3.1. Diurnal and Seasonal Variability of Inorganic and Organic Species
According to Figure 1
a, the highest daytime ozone levels in Moscow were observed from April to August, following closely the annual cycle of the total intensity of solar UV radiation measured at the MSU-IAP site [30
]. The observed abrupt increase of ozone levels in early spring, peaking in April, is a general feature of ozone climatology over the continent as evidenced from a variety of ozone measurements in both polluted and background environments [33
]. It has been suggested previously that the accumulation of ozone precursors during the cold season at northern latitudes provides a great potential for intense production of ozone under rapid increases of solar UV radiation in late winter and into early spring (see more discussion in [20
]). It is likely that the observed springtime ozone maximum at MSU-IAP has two primary contributions, which are the large-scale tropospheric chemistry and local ozone photochemical production in the polluted boundary layer under sufficient abundance of odd nitrogen species. Another distinct feature of the MSU-IAP ozone seasonal cycle is a prolonged spring–summer maximum [28
], which undoubtedly reflects the primary contribution of large local sources of ozone precursors in the photochemically active period (April–September) [34
] and not the downward ozone flux from the stratosphere and continental-scale transport from other potentially important areas of net photochemical production of ozone in the western part of North Eurasia. Over the whole observation period, the highest ozone levels were observed in July 2011, when daytime O3
mixing ratios exceeded the short-time MAC (80 ppb or 160 μg/m3
). These episodes are described in detail below.
Seasonal daytime NOx variation is opposite to that of O3
b) with the highest values being observed in winter (up to 130 ppbv). In summer, NOx levels usually did not exceed 30 ppbv. Evidently, the observed four-fold increase in wintertime NOx levels with respect to the summer values is a local manifestation of seasonal variations in vertical mixing intensity and associated atmospheric residence times of pollutants in the lower troposphere over the continent.
Since the origin of the observed enhanced levels of ozone in Moscow during the summer months is photochemical generation, we focus below on the summertime period of the observations to explore some basic features of the urban ozone photochemistry.
Meteorological observations at the MSU-IAP site during the summers of 2011–2013 (Figure 2
) show favorable conditions for accumulation of ozone precursors in the surface air and active photochemistry during a substantial part of the summer months. In the observed period, daytime air temperature often reached 25–30 °C. Wind speeds were ≤2 m/s on average and exceeded 3 m/s only in 5% of the whole observation time, thus evidencing generally stagnant meteorological conditions. The prevailing wind directions were from W and NW to N, which correspond to the transport of relatively clean air, subjected to a limited impact of regional pollution sources.
According to Figure 3
mixing ratio increases monotonically from early morning, reaching its maximum value of 34 ppbv (sample mean) at ~4 p.m. LT The diurnal variations of NOx, CO, and benzene, the latter being an important ozone precursor with anthropogenic origin, are opposite to that of O3
and characterized by a stable decrease from their morning peak values at 8 a.m. to late afternoon minimums at ~3–4 p.m. LT. Thus, the observed diurnal variation of the secondary pollutants was strongly and negatively correlated in the day time hours with the depth of the convective mixing layer, and the associated changes with vertical mixing intensity, over Moscow, which peaks at 1–3 p.m. LT, following a daily cycle of radiation and heat balance [37
]. Additionally, chemical distraction of the primary emitted VOCs, with benzene as an example, may contribute to their late afternoon decrease accompanied by significant accumulation of ozone and other secondary pollutants (Figure 4
). According to Figure 4
, the maximum O3
production rate is observed during the late morning hours (10–12 a.m. LT), which also coincides with diurnal peaks of the most abundant secondary VOCs (acetaldehyde, acetone, and acetic acid). The origin of the observed late morning maximum of the above species can be attributed to both photochemical oxidation of primary emitted VOCs after sunrise and entrainment of partially oxidized products from the upper layers under development of the convective boundary layer. The latter may be especially efficient for ozone accumulation in prolonged stagnation conditions leading to overnight retention of secondary organics above a nocturnal inversion layer [38
]. All compounds discussed except for O3
are not normally distributed (see Figure S1 in the Supplementary Materials
). Thus, we calculated the nonparametric Spearman’s correlation coefficients (Rs
) to measure the strength of association between maximal afternoon O3
(3–5 p.m. LT) and maximal morning (10–12 a.m. LT) mixing ratios of some secondary VOCs during the summer months from 2011–2013. High correlations were found between acetaldehyde, ethanol, acetone, and acetic acid (Rs
= 0.55–0.91) as well as between the above species and the products of isoprene oxidation, MVK and MACR (Rs
= 0.67–0.81) (Table 2
). The positive correlation of daytime O3
= 0.45–0.52) with secondary VOCs affirms an important role of photochemical O3
production in Moscow during the summers of 2011–2013.
According to the MSU-IAP observations, the toluene to benzene ratio (T/B) was mainly in the range of 1–2 (mean = 1.39; P75 = 1.50; P90 = 1.89) (Figure 5
). This ratio is known to be a safe indicator of vehicle pollution as well as proximity of the sampled air to the associated pollution sources [39
]. A T/B ratio approaching 1 indicates traffic-originated emission sources, and the value increases with the closeness of the pollution source [44
]. It suggests that the dominated anthropogenic VOCs source in the region of the MSU-IAP site during the summers of 2011–2013 was local vehicle emissions.
The diurnal cycles of biogenic VOCs, isoprene, and monoterpenes were generally weak (Figure 3
), which can be explained by the direction of the prevailing winds (Figure 2
) blowing mainly from areas with highly limited vegetation. Isoprene levels are the highest during the morning hours (07:00–10:00 a.m.) whereas monoterpenes increase during evening and nighttime hours owing to their emissions at these hours and accumulation in the stable atmosphere.
Since motor vehicle emissions dominate over other pollution sources in urban environments, VOCs primarily produced by motor vehicles (benzene, 1.3-butadiene etc.) are commonly used as suitable exhaust tracers. This allows for quantifying the traffic contributions to ambient isoprene and then separating biogenic isoprene from traffic emissions [45
]. The nighttime isoprene/anthropogenic VOC ratio in a vehicle-polluted atmosphere is supposed to characterize vehicle exhaust [46
]. We then used benzene concentration to estimate the traffic contribution to isoprene at the MSU-IAP site. Since biogenic isoprene emissions are strongly dependent on air temperature and the amount of local vegetation, nighttime winter isoprene concentrations are supposed to be of anthropogenic origin. Consequently, the monthly mean diurnal isoprene/benzene ratio was lowest during the cold season and reached its maximum during the summer months (mean = 0.94), thus following a seasonal cycle of biogenic isoprene emissions (Figure 6
). Yet, the observed high positive nonparametric Spearman’s correlation between isoprene and benzene on summer days (Rs
= 0.69) clearly indicates significant contribution of the anthropogenic signal in isoprene data during the warm season as well. A somewhat better correlation is observed for nighttime winter values (Rs
= 0.75). The nighttime winter isoprene/benzene ratio in the observed site was calculated to be 0.7 on average (see scatter diagram on Figure 6
). It was about 5% of the summer daytime isoprene/benzene ratios. Thus, we suppose that about 5% of the daytime summer isoprene in Moscow had an anthropogenic origin, presumably from vehicle exhaust.
3.3. High O3 Episodes in Moscow
In summer 2011, two high O3
concentration episodes at the MSU-IAP site were observed, these were from the 27 July (3–5 p.m. LT) and 28 July (2–5 p.m. LT) pollution events. In these events, hourly O3
mixing ratios exceeded 80 ppbv, which is well above the short-time MAC value (Figure 10
Both episodes were observed in the period of hot weather (daytime air temperature >30 °C). During the first episode (27.07), the daytime O3
mixing ratio increased stably and peaked at 4–5 p.m. LT whereas the observed non-methane hydrocarbons (NMHC) and NOx mixing ratios increased along with O3
until 4 p.m. LT with a subsequent decrease throughout late afternoon. During the second episode (28.07), a simultaneous increase in O3
with a decrease in NMHC and NOx, changed by some outliers, in the concentration graphs of all compounds was observed (Figure 10
, right graph) from 3:30 to 5 p.m. LT.
According to the meteorological data (Figure 11
), in July 2011 there was hot and dry weather because of strong anticyclonic conditions. From 27 to 28 July, the anticyclone decayed, which was accompanied by the respective drop in atmospheric pressure. In the evening (6–9 p.m. LT), thunderstorms with heavy rain (about 14 mm of precipitations) and squall winds were observed, accompanied by a change in wind direction from west to east.
The observed drop in the concentrations of the species at 28 July.2011 is assumed to be connected with the change in the weather conditions and ventilation of the boundary layer through the passage of the front system.
Some studies reported that daytime NMHC/NOx ratios lower than 10 are associated to VOC-sensitive ozone production regimes whereas daytime NMHC/NOx ratios greater than 20 correspond to NOx-sensitive ozone photochemistry [10
]. Figure 12
shows that during the first high ozone episode, the NMHC/NOx ratio was in the range of 10–20, which points to photochemical ozone production. During the second episode, O3
was produced mainly in NOx-sensitive (~in 80% of cases) and transitional (~in 20% of cases) regimes.
There was no significant increase in the concentrations of VOCs measured on 27 and 28 July 2011 compared to that of other days of the month, except for two compounds: acetone and acetic acid (Figure 13
). During the high ozone episodes, acetone, acetic acid, and isoprene concentrations exceeded their averages in July 2011 by 5.0, 4.6, and 0.3 ppbv, respectively. Acetone and acetic acid can be both biogenic and anthropogenic. This is confirmed by the high Spearman’s correlation of these compounds with biogenic isoprene (Rs
= 0.7–0.8), 2-methyl-3-buten-2-ol (MBO) (Rs
= 0.8–0.9), and anthropogenic benzene (Rs
= 0.5–0.6) according to our data.
3.4. VOCs Impact on O3 Generation in Moscow
To estimate the impact of the measured VOCs on ground-level ozone production in Moscow, we employed the widely used quantity, ozone-forming potential (OFP) [49
is a VOC concentration having the dimension of μg/m3
is a maximum incremental reactivity, a dimensionless quantity defined as grams of O3
produced per gram of the VOC [50
]. The method allows for estimating the maximum ozone concentration produced from the chemical destruction of the given VOC based on predefined MIRVOC
For all the measured VOCs in Moscow during the summers of 2011–2013, the OFP values did not exceed 115 μg/m3
for 90% of the measurement time (Figure 14
), with the highest OFP values observed during the summer of 2011 (mean 67.3 μg/m3
, 90-th percentile 103 μg/m3
). Acetaldehyde, 1.3-butadiene, and isoprene were found to play the leading roles in O3
generation, whereas benzene, styrene, phenol, and monoterpenes made a minor contribution to O3
formation in the city (Table 3
and Table 4
During the high July 2011 ozone events, the 90th percentile of OFPs for the total observed VOCs reached 110–136 μg/m3
(55–68 ppbv), with acetaldehyde (42–60 μg/m3
in 90% of cases) and 1.3-butadien (16–19 μg/m3
in 90% of cases) making the highest contribution to ozone generation. At the same time, the estimated inputs of acetone, acetic acid, and isoprene, which increased during the high O3
episodes (Figure 13
), to ozone production were about 2 times less than that of acetaldehyde and 1.3-butadien. The highest O3
production was found during the first episode on 27 July, when the anticyclonic meteorological conditions were not yet changed, as those changes took place during the second episode.
Thus, the total contribution of all the measured VOCs to daytime ozone levels in Moscow was found to be significant in hot and calm weather conditions. O3 production from VOCs was 31–67 μg/m3 on average and in 10% of cases exceeded 100 μg/m3. Anticyclonic conditions with high air temperature (above 30 °C), low cloud cover, and low wind speeds led to active ozone generation in the presence of local pollution sources and resulted in near-surface day-time ozone mixing ratios exceeding hazardous levels (OFPs from total VOCs was about 100–122 μg/m3 on average and were higher than 136 in 10% of calculations).