<p>Changes in the atmospheric composition in different periods of 2020 in Moscow, associated with the COVID-19 pandemic preventing measures of varying intensity and with corresponding reduction in emissions of pollutants, were investigated. Surface concentrations of nitrogen dioxide NO<sub>2</sub>, carbon monoxide CO, ozone O<sub>3</sub>, aerosol fraction PM<sub>10</sub> and meteorological parameters in different periods of 2020 are compared with similar data for the previous 5 years. The analysis of ground-based measurements, as well as high-resolution satellite distributions of CO and NO<sub>2</sub>, indicated that the content of major pollutants and its spatial distribution in the Moscow region were significantly affected by both restrictive measures and abnormal meteorological conditions in 2020. It is possible to obtain quantitative estimates of the contribution of both factors using transport and chemical modeling based on detailed inventory of anthropogenic emissions.</p><p>Additionally, some characteristics of atmospheric composition long-term trends in Moscow region are analyzed and discussed.</p><p>The study was supported by Russian Science Foundation under grant &#8470;21-17-00210.</p>
<p>Based on the analysis of orbital measurements, as well as GEOS-Chem model calculations, a study of trends in the total content of CO, CH4 and NO2 in different periods and seasons of 2003-2022 was made for the Eurasia domain, -20&#176;E - 180&#176;E, 0&#176;N - 80&#176;N. Data from the AIRS and OMI instruments were used as orbital information.</p>
<p>Before calculating the trend distributions, we compared the trend estimates obtained from orbital and ground-based measurements using only synchronous observations, i.e., days when the measurements were carried out by both orbital (AIRS, OMI) and ground-based (IAP RAS and NDACC stations) instruments.</p>
<p>A good agreement has been established between trend distributions obtained from the orbital data and the same distributions obtained from the GEOS-Chem model calculations.</p>
<p>In general, according to average annual estimates, trends over most of Eurasia regions in the period 2003-2022 was negative; however, after 2008 the downward trend slowed down, and in some areas the CO content began to rise.</p>
<p>Thus, a positive trend (change) in CO TC trends after about 2008 was established. In the entire domain under study, this change was about 2&#8211;2.5%/year. In the autumn months of 2008-2022 (including November) increase in CO TC was established over almost the entire Eurasia, including Arctic regions and Europe. This growth (at least in Europe) cannot be explained by either anthropogenic emissions or releases from fires. A possible reason for this rising may be the formation of additional CO from methane, the increase in concentrations of which began around the same time (after 2007), and change in the source/sink ratio for CO.</p>
<p>Based on GEOS-Chem calculations with different scenarios for specifying anthropogenic emissions and emissions from fires, the response of CO trends to climate change was calculated. The study was supported by Russian Science Foundation under grant &#8470;21-17-00210.</p>
<p>Carbon monoxide (CO) total column (TC) measurements of the TROPOMI high-resolution orbital spectrometer have been validated by ground-based spectroscopic measurements at sites of the A.M. Obukhov Institute of Atmospheric Physics RAS (OIAP RAS) in Moscow and Zvenigorod for the period from 06.28.2018 to 12.31.2021. Correlation coefficients (R) between TROPOMI orbital data and ground-based stationary data have been determined and analyzed. For different resolution of satellite data dependence of correlation parameters on the viewing orbital angles, underlying surface albedo and the height of atmospheric boundary layer (ABL) has been investigated. The high values of the correlation coefficient (R ~ 0.81 &#8211; 0.97) were obtained depending on the observation point, spatial averaging and applied filtration. The average systematic difference between TROPOMI and ground-based CO TC measurements was -1.1&#177;7.5% (ZSS) and 1.3&#177;5.7% (Moscow) for orbital data resolution 0.1&#176;&#215;0.1&#176;. The correlation coefficients depend on the viewing azimuthal angles and the height of the atmospheric boundary layer. Correlation increase was obtained during observations at viewing azimuthal angles of less than 40&#186; (up to R~0.97), as well as under increase of ABL height (up to R~0.90). For both sites no influence of surface albedo on the correlation parameters of orbital and ground-based measurements has been found. Also, no significant dependence of correlation on the viewing zenith angle has been detected.&#160;<br />The study was supported by Russian Science Foundation under grant &#8470;21-17-00210.</p>
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