Does Light Pollution Affect Nighttime Ground-Level Ozone Concentrations?

: Ground-level ozone (O 3 ) is mainly produced during daytime in the presence of ultraviolet (UV) light and later destroyed by nitrogen oxides during nighttime. However, light pollution caused by the excessive use of artiﬁcial lights may disrupt the chemistry of night-time ground-level O 3 by providing enough energy to initiate nighttime ground-level O 3 production. In this study, nighttime (7 p.m. to 7 a.m.) ground-level O 3 , nitrogen oxide (NO), and nitrogen dioxides (NO 2 ) concentrations were observed for three years (2013, 2014, and 2015). The existence of O 3 was found during nighttime, especially in urban areas with a concentration range of 8–20 ppb. The results suggested that nighttime variations of ground-level O 3 concentrations were higher in urban areas than in suburban areas. The mean nighttime O 3 concentration at urban sites varied, possibly because the distribution of anthropogenic lights around the urban sites is brighter than in suburban locations, as indicated by the data from the light-pollution map. This anthropogenic light has not caused the suspected nighttime photolysis processes, which directly slowed nighttime oxidation. The photochemistry rate of J NO 2 /k 3 was supposed to be near zero because of the absence of photochemical reactions at night. However, the minimum concentration in all urban and suburban sites ranged from 2–3 ppb, indicating that O 3 might also form at night, albeit not due to light pollution.


Introduction
Artificial light has become an essential part of modern civilisation in everyday life and culture because of industrialisation and modernisation [1]. Streets, roads, bridges, airports, commercial and industrial buildings, parking lots, sports centres, and homes are artificial light sources [2]. The use of artificial lights directly degrades natural light in the environment because the former adversely affects not only the health of humans [3], animals, and ecosystems, especially nocturnal wildlife, but also the quality of the sky and air [4]. Stark et al. [4] indicated that street lights are dimmer than the sun and up to 25 times brighter than the full moon.
Ground-level ozone (O 3 ) exists as one of the secondary air pollutants in the atmosphere and is listed as one of the vital air pollutants affecting Malaysia's air quality [5]. The toxicity of O 3 may affect human health and vegetation [6] Chemically, O 3 formation is induced by ultraviolet (UV) light in the presence of its precursors, such as nitrogen oxides (NO x ) and volatile organic compounds (VOCs) [7,8], and this daytime reaction is called a photochemical reaction. In addition to the variability of its precursors, UV intensities and meteorological factors influence O 3 formation [3]. During the day, ground-level O 3 formation is usually related to hydroxyl radical (OH) chemistry. OH is inherently linked to the chemistry of nitric oxide (NO) and nitrogen dioxides (NO 2 ); meanwhile, at night, nitrate radical (NO 3 ) serves as a night-time oxidant [9,10].
O 3 transformation during daytime and nighttime should be extensively explored. Previous measurement and modelling studies have investigated the effects of anthropogenic sources on diurnal variation during daytime and nighttime [11][12][13]. Overall, variations in O 3 at night have been studied, and the possible factors that contribute to the presence of O 3 at night have been analysed [14][15][16][17][18]. Nighttime O 3 variations are attributed to the weakening of vertical mixing at night [18] and the effects of emissions and meteorological factors [14][15][16]19,20].
Yan et al. [18] observed the O 3 variability over 1000 US sites from 1990 to 2014 and found that NO x reduction led to changes in O 3 concentrations at night because of a decrease in O 3 titration rates. Another oxidant, NO 3 , generated at night by the reaction of NO 2 with O 3 , directly acts as a cleansing compound because it neutralises some NOx that pollutes daytime air and leads to wheeze-inducing levels of O 3 [21]. NO 3 further reacted with NO 2 to establish a chemical equilibrium with N 2 O 5 [22]. However, further research has shown that this nightly cleansing action is not as effective as expected because NO 3 is being destroyed by the light reflected in the sky by outdoor lighting on the ground [4,23]. In the presence of anthropogenic lights, NO 3 is destroyed, and O 3 concentrations increase through possible chemical reactions occurring at night as in reactions (R1) and (R2). Reaction (R1) can occur during daytime; however, NO 3 is quickly photolysed by daylight (R3), and NO 3 and N 2 O 5 are heavily suppressed during the day.
Stark et al. [4] revealed that this nighttime phenomenon is related to increasing the number of photons in the atmosphere and directly enhancing O 3 photochemical reactions. Stark et al. [23] also performed aircraft measurements and found that UV intensities from anthropogenic light sources at night enhance O 3 formation in brighter cities. O 3 production is not limited to daytime; this phenomenon likely occurs all day. Nevertheless, the nighttime production rate is very much lower than the daytime photochemical reactions. The loss of NO 3 significantly affects nighttime pollutant levels and next-day O 3 production because light pollution exacerbates due to rapid urbanisation. Demands for light during nighttime will continuously increase in the future because more locations are being transformed into urbanised cities, where nighttime light intensities are higher. Thus, this study intended to critically investigate the possible relationship between nighttime light pollution and ground-level O 3 variations in urban and suburban areas in Malaysia. Differences in light intensities in urban and suburban areas are crucial to this study to establish the possible impact of light pollution on nighttime O 3 concentrations.

Light Pollution in Urban and Sub-Urban Areas
Specific sites in urban and suburban areas were selected to compare their nighttime O 3 concentrations. Putrajaya (PT), Seremban (SB), and Bandaraya Melaka (BM) were chosen as representatives of urban sites. These sites were situated in the cities where the population density was higher than that of suburban sites, i.e., Muar (MR), Indera Mahkota (IM), and Tanjung Malim (TM). A city is considered a significant contributor to light pollution because of more artificial light sources, such as street lights, security lights, lights on vehicles, and lighted buildings and towers that vary to many degrees [1,24]. Figure 1 illustrates the irradiance map of peninsular Malaysia. Table 1 depicts the Bortle dark-sky scale [25], artificial brightness (artificial lights that increase the night sky luminance), and brightness (natural brightness of the night sky) recorded at different location profiles by referring to a light-pollution map [26]. Amongst the selected sites, PT had the highest artificial brightness (6350 µcd/m 2 ) and brightness (6520 µcd/m 2 ). It was classified as class 8-9, a city sky by the Bortle dark-sky scale. The lowest artificial brightness and brightness were recorded at the TM site, representing the suburban site with 713 and 884 µcd/m 2 , respectively. TM was classified as class 5 (suburban sky) based on the Bortle dark-sky scale.
sen as representatives of urban sites. These sites were situated in the cities where the population density was higher than that of suburban sites, i.e., Muar (MR), Indera Mahkota (IM), and Tanjung Malim (TM). A city is considered a significant contributor to light pollution because of more artificial light sources, such as street lights, security lights, lights on vehicles, and lighted buildings and towers that vary to many degrees [1,24]. Figure 1 illustrates the irradiance map of peninsular Malaysia. Table 1 depicts the Bortle dark-sky scale [25], artificial brightness (artificial lights that increase the night sky luminance), and brightness (natural brightness of the night sky) recorded at different location profiles by referring to a light-pollution map [26].Amongst the selected sites, PT had the highest artificial brightness (6350 µcd/m 2 ) and brightness (6520 µcd/m 2 ). It was classified as class 8-9, a city sky by the Bortle dark-sky scale. The lowest artificial brightness and brightness were recorded at the TM site, representing the suburban site with 713 and 884 µcd/m 2 , respectively. TM was classified as class 5 (suburban sky) based on the Bortle dark-sky scale.   [25]; ** referring to light-pollution map [26].  (12.00 a.m.-12.00 a.m.) consecutively for 3 years. The data were grouped as daytime (7 a.m. to 7 p.m.) and nighttime (7 p.m. to 7 a.m.) to differentiate between daytime and nighttime analysis. The secondary data's quality control and quality assurance were carried out properly according to the standard [29]. Table 2 shows the details of the monitoring instruments, monitoring periods, and total durations to collect the data on O 3 , NO, and NO 2 concentrations.

Nighttime Chemistry and O 3 Formation
Clapp and Jenkin [22] revealed a point where NO 2 is destroyed and reproduced fast to maintain a cycle. The rate of NO 2 photolysis (photolyis, J NO 2 ) as a coefficient of the reaction of NO with O 3 (k 3 ) was calculated using Equation (1) to determine the variations in O 3 production rates during daytime and nighttime [21,22].
This result provided the value of O 3 production and destruction during daytime and nighttime.  [34]. PT had the highest variation of O 3 , NO, and NO 2 because it was located in Malaysia's urban city and federal administrative capital. It is also an intelligent garden city with a well-lit area for night sightseeing activities. TM had the lowest trend of O 3 , NO, and NO 2 and represented the suburban site trend. The trend exhibited similarities to previous findings in urban and suburban sites, peaked during daytime (2 p.m.), and decreased during nighttime, but the concentrations differed [14,16,17,35]. Faid et al. [24] investigated the profile of the night sky in Malaysia. They found that population and location distance from a city are the significant variables of light pollution because Kuala Lumpur is five times brighter than Teluk Kemang, a suburban sky.    The minimum mean O 3 concentration during nighttime did not reach zero, and the value remained stable at 8-20 ppb at an urban site until the following day. In a suburban area, the concentrations were between 8 and 14 ppb. Even though the values were not high, the O 3 concentrations should be at zero during nighttime because of the absence of sunlight. This is due to the decreased hydroxyl radical (as they are produced mainly OH) concentrations from the photolysis of stable molecules, and supposedly no further reaction occurs [16]. HONO photolysis contributes more than 50% of production on average daytime, while nighttime OH production is mainly from the O 3 reactions with alkenes [36]. The radical propagation is efficient due to the abundance of NO in an urban environment, which results in O 3 production. Khan et al. [37] found that NO 3 and OH levels during nighttime were in the range of 0.01 to 10 ppt and 1 × 10 4 to 1 × 10 6 molecule cm −3 , respectively. The OH and NO 3 concentration variations in different locations depended on the air and tended to be lower in clean air masses and higher in polluted areas (urban areas). A NO 3 radical, another oxidant, is generated during nighttime via the reaction of NO 2 and O 3 . NO 3 radicals react with NO 2 to establish a chemical equilibrium with N 2 O 5 [38].

Hourly Diurnal Variation in Urban and Suburban Area
Besides anthropogenic sources and O 3 precursors [14][15][16], mean nighttime O 3 concentrations in urban sites vary because the distribution of anthropogenic lights around the metropolitan area had more brightness than that in the suburban locations. The brightness of light with specific wavelength intensities disturbs the reaction of nocturnal nitrogen oxides (NO 3 and N 2 O 5 ) that have the potential to either remove nitrogen from the atmosphere or store it and then release it during daytime. Even though the effect is small, this parameter is measurable and accounts for 2-3% of the NO 3 loss in some brighter sites Still, this phenomenon has different consequences for O 3 formation [39], where the lighting in urban sites influences NO 3 photolysis as a sink for NO 3 and N 2 O 5 at night.

Nighttime Ozone Formation
The photochemistry rates of O3, NO, and NO2 concentrations at urban and suburban sites depend on the rate of NO2 photolysis (JNO2) and NO titration (k3), and their reaction rates can be depicted in a timescale over a few minutes [21]. The average diurnal variations in JNO2/k3 rates in urban and suburban areas are illustrated in Figure 5. The JNO2/k3 shows the average hourly formation of O3 concentrations. The figure also demonstrates

Nighttime Ozone Formation
The photochemistry rates of O 3 , NO, and NO 2 concentrations at urban and suburban sites depend on the rate of NO 2 photolysis (J NO 2 ) and NO titration (k 3 ), and their reaction rates can be depicted in a timescale over a few minutes [21]. The average diurnal variations in J NO 2 /k 3 rates in urban and suburban areas are illustrated in Figure 5. The J NO 2 /k 3 shows the average hourly formation of O 3 concentrations. The figure also demonstrates that the diurnal variations in J NO 2 /k 3 differ in urban and suburban areas, and this phenomenon is similar to that described by Han et al. [21] in Tianjin, China.

Conclusions
The secondary data in 2013, 2014, and 2015 from the Department of Environment, Malaysia has been used to investigate the possible relationship between nighttime light pollution and ground-level O3 variations between two sites: urban and sub-urban areas in Malaysia. The data were grouped as daytime (7 a.m. to 7 p.m.) and nighttime (7 p.m. to 7 a.m.) to analyze the variations. An hourly diurnal plot was used to depict the variations during daytime and nighttime. Remarkably, from the results, urban sites (PT) had the highest trend of nighttime O3, NO, and NO2 because they were located in the urban area, with mean nighttime O3 concentrations ranging from 8 ppb to 20 ppb. Urban sites (PT) also had the highest artificial brightness (6350 µcd/m 2 ) and brightness (6520 µcd/m 2 ), respectively. The mean nighttime O3 concentration in urban sites varied, possibly because the distribution of anthropogenic lights around the metropolitan area had more brightness than in suburban locations. Suburban sites (TM) had the lowest trend of O3, NO, and NO2, with the mean nighttime concentration ranging from 8 ppb to 14 ppb. However, the minimum JNO2/k3 in all urban and suburban sites was two ppb, indicating that O3 was formed during nighttime. The highest photochemistry rate was 14.8 ppb, recorded in the urban area (PT) in 2013. Thus, O3 production during daytime could be increased by In urban sites, the O 3 concentrations in SB increased after 11 p.m. Likewise, the O 3 concentrations in BM also increased, but instead the increment was recorded at 9 p.m. In suburban sites (IM and TM), the O 3 production rates increased after 9 p.m. These findings might indicate some light pollution contribution to O 3 formation. Theoretically, during nighttime, the value of J NO 2 /k 3 should be zero because of the absence of photochemical reactions. However, the minimum photochemistry rate in all urban and suburban sites was two ppb, indicating that O 3 was also formed at night, even with minimal value. This result tends to be higher than those Awang et al. [40] found in the urban site (0.10 ppb). This difference is due to the variation in the weather parameters, including different locations (urban port cities) and climate. The highest nighttime photochemistry rate was 14.8 ppb, recorded at PT in 2013.
The obtained result showed that during the night, O 3 photochemical reactions failed to stop. Stark et al. [4] stated that nighttime oxidation could be slower due to lights.