air temperature approximately matched.
The rate of increase over time in the monthly average wind speed for each month is shown in Figure 12. A declining trend is seen in every month. During the period April-October, when PSW are issued, the downward trend is greater than the annual average in May, June, August, and September and smaller than the annual average in July.

Solar Radiation
The trends in the annual averages of solar radiation compared with those of air temperature are shown in Figure 13. Solar radiation exhibited a rising trend between 1990 and 2019, and it could be said that there has been an increase in the tendency of photochemical reactions to occur. Trends of an increase/decrease in solar radiation and air temperature approximately matched. The rate of increase in solar radiation for each month is shown in Figure 14. An increasing trend is observed in all the months, with a particularly large rate of increase in The rate of increase in solar radiation for each month is shown in Figure 14. An increasing trend is observed in all the months, with a particularly large rate of increase in March and May. During April-October, when PSW are issued, the rate of increase is the highest in May, followed by April. March and May. During April-October, when PSW are issued, the rate of increase is the highest in May, followed by April. As shown above, having analyzed the air temperature, solar radiation, and wind speed in Kawasaki between 1990 and 2019, because air temperature and solar radiation exhibited an upward trend and wind speed exhibited a downward trend, it can be said that for each of the parameters air temperature, solar radiation, and wind speed, during As shown above, having analyzed the air temperature, solar radiation, and wind speed in Kawasaki between 1990 and 2019, because air temperature and solar radiation exhibited an upward trend and wind speed exhibited a downward trend, it can be said that for each of the parameters air temperature, solar radiation, and wind speed, during the period from April to October, when PSW are issued, from these meteorological points of view, the Ox formation potential has been increasing.

Precursors
Nitrogen oxides from anthropogenic sources are mainly generated through the combustion of fossil fuels and are mainly emitted by stationary sources such as factories and workplaces and mobile sources such as motor vehicles.
The trend over time in the emissions of NOx from factories and workplaces (which contain facilities that emit soot and smoke (boilers, etc.) defined by the Air Pollution Control Act) in Kawasaki is shown in Figure 15. For the volume of NOx emissions from facilities that emit soot and smoke, aggregated results of reported emissions from stationary sources in Kawasaki were used. The volume of NOx emitted from factories and workplaces in FY2018 declined by 33% compared with that in FY1990. Furthermore, in FY2000, the Kawasaki city government established its own comprehensive total emissions control scheme (Appendix D). Full implementation of the scheme was achieved in FY2005, and reductions in NOx emission have increased substantially.

Figure 14.
Rate of increase over time in the monthly average of solar radiation for each month. Solar radiation: same as Figure 13. Rate of increase in solar radiation: slope of linear regression line for the monthly average for each month over 30 years.
As shown above, having analyzed the air temperature, solar radiation, and wind speed in Kawasaki between 1990 and 2019, because air temperature and solar radiation exhibited an upward trend and wind speed exhibited a downward trend, it can be said that for each of the parameters air temperature, solar radiation, and wind speed, during the period from April to October, when PSW are issued, from these meteorological points of view, the Ox formation potential has been increasing.

Emissions of NOx
Nitrogen oxides from anthropogenic sources are mainly generated through the combustion of fossil fuels and are mainly emitted by stationary sources such as factories and workplaces and mobile sources such as motor vehicles.
The trend over time in the emissions of NOx from factories and workplaces (which contain facilities that emit soot and smoke (boilers, etc.) defined by the Air Pollution Control Act) in Kawasaki is shown in Figure 15. For the volume of NOx emissions from facilities that emit soot and smoke, aggregated results of reported emissions from stationary sources in Kawasaki were used. The volume of NOx emitted from factories and workplaces in FY2018 declined by 33% compared with that in FY1990. Furthermore, in FY2000, the Kawasaki city government established its own comprehensive total emissions control scheme (Appendix D). Full implementation of the scheme was achieved in FY2005, and reductions in NOx emission have increased substantially.

Jan
Feb The trend over time in the emissions of NOx from motor vehicles driven in Kawasaki is shown in Figure 16. The value of the emissions of NOx was produced from the estimates reported by the Kawasaki city government, though it only covers fiscal years for which data on traffic density could be obtained. The emissions of NOx from motor vehicles remained steady from FY1993 to FY1997 but began to decline in FY1997. The emissions in FY2015 dropped by 27% compared with those in FY1997. In Japan, vehicle-specific regulations that apply at the time of new-vehicle registrations and vehicle-type regulations that apply at the time of vehicle inspections are being tightened in phases. In the TMA, regulations concerning the use of diesel vehicles were introduced in FY2003. As a result of these regulations (Appendix E), progress has been made in reducing NOx emissions.
FY2015 dropped by 27% compared with those in FY1997. In Japan, vehicle-specific regulations that apply at the time of new-vehicle registrations and vehicle-type regulations that apply at the time of vehicle inspections are being tightened in phases. In the TMA, regulations concerning the use of diesel vehicles were introduced in FY2003. As a result of these regulations (Appendix E), progress has been made in reducing NOx emissions.

NOx Concentrations
The trends in the annual averages of NOx, NO, and NO2 are shown in Figure 17. NOx was measured at nine General sta. and nine Roadside sta. The annual averages of all three pollutants have been decreasing, and the decline at Roadside sta. from the 2000s is especially striking.

NOx Concentrations
The trends in the annual averages of NOx, NO, and NO 2 are shown in Figure 17. NOx was measured at nine General sta. and nine Roadside sta. The annual averages of all three pollutants have been decreasing, and the decline at Roadside sta. from the 2000s is especially striking. The trends in the annual averages of NO2/NOx are shown in Figure 18. Although the ratio of NO to NO2 immediately after the release of NOx from combustion sources is around 9:1 [24,25], in flue gas treatment devices as well as in the atmosphere, NO is oxidized to NO2, increasing NO2 as a proportion of NOx. As a result, NO2 as a proportion of NOx tends to be higher at General sta. than at Roadside sta. In addition, the annual averages of NO2/NOx have been on an upward trend since the year 2000.  The trends in the annual averages of NO 2 /NOx are shown in Figure 18. Although the ratio of NO to NO 2 immediately after the release of NOx from combustion sources is around 9:1 [24,25], in flue gas treatment devices as well as in the atmosphere, NO is oxidized to NO 2 , increasing NO 2 as a proportion of NOx. As a result, NO 2 as a proportion of NOx tends to be higher at General sta. than at Roadside sta. In addition, the annual averages of NO 2 /NOx have been on an upward trend since the year 2000.
Because NO 2 is an important precursor to O 3 , it is extremely significant to know the ratio of NO and NO 2 in NOx from the source. Because the oxidation of NO to NO 2 in the atmosphere occurs by ozone or RO 2 radicals, we analyzed changes over time at night (from 8 p.m. until 5 a.m.) during the cold season (from October to March), when O 3 or RO 2 radicals reduce. The results of this analysis are shown in Figure 19.
The trends in the annual averages of NO2/NOx are shown in Figure 18. Although the ratio of NO to NO2 immediately after the release of NOx from combustion sources is around 9:1 [24,25], in flue gas treatment devices as well as in the atmosphere, NO is oxidized to NO2, increasing NO2 as a proportion of NOx. As a result, NO2 as a proportion of NOx tends to be higher at General sta. than at Roadside sta. In addition, the annual averages of NO2/NOx have been on an upward trend since the year 2000. Because NO2 is an important precursor to O3, it is extremely significant to know the ratio of NO and NO2 in NOx from the source. Because the oxidation of NO to NO2 in the atmosphere occurs by ozone or RO2 radicals, we analyzed changes over time at night (from 8 p.m. until 5 a.m.) during the cold season (from October to March), when O3 or RO2 radicals reduce. The results of this analysis are shown in Figure 19.  To investigate the trend in NOx emissions from motor vehicles in Kawasaki, the average of NOx, NO2, and NO at Roadside sta. less that at General sta. at night during the cold season is shown in Figure 20. Although NOx values stayed fairly level until FY2000, they decreased sharply after that. To investigate the trend in NOx emissions from motor vehicles in Kawasaki, the average of NOx, NO 2 , and NO at Roadside sta. less that at General sta. at night during the cold season is shown in Figure 20. Although NOx values stayed fairly level until FY2000, they decreased sharply after that.
To estimate the trend in NO 2 /NOx in NOx emitted from motor vehicles, changes over time in the average for Roadside sta. less that for General sta. at night during the cold season in Kawasaki are shown in Figure 21.
NOx emissions from motor vehicles shown in Figure 17 declined from FY1997. However, NO 2 /NOx emissions increased significantly from FY2005. Therefore, it seems that as NOx emissions from motor vehicles fell, NO 2 as a proportion of NOx rose substantially.
Until about FY2005, NO 2 /NOx emissions were around 0.1, but after that, they increased to 0.3 or thereabouts, and the fact that this was identified for the first time is significant. Until now, 0.1 has generally been used as an input ratio value for NO 2 as a proportion of NOx in NO emissions from motor vehicles with the atmospheric chemical transport model (CTM), but this finding shows that it needs to be altered to approximately 0.3. NO2: annual averages of hourly NOx, NO, and NO2 at nine General sta. and nine Roadside sta. in the cold season.
To investigate the trend in NOx emissions from motor vehicles in Kawasaki, the average of NOx, NO2, and NO at Roadside sta. less that at General sta. at night during the cold season is shown in Figure 20. Although NOx values stayed fairly level until FY2000, they decreased sharply after that. To estimate the trend in NO2/NOx in NOx emitted from motor vehicles, changes over time in the average for Roadside sta. less that for General sta. at night during the cold season in Kawasaki are shown in Figure 21.
NOx emissions from motor vehicles shown in Figure 17 declined from FY1997. However, NO2/NOx emissions increased significantly from FY2005. Therefore, it seems that as NOx emissions from motor vehicles fell, NO2 as a proportion of NOx rose substantially.  Until about FY2005, NO2/NOx emissions were around 0.1, but after that, they increased to 0.3 or thereabouts, and the fact that this was identified for the first time is significant. Until now, 0.1 has generally been used as an input ratio value for NO2 as a proportion of NOx in NO emissions from motor vehicles with the atmospheric chemical transport model (CTM), but this finding shows that it needs to be altered to approximately 0.3.

Emissions of NMHC
Non-methane hydrocarbons (NMHC) have been monitored as an indicator of VOCs. Estimates of emissions of VOCs from stationary sources have been published by the MOE for each prefecture since FY2000. Here, stationary sources refer to more than just factories. For example, they also include paint during construction work and vapor when filling a motor vehicle at a gas station. Trends in VOC emissions from stationary sources in Kanagawa Prefecture are shown in Figure 22. The emissions dropped sharply between FY2000 and FY2010, with the reduction in FY2010 standing at approximately 43% compared with FY2000 levels. The downward trend slowed after FY2010, yet the reduction in FY2018 compared with FY2010 was approximately 20%.
As a countermeasure to VOCs, in FY2006, the Air Pollution Control Act was amended to include new legal regulations for stationary sources that emit VOCs in large quantities. Voluntary steps to curb emissions are also being taken by business operators. This curtailment of VOC emissions through a combination of legal regulations and voluntary initiatives is referred to as the best-mix approach. The regulations for controlling VOC emissions were the first regulations to incorporate the idea of a best mix of legal regulations and voluntary cuts by businesses. For that reason, in FY2006, the Air Pollution Control Act was amended to include new legal regulations for stationary sources that emit VOCs in large quantities (Appendix F).
At the same time, a numerical target of reducing emissions by 30% by FY2010 compared with FY2000 was also set. In addition, the Kawasaki city government is working with adjacent cities in the TMA to raise awareness about reducing VOCs. These initiatives

Emissions of NMHC
Non-methane hydrocarbons (NMHC) have been monitored as an indicator of VOCs. Estimates of emissions of VOCs from stationary sources have been published by the MOE for each prefecture since FY2000. Here, stationary sources refer to more than just factories. For example, they also include paint during construction work and vapor when filling a motor vehicle at a gas station. Trends in VOC emissions from stationary sources in Kanagawa Prefecture are shown in Figure 22. The emissions dropped sharply between FY2000 and FY2010, with the reduction in FY2010 standing at approximately 43% compared with FY2000 levels. The downward trend slowed after FY2010, yet the reduction in FY2018 compared with FY2010 was approximately 20%.
As a countermeasure to VOCs, in FY2006, the Air Pollution Control Act was amended to include new legal regulations for stationary sources that emit VOCs in large quantities. Voluntary steps to curb emissions are also being taken by business operators. This curtailment of VOC emissions through a combination of legal regulations and voluntary initiatives is referred to as the best-mix approach. The regulations for controlling VOC emissions were the first regulations to incorporate the idea of a best mix of legal regulations and voluntary cuts by businesses. For that reason, in FY2006, the Air Pollution Control Act was amended to include new legal regulations for stationary sources that emit VOCs in large quantities (Appendix F).
At the same time, a numerical target of reducing emissions by 30% by FY2010 compared with FY2000 was also set. In addition, the Kawasaki city government is working with adjacent cities in the TMA to raise awareness about reducing VOCs. These initiatives are likely contributing to the reduction in emissions.

NMHC Concentrations
The trend in the annual average NMHC is shown in Figure 23. NMHC are measured at seven General sta., namely Daishi, Tajima, Kawasaki, Saiwai, Nakahara, Takatsu, and Tama. With regard to NMHC, in 1976, the Environment Agency established guidelines concerning atmospheric hydrocarbon concentrations as a means of controlling the generation of Ox. Specifically, the guidelines stated that "the three-hour average of NMHC between 6 a.m. and 9 a.m. corresponding to a daily maximum hourly concentration of photochemical oxidants of 0.06 ppm should be in the range of 0.20 ppmC to 0.31 ppmC." Therefore, the trend in the annual average of NMHC concentration was estimated using the 3 h average between 6 a.m. and 9 a.m., with that of the 24 h average as a reference. Both show a downward trend, and the figures are almost the same. Over the 30 years that are the focus of this article, the chemical component ratios in VOCs and NMHC were considerably changed. This change possibly affected the reactivity of VOCs.
It is important to consider the possibility of the effect of a change in the chemical component ratio. However, so far, the monitoring data on the chemical species are insufficient in Kawasaki.

NMHC Concentrations
The trend in the annual average NMHC is shown in Figure 23. NMHC are measured at seven General sta., namely Daishi, Tajima, Kawasaki, Saiwai, Nakahara, Takatsu, and Tama. With regard to NMHC, in 1976, the Environment Agency established guidelines concerning atmospheric hydrocarbon concentrations as a means of controlling the generation of Ox. Specifically, the guidelines stated that "the three-hour average of NMHC between 6 a.m. and 9 a.m. corresponding to a daily maximum hourly concentration of photochemical oxidants of 0.06 ppm should be in the range of 0.20 ppmC to 0.31 ppmC." Therefore, the trend in the annual average of NMHC concentration was estimated using the 3 h average between 6 a.m. and 9 a.m., with that of the 24 h average as a reference. Both show a downward trend, and the figures are almost the same. Over the 30 years that are the focus of this article, the chemical component ratios in VOCs and NMHC were considerably changed. This change possibly affected the reactivity of VOCs.

NMHC Concentrations
The trend in the annual average NMHC is shown in Figure 23. NMHC are measured at seven General sta., namely Daishi, Tajima, Kawasaki, Saiwai, Nakahara, Takatsu, and Tama. With regard to NMHC, in 1976, the Environment Agency established guidelines concerning atmospheric hydrocarbon concentrations as a means of controlling the generation of Ox. Specifically, the guidelines stated that "the three-hour average of NMHC between 6 a.m. and 9 a.m. corresponding to a daily maximum hourly concentration of photochemical oxidants of 0.06 ppm should be in the range of 0.20 ppmC to 0.31 ppmC." Therefore, the trend in the annual average of NMHC concentration was estimated using the 3 h average between 6 a.m. and 9 a.m., with that of the 24 h average as a reference. Both show a downward trend, and the figures are almost the same. Over the 30 years that are the focus of this article, the chemical component ratios in VOCs and NMHC were considerably changed. This change possibly affected the reactivity of VOCs.
It is important to consider the possibility of the effect of a change in the chemical component ratio. However, so far, the monitoring data on the chemical species are insufficient in Kawasaki. It is important to consider the possibility of the effect of a change in the chemical component ratio. However, so far, the monitoring data on the chemical species are insufficient in Kawasaki.

Relationship between Ox, NOx, and NMHC
The trend in the annual averages of NMHC/NOx (ppmC/ppm) between 6 a.m. and 9 a.m. is shown in Figure 24. NMHC/NOx decreased from FY1990 to FY2004 but from FY2005 exhibited an increasing trend. The increase in NMHC/NOx from FY2005 is attributed more to the ratio of decreasing NOx than that of NMHC from FY2005.
Atmosphere 2021, 12, x FOR PEER REVIEW 16 of 26 The trend in the annual averages of NMHC/NOx (ppmC/ppm) between 6 a.m. and 9 a.m. is shown in Figure 24. NMHC/NOx decreased from FY1990 to FY2004 but from FY2005 exhibited an increasing trend. The increase in NMHC/NOx from FY2005 is attributed more to the ratio of decreasing NOx than that of NMHC from FY2005.

Changes over Time in Meteorological Conditions Apt to Lead to High Concentrations
It is thought that the amount and rate of Ox production are determined by the intensity of sources of NOx and VOCs and the proportion of the two. It is conceivable that daily maximum Ox concentrations are determined by the amount of NOx emissions while the rate of Ox production is determined by the amount of VOC emissions. However, even under the same conditions of sources of NOx and VOCs, a variation in meteorological The trend in the annual averages of NMHC/NOx (ppmC/ppm) between 6 a.m. and 9 a.m. is shown in Figure 24. NMHC/NOx decreased from FY1990 to FY2004 but from FY2005 exhibited an increasing trend. The increase in NMHC/NOx from FY2005 is attributed more to the ratio of decreasing NOx than that of NMHC from FY2005. The relationship between Ox on the one hand and NOx and NMHCs on the other from 1990 to 2019 is shown in Figure 25. In terms of decade units, both NMHC concentration and NOx concentration have been declining. On the other hand, Ox concentration has been gently rising.

Changes over Time in Meteorological Conditions Apt to Lead to High Concentrations
It is thought that the amount and rate of Ox production are determined by the intensity of sources of NOx and VOCs and the proportion of the two. It is conceivable that daily maximum Ox concentrations are determined by the amount of NOx emissions while the rate of Ox production is determined by the amount of VOC emissions. However, even under the same conditions of sources of NOx and VOCs, a variation in meteorological

Changes over Time in Meteorological Conditions Apt to Lead to High Concentrations
It is thought that the amount and rate of Ox production are determined by the intensity of sources of NOx and VOCs and the proportion of the two. It is conceivable that daily maximum Ox concentrations are determined by the amount of NOx emissions while the rate of Ox production is determined by the amount of VOC emissions. However, even under the same conditions of sources of NOx and VOCs, a variation in meteorological conditions such as air temperature, solar radiation, and wind speed leads to differences in temporal changes and in the geographical distribution of Ox concentrations [26][27][28][29]. Therefore, when assessing the impact of measures on sources, it is necessary to exclude variable factors due to meteorological conditions. Only the first half may be quoted. The second half is still being confirmed.
As already stated, even though both NOx concentrations and NMHC concentrations, which are causes of Ox, are declining, no downward trend in Ox concentrations is seen. However, the relationship between Ox and its precursors may not be accurately assessed under different meteorological conditions because of variations in meteorological parameters year to year, as described in Section 3.3. Consequently, the relationship between Ox and its precursors was analyzed to eliminate meteorological bias by using data of days selected by certain meteorological conditions.

Method of Selection of Meteorological Conditions Apt to Lead to High Ox Concentrations
We selected certain meteorological conditions that are likely to result in high concentrations of Ox. These meteorological conditions, air temperature, wind speed, and solar radiation, are presented in Table 2. Pattern A meets only (1); pattern B meets (1) and (2); and pattern C meets all of (1), (2), and (3). Table 2. Meteorological selection conditions.

Conditions
(1) Day on which the average *1 daily maximum air temperature was 30 • C or higher (2) Day on which the average *1 daily maximum wind speed was 3 m/s or lower (3) Day on which the average *2 daily total solar radiation exceeded 10 MJ/m 2 *1 Average for all nine General sta. *2 Average for two stations Tajima and Saiwai (however, figures for 1990-2006 are for Tajima only).
The trend over time in the number of days matching each pattern and the trend over time in the daily maximum Ox are shown in Figure 26. As for the number of days, B had fewer days than A, but B and C were the same mostly. Below, days corresponding to pattern C will be referred to as extracted days.
The three-year average of the daily maximum Ox peaked in FY2007, after which it began to decline. From FY2012, however, it turned upward once again, before dipping again in FY2015 and remaining steady thereafter. The number of extracted days has been on an upward trend, and from FY2010, the number of extracted days consecutively topped 30 days. However, no correlation was observed between the number of extracted days and the single-year daily maximum Ox. tern C will be referred to as extracted days.
The three-year average of the daily maximum Ox peaked in FY2007, after which it began to decline. From FY2012, however, it turned upward once again, before dipping again in FY2015 and remaining steady thereafter. The number of extracted days has been on an upward trend, and from FY2010, the number of extracted days consecutively topped 30 days. However, no correlation was observed between the number of extracted days and the single-year daily maximum Ox.

Relationship between Ox, NOx, and NMHC on Extracted Days
On extracted days, the relationship between Ox, NOx, and NMHC is shown in Figure 27. In terms of decade units, both NMHC and NOx declined. However, no correlation was observed between Ox, NOx, and NMHCs.

Relationship between Ox, NOx, and NMHC on Extracted Days
On extracted days, the relationship between Ox, NOx, and NMHC is shown in Figure  27. In terms of decade units, both NMHC and NOx declined. However, no correlation was observed between Ox, NOx, and NMHCs. A relationship diagram of NMHC and NOx all year ( Figure 25) and on extracted days ( Figure 27) between FY1990 and FY2019 is also shown in Figure 28. Extracted days, upon which meteorological conditions are likely to lead to high concentrations of Ox, tend to exhibit high NMHC concentrations and low NOx concentrations compared with the allyear figures. The NMHC concentration is susceptible to the influence of stationary sources; increases in air temperature raise the volatilization of VOCs, and the concentration elevates when dispersion is suppressed due to the emergence of inversion layers or low wind speed. NMHC concentrations on extracted days, which are limited to days on which the air temperature is high and wind speed is low, tend to be higher than is the case all year round, as the entire year includes autumn and winter, when emergences of inversion layers are a frequent occurrence. Such days can, therefore, be said to be highly susceptible to the influence of rising air temperatures. In contrast, NOx concentrations are apt to rise in autumn and winter, when inversion layers often emerge, so because ex- A relationship diagram of NMHC and NOx all year ( Figure 25) and on extracted days ( Figure 27) between FY1990 and FY2019 is also shown in Figure 28. Extracted days, upon which meteorological conditions are likely to lead to high concentrations of Ox, tend to exhibit high NMHC concentrations and low NOx concentrations compared with the all-year figures. The NMHC concentration is susceptible to the influence of stationary sources; increases in air temperature raise the volatilization of VOCs, and the concentration elevates when dispersion is suppressed due to the emergence of inversion layers or low wind speed. NMHC concentrations on extracted days, which are limited to days on which the air temperature is high and wind speed is low, tend to be higher than is the case all year round, as the entire year includes autumn and winter, when emergences of inversion layers are a frequent occurrence. Such days can, therefore, be said to be highly susceptible to the influence of rising air temperatures. In contrast, NOx concentrations are apt to rise in autumn and winter, when inversion layers often emerge, so because extracted days are concentrated in the summer, the all-year figures, which include autumn and winter, tend to be higher. It is likely that due to these reasons, on extracted days, NMHC concentrations were higher than those for all year and NOx concentrations were lower than those for all year.
Furthermore, comparing Ox concentrations in Figures 25 and 27

A New Indicator for the Impact of Countermeasures to Ox in Kawasaki
The EQS for Ox is one of the toughest standards in the world, and it is difficult to assess the impact of countermeasures simply by evaluating the EQS. Therefore, to assess the effectiveness of Ox-related environmental improvements, the MOE proposed a new indicator focusing on high Ox concentrations. The trends in the indicators from the MOE in Kawasaki are shown in Figure 8, and it was found that appropriately assessing the impact of countermeasures was not necessarily possible by the influence of transboundary transport in a case such as May 2019, when transport of pollution nationwide gave rise to significantly high Ox concentrations.
Furthermore, in the previous section, analysis limited to meteorological conditions under which Ox concentrations tend to be higher was conducted and the relationship between Ox, NOx, and NMHC was not clear. Therefore, an analysis by another method was conducted.
As shown in Figure 3, annual averages of Ox concentrations during the daytime and those during the nighttime both showed an increasing trend. The increase in annual averages during the nighttime can possibly be attributed to an increase in global background concentrations, a decline in NO titration, the effect of transboundary transport, and other things [7]. The photochemical reaction of NOx and VOCs during the daytime leads to high Ox concentrations; so to assess the impact of countermeasures to precursors, it is valid to estimate the amount of Ox in the Kawasaki area during the daytime by excluding the effect of the previous day's nighttime Ox concentrations. Moreover, it is necessary to evaluate the impact of countermeasures during warm times of the year, when peak Ox levels typically occur, rather than all year around.

A New Indicator for the Impact of Countermeasures to Ox in Kawasaki
The EQS for Ox is one of the toughest standards in the world, and it is difficult to assess the impact of countermeasures simply by evaluating the EQS. Therefore, to assess the effectiveness of Ox-related environmental improvements, the MOE proposed a new indicator focusing on high Ox concentrations. The trends in the indicators from the MOE in Kawasaki are shown in Figure 8, and it was found that appropriately assessing the impact of countermeasures was not necessarily possible by the influence of transboundary transport in a case such as May 2019, when transport of pollution nationwide gave rise to significantly high Ox concentrations.
Furthermore, in the previous section, analysis limited to meteorological conditions under which Ox concentrations tend to be higher was conducted and the relationship between Ox, NOx, and NMHC was not clear. Therefore, an analysis by another method was conducted.
As shown in Figure 3, annual averages of Ox concentrations during the daytime and those during the nighttime both showed an increasing trend. The increase in annual averages during the nighttime can possibly be attributed to an increase in global background concentrations, a decline in NO titration, the effect of transboundary transport, and other things [7]. The photochemical reaction of NOx and VOCs during the daytime leads to high Ox concentrations; so to assess the impact of countermeasures to precursors, it is valid to estimate the amount of Ox in the Kawasaki area during the daytime by excluding the effect of the previous day's nighttime Ox concentrations. Moreover, it is necessary to evaluate the impact of countermeasures during warm times of the year, when peak Ox levels typically occur, rather than all year around.

Definition of Daytime Photochemical Oxidant Production
Not only peak Ox levels but also the amount of generated Ox is an important indicator. A new method by which the amount of Ox generated during the daytime is estimated was devised. The previous day's nighttime Ox concentrations are defined by the averaged Ox monitored at all General sta. over a 9 h period, from one day's 8 p.m. to the next day's 5 a.m. The daytime Ox concentrations are defined by the averaged Ox monitored at all General sta. over a 15 h period, from 5 a.m. to 8 p.m. The daytime production of photochemical oxidant (DPOx) is defined by the daytime Ox concentrations less the previous day's nighttime Ox concentrations. An image of DPOx is shown in Figure 29. Note that if the daytime Ox concentrations are lower than the previous day's nighttime Ox concentrations, DPOx is zero on the assumption that there has been no Ox production during the daytime.

Trend in the DPOx
Calculation of the averaged DPOx for each fiscal year from April to October was conducted. PSW are issued during April to October. Three-year moving averages are employed similarly to the new indicator from the MOE. The trend in the three-year moving averages of DPOx is shown in Figure 30. DPOx was trending upward until FY2004-FY2006 and after that trending downward.

Trend in the DPOx
Calculation of the averaged DPOx for each fiscal year from April to October was conducted. PSW are issued during April to October. Three-year moving averages are employed similarly to the new indicator from the MOE. The trend in the three-year moving averages of DPOx is shown in Figure 30. DPOx was trending upward until FY2004-FY2006 and after that trending downward.
Furthermore, the trends in NOx and NMHC from April to October are shown in Figure 31. Similar to Figures 17 and 23, showing annual averages, the trends in NOx and NMHC from April to October are decreasing. The trends agree with the trends in DPOx after FY2004-FY2006.
Although trends in precursors decreased over 30 years, the averaged DPOx from April to October increased until around FY2006. Similar increasing trends until FY2006 are shown not only in DPOx but also in the annual averages of the daily maximum Ox, the annual average of hourly Ox, and the new indicator from the MOE.
When NOx concentrations are high, the effect of a quencher of Ox through NO titration overcomes the effect of Ox formation by NO 2 . On the other hand, when a decrease in NOx emissions results in a decrease in NO concentrations, the titration effect by NO is not dominant, resulting in an increase in Ox concentrations in spite of a decrease in NOx concentrations. However, a further decrease in NO by a further reduction in NOx emissions will negate the titration effect and a decrease in Ox will be expected [20].
Calculation of the averaged DPOx for each fiscal year from April to October was conducted. PSW are issued during April to October. Three-year moving averages are employed similarly to the new indicator from the MOE. The trend in the three-year moving averages of DPOx is shown in Figure 30. DPOx was trending upward until FY2004-FY2006 and after that trending downward. Figure 30. Trend in three-year moving averages of DPOx from April to October. Note: If the daytime Ox concentrations are lower than the previous day's nighttime Ox concentrations, the day's DPOx is zero, and the day's DPOx is included in the averaged DPOx. Note: If the total number of hours with no measurement data is over 5 between the one day's 9 p.m. and the next day's 8 p.m., the day's DPOx is excluded from averaged DPOx.   Figures 17 and 23, showing annual averages, the trends in NOx and NMHC from April to October are decreasing. The trends agree with the trends in DPOx after FY2004-FY2006.
Although trends in precursors decreased over 30 years, the averaged DPOx from April to October increased until around FY2006. Similar increasing trends until FY2006 are shown not only in DPOx but also in the annual averages of the daily maximum Ox, the annual average of hourly Ox, and the new indicator from the MOE.
When NOx concentrations are high, the effect of a quencher of Ox through NO titration overcomes the effect of Ox formation by NO2. On the other hand, when a decrease in NOx emissions results in a decrease in NO concentrations, the titration effect by NO is not dominant, resulting in an increase in Ox concentrations in spite of a decrease in NOx concentrations. However, a further decrease in NO by a further reduction in NOx emissions will negate the titration effect and a decrease in Ox will be expected [20].
It was suggested that the suppression of Ox formation by a further decrease in NOx led to the decreasing trend in DPOx after FY2006.   In FY2013, the summer high-pressure system over the Pacific was coupled with a Tibetan high-pressure system that covered the whole of Japan. Air temperatures were high, solar radiation was intense, and wind speeds were low, making it easy for high concentrations of Ox to emerge. These meteorological conditions continued throughout July and August unusually [9]. In FY2019, high Ox concentrations occurred nationwide due to transboundary pollution from the continent [10], and in Kawasaki, under the influence of the pollution, PSW were unusually issued on two consecutive days in May, when PSW are hardly issued in an ordinary year. Although the new indicator from the MOE decreases the yearly fluctuation by using the three-year moving average, the indicator is more sensitive to high Ox concentrations due to meteorology and transboundary pollution, not due to emissions of precursors, because the indicator is focused on the high-Ox-concentration zone.
Therefore, the correlation between DPOx from April to October and precursors' concentrations was closer than the one between the new indicator from the MOE and precursors' concentrations. As suggested above, DPOx is a suitable indicator for showing the effect of emission reduction of precursors in the Kawasaki area.
(a) (b) Figure 31. Trends in NOx and NMHC from April to October. (a) NOx: annual averages of hourly NOx for all nine General sta. from April to October. (b) NMHC: annual averages of hourly NMHC for seven General sta. from April to October.

Comparison between DPOx and the New Indicator from the MOE
The trends in DPOx with the new indicator from the MOE are shown in Figure 32. The trends in DPOx and the new indicator from the MOE are similar until FY2010-FY2012. On the other hand, after FY2011-FY2013, DPOx shows a decreasing trend, while the new indicator from the MOE shows a flat trend. In fiscal years including FY2013 or FY2019, such as FY2011-FY2013, the difference between DPOx and the new indicator from the MOE is larger.

Conclusions Results
To evaluate the impact of countermeasures taken in Kawasaki on air pollution, hourly atmospheric concentrations of Ox and its precursors NOx and NMHC were analyzed with meteorological data obtained at 18 monitoring stations during 30 years from 1990 to 2019.
(1) The annual average Ox concentration in Kawasaki increased from 1990 to 2013 and remained flat after that. In terms of seasons, the increase in spring was remarkable.
Since the formation and distribution of Ox are affected by meteorology, trends in atmospheric temperature, wind speed, and solar radiation in Kawasaki were analyzed. The temperature showed an upward trend over 30 years. Seasonally, the rate of increase in May was remarkable. The trend in wind speed was downward, and the rates of decline between spring and autumn were larger than in winter. Meanwhile, solar radiation showed an upward trend, with the rate of increase in March and May higher than in other months. From these meteorological points of view, Ox formation potential has been increasing. (2) Emissions of NOx and VOCs reduced dramatically, and their environmental concentrations were also gradually decreasing. NOx emissions from stationary sources declined in a linear fashion due to such factors as the comprehensive total emissions control scheme conducted by the government of Kawasaki City. Emissions from motor vehicles also decreased substantially, and since 2003, when restrictions on the operation of diesel vehicles were imposed, the rate of decrease has been especially marked. The drop in NO has been especially conspicuous, resulting in a substantial change in the ratio of NO to NO 2 . The large fall in NO has probably been a factor in the significant drop in emissions from motor vehicles. As a result, the ratio of NO 2 in the NOx emitted from motor vehicles in Kawasaki increased from 0.1 to 0.3 from FY2004 to FY2019, though its ratio had remained around 0.1 until FY2000. Both emissions and environmental concentrations of NMHC declined monotonously, but in recent years, the rate of decline in NOx concentrations has outstripped the rate of decline in NMHC concentrations. Until around 2005, the NMHC/NOx (ppmC/ppm) ratio remained in the vicinity of 5.5, but from 2005, it showed a rising trend such that by 2019, it had reached around 6.5.
(3) The amount of generated Ox is another important indicator. Methods to estimate the amount of generated Ox during the daytime were introduced. This attempt was developed into DPOx as a new indicator for assessing the effectiveness of countermeasures for Ox reduction. This indicator involves deducting one day's night concentration from the next day's daytime concentration, allowing only the Ox generated in the area during the day to be assessed. The three-year moving average of the average DPOx during April to October exhibited a declining trend from FY2006, much like the new indicator from the MOE.

Discussion
The annual average concentration of Ox has been increasing in spite of the decrease in ambient NOx and VOCs since the 1990s. However, from the 2000s, a decrease in in situ photochemical production occurred in four metropolitan areas in Japan. It is expected that the reduction in NOx over a certain limit should bring further reduction in Ox, which will overcome the increase in Ox due to the reduction in NO titration [20]. Moreover, it has been shown that high Ox concentrations in Tokyo have fallen since the decline in NO 2 after 2000 and the introduction of restrictions on VOC emissions in 2006 [18]. Because of these, a reduction in the three-year moving average of the averages of DPOx in Kawasaki for April-October from FY2006 was assumed to reflect a decrease in photochemical production in the city from around FY2006, because the declines in the NOx concentration and the NO 2 concentration from the 2000s shown in Figure 18, the decline in the NMHC concentration shown in Figure 24, and the increase in NMHC/NOx shown in Figure 25 exceeded a certain limit. Therefore, it is supposed that DPOx may be a suitable indicator that reflects the impact of reductions in sources of NOx and VOCs in Kawasaki.
In light of the above, though meteorological conditions during the past 30 years have made it more likely for high concentrations of Ox to occur, it is considered that the amount of daily production of Ox has progressively decreased with reducing NOx and VOCs, which are Ox precursors, since FY2006. The same trend of reduction can also be shown in the number of PSW days and the new indicator from the MOE. However, because the new indicator from the MOE reflects zones with high Ox concentrations, it will also be affected by factors other than sources within the area, such as meteorological factors and transboundary transport of Ox. The effects of countermeasures for Ox control are thought to occur daily, not only on days of high concentration, but also on days of low concentration of Ox. Therefore, it is suggested that DPOx from April to October, as assessed based on the daytime production of Ox, is an indicator that reflects the impact of measures to reduce Ox precursors in sources within the area. Nevertheless, because this indicator could show unique tendencies due to sources being in close proximity in Kawasaki, we hope that analysis will be performed for various regions in which this indicator is being applied.
Author Contributions: This paper was written jointly by all the authors. S.W. designed the study and exercised overall supervisory control. A.F. analyzed the data on meteorology, NMHC, and relationships between Ox and its precursors involved in high Ox concentrations, while T.S. analyzed the data on NOx. K.F. contributed to the analysis of the Ox data and designed DPOx. D.Y. and S.I. edited the entire manuscript. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

Data Availability Statement:
Publicly available datasets were analyzed in this study. This data can be found here: http://sc.city.kawasaki.jp/taiki/DOWNLOAD/YMSelectsk.html (accessed on 27 March 2021).