Elena V. Berezina

Results of the analysis of interannual, seasonal, and diurnal variations in the ratios of the mixture of CH₄, CO₂, CO, benzene, and δ13C–CH₄ in the surface air of Moscow based on the measurements at the station of the Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences (IAP RAS), in 2018–2020 are presented. The annual maximum concentration of CH₄, CO₂, and CO (>2.2, 430, and 0.20 ppm, respectively) occurs in the winter months due to the seasonal increase in the anthropogenic load from the main sources of urban pollution: motor transport and thermal power plants and a decrease in the role of vertical air mixing. The greatest contribution of local and remote microbial sources to the CH₄ concentration is noted in the summer months against the background of low δ13C–CH₄ values (–50 to –60‰). In all seasons, CH₄, CO₂, and CO ground level peaks lasting up to several hours are due to the transport from industrial sites in the E–SE sector. The calculated mean emission ratios in urban air were CH₄/benzene = 0.52–0.54 ppm/ppb, CH₄/СО = 0.56–0.75 ppm/ppm, CO₂/benzene = 77–93 ppm/ppb, CO₂/СО = 81–131 ppm/ppm, and CO/benzene = 0.65–1.11 ppm/ppb. These ratios characterize the predominant contribution of emissions from motor transport and the heat and power equipment and can be used to refine the absolute values of emissions, including those on the basis of existing inventories of sources of anthropogenic air pollution.

Near-surface observations of air mixing ratios of CH4, CO2, CO, benzene, and δ13C–СH4 at the IAP-RAS site in Moscow for years 2018–2020 are analyzed to describe typical interannual, seasonal, and diurnal variations. The highest mixing ratios of CH4, CO2, and CO (above 2.2, 430, and 0.2 ppmv, respectively) are mostly observed in winter as a result of the seasonal maxima in the emissions of these gases from motor transport and energy sectors and the slow removal of the emissions from the near-surface air due to suppressed turbulent vertical mixing in the cold season. The highest impact of local and distant microbial emissions on the CН4 mixing ratios is observed in summer, as follows from the low δ13C–СH4 values from –50 to –60‰. The highest increase in the mixing ratios of all the measured species is associated with air transport from the industrial area located at the east – southeast from the site. The estimated emission ratios CH4/benzene = = 0.52–0.54 ppmv/ppbv, СH4/СО = 0.56–0.75 ppmv/ppmv, СО2/benzene = 77–93 ppmv/ppbv, СО2/СО = = 81–131 ppmv/ppmv, СО/benzene = 0.65–1.11 ppmv/ppbv show the prevailing contribution of emissions from motor transport and energy sectors to the content of trace gases in the near-surface air in Moscow and are consistent with other similar estimates published on the basis of observations in large cities.

Measurements of near-surface methane (CH₄) mixing ratio and its stable isotope ¹³C were carried out from January 2018 to December 2020 at the A.M. Obukhov Institute of Atmospheric Physics (IAP) research site in the center of Moscow city. The data show moderate interannual variations in monthly mean CH₄ with maximum values being observed predominantly in winter (2.05–2.10 ppmv on average). The most δ¹³C depleted CH₄ (up to −56‰) is observed in summer and autumn following seasonal decrease in traffic load in the city. The highest CH₄ concentrations (>2.2 ppmv) were likely to be caused by air transport from the E–SE sector where potentially large microbial CH₄ sources are located (landfills and water treatment plants, Moscow River). Keeling plots of these episodes in different seasons of 2018–2020 showed δ¹³C isotopic signatures of about −58–−59‰ for the spring–autumn period and −67‰ for winter. A good correlation was observed between CH₄ and other pollutants: CO₂, CO, and benzene in daytime (10:00–19:00) hours (R > 0.7). Contribution of urban methane emissions due to vehicle exhausts (∆[CH₄]_auto) and microbial activity (∆[CH₄]_micro+) along with regional baseline mixing ratios of CH₄ ([CH₄]_base) and CO ([CO]_base) were estimated from the linear orthogonal regression analyses of the measured daytime mixing ratios. A significant role of microbial methane in the formation of CH₄ maximums in Moscow was revealed. Contributions of the upwind continental CH₄ and CO sources to the measured species levels were estimated through comparison with the Mace Head site data representative for the Northern Hemisphere baseline air. The study provides, for the first time, important insights into the long- and short-term variations of CH₄ levels in Moscow in connection to the local (urban) emissions and long-range transport from upwind continental sources. The results will contribute to elaboration of a default emission inventory in air quality modeling and help to identify the areas for targeted mitigation efforts.

Measurements of CO and 15 volatile organic compounds (VOCs) at the IAP-RAS (A.M. Obukhov Institute of Atmospheric Physics) site located in the center of Moscow were analyzed. Acetaldehyde, ethanol, 1.3-butadiene, isoprene, toluene and C-8 aromatics were established to be the main ozone precursors in the observed area, providing up to 82% of the total ozone formation potential of the VOCs measured. Diurnal and seasonal variations of the compounds are discussed. The concentrations of anthropogenic VOCs (acetaldehyde, benzene, 1.3-butadiene, toluene, and C-8 aromatics) did not exceed their maximum permissible levels, reaching their maxima in summer and autumn in the morning and evening hours. Biogenic ethanol and isoprene were the highest in summer midday but their concentrations were low enough (up to 4 and 0.4 ppbv, respectively) due to small vegetation area around the site. Emission ratios (ERs) for the main ozone precursors—acetaldehyde, ethanol, 1.3-butadiene, isoprene, toluene, and C-8 aromatics—were estimated from two-sided linear regression fits using benzene and CO as tracers for anthropogenic emissions, with spatial and temporal filters being applied to account for the influence of chemistry and local emission sources. The best estimates of ERs were obtained using benzene as a reference species. Anthropogenic fractions of VOCs (AFs) were then estimated. As expected, acetaldehyde, toluene, 1.3-butadiene, and C8aromatics were entirely anthropogenic and emitted mainly from urban vehicle exhausts throughout the day, both in summer and in winter. AFs of isoprene and ethanol did not exceed 30% and 50% in summer, respectively, during both daytime and nighttime hours. In winter, the anthropogenic fractions of isoprene and ethanol were slightly higher (up to 35% and 60%, respectively).

Volatile organic compounds (VOCs), ozone (O₃), nitrogen oxides (NOx), carbon monoxide (CO), meteorological parameters, and total non-methane hydrocarbons (NMHC) were analyzed from simultaneous measurements at the MSU-IAP (Moscow State University—Institute of Atmospheric Physics) observational site in Moscow from 2011–2013. Seasonal and diurnal variability of the compounds was studied. The highest O₃ concentration in Moscow was observed in the summer daytime periods in anticyclonic meteorological conditions under poor ventilation of the atmospheric boundary layer and high temperatures (up to 105 ppbv or 210 μg/m³). In contrast, NOx, CO, and benzene decreased from 8 a.m. to 5–6 p.m. local time (LT). The high positive correlation of daytime O₃ with secondary VOCs affirmed an important role of photochemical O₃ production in Moscow during the summers of 2011–2013. The summertime average concentrations of the biogenic VOCs isoprene and monoterpenes were observed to be 0.73 ppbv and 0.53 ppbv, respectively. The principal source of anthropogenic VOCs in Moscow was established to be local vehicle emissions. Yet, only about 5% of the observed isoprene was safely attributed to anthropogenic sources, suggesting significant contribution of biogenic sources into the total levels of ozone precursors. The non-linear O₃–NOx dependence shows a decrease in ground-level O₃ with an increase in NOx during the summers of 2011–2013, which is typical for the VOC-sensitive photochemical regime of O₃ formation. Nevertheless, during the elevated ozone episodes in July 2011, the photochemical regime of ozone production was either transitional or NOx-sensitive. Contribution of various anthropogenic and biogenic VOCs into the measured ozone values was evaluated. The ozone-forming potential (OFP) of total VOCs was 31–67 μg/m³ on average and exceeded 100 μg/m³ in the top 10% of high ozone events, reaching 136 μg/m³. Acetaldehyde, 1.3-butadiene, and isoprene have the highest ozone production potential in Moscow compared to that of other measured VOCs.

Spatial and temporal distributions of isoprene and its oxidation products, methyl vinyl ketone and methacrolein in the Far East region of Russia were investigated. The measurement data were obtained from a mobile laboratory, which moved along the Trans-Siberian railway and from WRF-CHEM (Weather Research and Forecasting Chemical Model) and GEOS-CHEM (Goddard Earth Observing System Chemical Model) simulations. During the simulations, the RACM-MIM and MOZART mechanisms, included in the mesoscale WRF-CHEM model, as well as the Caltech Isoprene Scheme (CIS), built in the global GEOS-CHEM model, have been used. We found that the temporal distribution of the measured isoprene is in good agreement with the simulations. The measured isoprene, methyl vinyl ketone (MVK) and methacrolein (MACR) concentrations demonstrate pronounced diurnal variations. The correlation between the measured isoprene and MVK + MACR was good (R ~ 0.60–0.86). However, the simulated correlation between MVK + MACR and isoprene is very low, with the data for the night-time and daytime values varying. The simulated MVK + MACR to isoprene ratio, in comparison with the experimental result, has pronounced diurnal variations. During twilight and the night-time, the simulated MVK + MACR to isoprene ratio is more than 10. We propose that, due to the validity of the kinetic equations only in the homogeneous system, all chemical and transport (CTM) models, based on these kinetic equations, are not able to show an adequate simulation at night in the weak mixing atmosphere, when the atmospheric structure becomes heterogeneous. At moderate latitudes, we recommend the use of the turbulent Damköhler number and the Kolmogorov Damköhler numbers, which characterize the limits of CTM applicability, as the quality flags at the air quality forecast simulations.

Volatile organic compounds (VOCs) were measured by proton transfer reaction mass spectrometry (PTR-MS) on a mobile laboratory in a transcontinental TROICA-12 (21 July–4 August 2008) campaign along the Trans-Siberian Railway from Moscow to Vladivostok. Surface concentrations of benzene (C₆H₆) and toluene (C₇H₈) along with non-methane hydrocarbons (NMHCs), CO, O₃, SO₂, NO, NO₂ and meteorology are analyzed in this study to identify the main sources of benzene and toluene along the Trans-Siberian Railway. The most measurements in the TROICA-12 campaign were conducted under low-wind/stagnant conditions in moderately ( ∼ 78 % of measurements) to weakly polluted ( ∼ 20 % of measurements) air directly affected by regional anthropogenic sources adjacent to the railway. Only 2 % of measurements were identified as characteristic of highly polluted urban atmosphere. Maximum values of benzene and toluene during the campaign reached 36.5 and 45.6 ppb, respectively, which is significantly less than their short-term exposure limits (94 and 159 ppb for benzene and toluene, respectively). About 90 % of benzene and 65 % of toluene content is attributed to motor vehicle transport and 10 and 20 %, respectively, provided by the other local- and regional-scale sources. The highest average concentrations of benzene and toluene are measured in the industrial regions of the European Russia (up to 0.3 and 0.4 ppb for benzene and toluene, respectively) and south Siberia (up to 0.2 and 0.4 ppb for benzene and toluene, respectively). Total contribution of benzene and toluene to photochemical ozone production along the Trans-Siberian Railway is about 16 % compared to the most abundant organic VOC – isoprene. This contribution, however, is found to be substantially higher (up to 60–70 %) in urbanized areas along the railway, suggesting an important role of anthropogenic pollutant sources in regional ozone photochemistry and air quality.

This paper reports proton mass spectrometry data on aromatic volatile organic compounds (VOCs) (benzene, toluene, phenol, styrene, xylene, and propylbenzene) obtained in different Russian regions along the Trans-Siberian Railway from Moscow to Vladivostok, based on expedition data retrieved using the TRO-ICA-12 mobile laboratory in the summer of 2008. The contribution of aromatic VOCs to ozone formation in the cities and regions along the measurement route has been estimated quantitatively. The greatest contribution of aromatic VOCs to ozone formation is characteristic of large cities along the Trans-Siberian Railway (up to 7.5 ppbv O₃) specified by the highest concentrations of aromatic VOCs (1–1.7 ppbv) and nitrogen oxides (>20 ppbv). The results obtained are indicative of a considerable contribution (30–50%) of anthropogenic emissions of VOCs to photochemical ozone generation in the large cities along the Trans-Siberian Railway in hot and dry weather against the background of a powerful natural factor such as isoprene emissions controlling the regional balance of ground-level ozone in warm seasons.

Volatile organic compounds (VOCs) were measured by proton transfer reaction – mass spectrometry (PTR-MS) on a mobile laboratory in a transcontinental TROICA-12 (21.07.2008–04.08.2008) campaign along the Trans-Siberian railway from Moscow to Vladivostok. Surface concentrations of benzene (C₆H₆) and toluene (C₇H₈) along with non-methane hydrocarbons (NMHCs), CO, O₃, SO₂, NO, NO₂ and meteorology are analyzed in this study to identify the main sources of benzene and toluene along the Trans-Siberian railway. The most measurements in the TROICA-12 campaign were conducted under low-wind/stagnant conditions in moderately (~ 78 % of measurements) to weakly polluted (~ 20 % of measurements) air directly affected by regional anthropogenic sources adjacent to the railroad. Only 2 % of measurements were identified as characteristic of highly polluted urban atmosphere. Maximum values of benzene and toluene during the campaign reached 36.5 ppb and 45.6 ppb, correspondingly, which is significantly less than their one-time maximum permissible concentrations (94 and 159 ppb for benzene and toluene, correspondingly). About 90 % of benzene and 65 % of toluene content is attributed to motor vehicle transport and 10 % and 20 %, correspondingly, provided by the other local and regional-scale sources. The highest average concentrations of benzene and toluene are measured in the industrial regions of the European Russia (up to 0.3 and 0.4 ppb for benzene and toluene, correspondingly) and south Siberia (up to 0.2 and 0.4 ppb for benzene and toluene, correspondingly). Total contribution of benzene and toluene to photochemical ozone production along the Trans-Siberian railway is about 16 % compared to the most abundant biogenic VOC – isoprene. This contribution, however, is found to be substantially higher (up to 60–70 %) in urbanized areas along the railroad suggesting important role of anthropogenic pollutant sources in regional ozone photochemistry and air quality.

On the basis of simultaneous measurements of surface concentrations of CH₄, CO₂, O₃, and ²²²Rn during 6 TROICA (TRanscontinental Observations Into the Chemistry of the Atmosphere) expeditions along the Trans-Siberian Railway from Moscow to Vladivostok in 1999–2008, we have estimated the biogenic emissions of CH₄ and CO₂ from terrestrial ecosystems and the nighttime sink of O₃ on the underlying surface from data on the accumulation rate of ²²²Rn under conditions of near-surface temperature inversion. The regional emissions of CH₄ were the highest in summer in the Far East (0.87 ± 0.52 μg m⁻² s⁻¹) and in West Siberia (0.77 ± 0.41 μg m⁻² s⁻¹), which is associated with significant methane flows from wetlands and moist soils that are typical for these regions. The biogenic emissions of CO₂ vary on average from 0.18 ± 0.04 μg m⁻² s⁻¹ in West Siberia to 0.89 ± 0.07 μg m⁻² s⁻¹ in East Siberia. The nighttime sink of O₃ has a seasonal maximum in summer and varies from 0.05 ± 0.01 μg m⁻² s⁻¹ in West Siberia to 0.07 ± 0.01 μg m⁻² s⁻¹ in Central Siberia; the dry deposition rate of O₃ varies from 0.10 ± 0.08 cm s⁻¹ in West Siberia to 0.33 ± 0.21 cm s⁻¹ in East Siberia and the Far East.