Studies of urban aerosol particle number and mass concentrations with a mobile platform: The roles of COVID-19, traffic, and meteorology

<p>Although urban air pollution is on the decline in central Europe, it still causes several hundreds of thousands of premature deaths per year. The EU standards of atmospheric aerosol particle mass concentrations PM10 and PM2.5 (µg m<sup>-3</sup>) have not been exceeded anymore in Germany in 2020, yet there is a rather large uncertainty about the toxicity of particle number concentrations PN (cm<sup>-3</sup>), for which no legal limits are established. High PN concentrations are typically caused by the exhaust of motorized road vehicles. From 2019 through 2021, national lockdowns in response to the COVID-19 pandemic resulted in reduced human activity. The traffic intensity was heavily reduced, which should have led to an equally strong reaction of the urban aerosol particle concentrations, specifically the PN concentrations. For NO<sub>x</sub> and PM10, it has been shown for sections of central Europe that the decrease of urban concentrations was not as intense as expected by traffic reduction, because lockdowns coincided with periods of low wind speeds and poor atmospheric exchange conditions. We performed meteorological and air chemistry measurements with an instrumented cargo bicycle before, during, and after the COVID-19 lockdown periods in Münster, Germany. During each ride, two circular routes around the city center were realized, a high-traffic route and a low-traffic route. A complex picture emerged with varying impact of the day of the week, selection of route, meteorological conditions, and traffic intensity driving the PN and PM concentrations. Single-ride high-resolution analysis showed convincingly that the multitude of exhaust plumes from motorized vehicles exerted a strong impact on the PN concentrations. A relative importance analysis was performed on the entire dataset. According to the statistical analysis, PM10 responded most to the day of the week. Although the traffic intensity was also low on weekends, the impact of traffic on PM10 was rather low. Presumably, PM10 responded either to a specific traffic component such as commercial, low-duty vehicles, or to other business with weakly cycles such as construction activity. The meteorological conditions exert impact mostly through the relative humidity, which affects particle growth and reduction of the PN concentration. The role of the lockdowns was quite little overall. For future research, a more complete coverage of the seasons of the year is recommended as well as the inclusion of NO<sub>x</sub> measurements on board of the cargo bicycle. </p>

Cloud activation properties of aerosol particles in a continental Central European urban environment

Collocated measurements using a condensation particle counter, differential mobility particle sizer and cloud condensation nuclei counter were realised in parallel in central Budapest from 15 April 2019 to 14 April 2020 to gain insight into the cloud activation properties of urban aerosol particles. The median total particle number concentration was 10.1 × 103 cm−3. The median concentrations of cloud condensation nuclei (CCN) at water vapour supersaturation (S) values of 0.1 %, 0.2 %, 0.3 %, 0.5 % and 1.0 % were 0.59, 1.09, 1.39, 1.80 and 2.5 × 103 cm−3, respectively. The CCN concentrations represented 7–27 % of all particles. The CCN concentrations were considerably larger but the activation fractions were systematically substantially smaller than observed in regional or remote locations. The effective critical dry particle diameters (dc,eff) were derived utilising the CCN concentrations and particle number size distributions. Their median values at the five supersaturation values considered were 207, 149, 126, 105 and 80 nm, respectively; all of these diameters were positioned within the accumulation mode of the typical particle number size distribution. Their frequency distributions revealed a single peak for which the geometric standard deviation increased monotonically with S. This broadening indicated high time variability in the activating properties of smaller particles. The frequency distributions also showed fine structure, with several compositional elements that seemed to reveal a consistent or monotonical tendency with S. The relationships between the critical S and dc,eff suggest that urban aerosol particles in Budapest with diameters larger than approximately 130 nm showed similar hydroscopicity to corresponding continental aerosol particles, whereas smaller particles in Budapest were less hygroscopic than corresponding continental aerosol particles. Only modest seasonal cycling in CCN concentrations and activation fractions was seen, and only for large S values. This cycling likely reflects changes in the number concentration, chemical composition and mixing state of the particles. The seasonal dependencies of dc,eff were featureless, indicating that the droplet activation properties of the urban particles remained more or less the same throughout the year. This is again different from what is seen in non-urban locations. Hygroscopicity parameters (κ values) were computed without determining the time-dependent chemical composition of the particles. The median values for κ were 0.15, 0.10, 0.07, 0.04 and 0.02, respectively, at the five supersaturation values considered. The averages suggested that the larger particles were considerably more hygroscopic than the smaller particles. We found that the κ values for the urban aerosol were substantially smaller than those previously reported for aerosols in regional or remote locations. All of these characteristics can be linked to the specific source composition of particles in cities. The relatively large variability in the hygroscopicity parameters for a given S emphasises that the individual values represent the CCN population in ambient air while the average hygroscopicity parameter mainly corresponds to particles with sizes close to the effective critical dry particle diameter.

Urban aerosol size distributions: a global perspective

Urban aerosol measurements are necessary to establish associations between air pollution and human health outcomes and to evaluate the efficacy of air quality legislation and emissions standards. The measurement of urban aerosol particle size distributions (PSDs) is of particular importance as they enable characterization of size-dependent processes that govern a particle's transport, transformation, and fate in the urban atmosphere. PSDs also improve our ability to link air pollution to health effects through evaluation of particle deposition in the respiratory system and inhalation toxicity. To inform future measurements of urban aerosol observations, this paper reviews and critically analyzes the current state of knowledge on urban aerosol PSD measurements by synthesizing 737 PSD observations made between 1998 to 2017 in 114 cities in 43 countries around the globe. Significant variations in the shape and magnitude of urban aerosol number and mass PSDs were identified among different geographical regions. In general, number PSDs in Europe (EU) and North America, Australia, and New Zealand (NAAN) are dominated by nucleation- and Aitken-mode particles. PSDs in Central, South, and Southeast Asia (CSSA) and East Asia (EA) are shifted to larger sizes, with a meaningful contribution from the accumulation mode. Urban mass PSDs are typically bimodal, presenting a dominant mode in the accumulation mode and a secondary mode in the coarse mode. Most PSD observations published in the literature are short-term, with only 14 % providing data for longer than 6 months. There is a paucity of PSDs measured in Africa (AF), CSSA, Latin America (LA), and West Asia (WA), demonstrating the need for long-term aerosol measurements across wide size ranges in many cities around the globe. Geographical variations in urban aerosol effective densities were also reviewed. Size-resolved urban aerosol effective density functions from 3 to 10 000 nm were established for different geographical regions and intra-city sampling locations in order to accurately translate number PSDs to mass PSDs, with significant variations observed between near-road and urban background sites. The results of this study demonstrate that global initiatives are urgently needed to develop infrastructure for routine and long-term monitoring of urban aerosol PSDs spanning the nucleation to coarse mode. Doing so will advance our understanding of spatiotemporal trends in urban PSDs throughout the world and provide a foundation to more reliably elucidate the impact of urban aerosols on atmospheric processes, human health, and climate.

Cloud droplet activation in a continental Central European urban environment

Collocated measurements by condensation particle counter, differential mobility particle sizer and cloud condensational nuclei counter instruments were realised in parallel in central Budapest from 15 April 2019 to 14 April 2020 to gain insight into the droplet activation behaviour of urban aerosol particles. The median total particle number concentration was 10.1 × 103 cm−3. The median concentrations of cloud condensation nuclei (CCN) at water vapour supersaturations (Ss) of 0.1, 0.2, 0.3, 0.5 and 1.0 % were 0.59, 1.09, 1.39, 1.80 and 2.5 × 103 cm−3, respectively. They represented from 7 to 27 % of the total particles. The effective critical dry particle diameters (dc,eff) were derived utilising the CCN concentrations and particle number size distributions. Their medians were 207, 149, 126, 105 and 80 nm, respectively. They were all positioned within the accumulation mode of the typical particle number size distribution. Their frequency distributions revealed a single peak, which geometric standard deviation increased monotonically with S. The broadening indicated larger time variability in the activation properties of smaller particles. The frequency distributions also showed a fine structure. Its several compositional elements seemed to change in a tendentious manner with S. They were related to the size-dependent chemical composition and external mixtures of particles. The relationships between the critical S and dc,eff suggested that the urban aerosol particles in Budapest with a diameter larger than approximately 130 nm showed similar hygroscopicity than the continental aerosol in general, while the smaller particles appeared to be less hygroscopic than that. Seasonal cycling of the CCN concentrations and activation fractions implied modest alterations and for the larger Ss only. They likely reflected the changes in particle number concentrations, chemical composition and mixing state of particles. The seasonal dependencies for dc,eff were featureless, which indicated that the urban particles exhibited more or less similar droplet activation properties over the measurement year. This is different from non-urban locations. The hygroscopicity parameters (κ values) were computed without determining time-dependent chemical composition of particles. Their medians were 0.16, 0.10, 0.07, 0.04 and 0.02, respectively. The averages suggested that the larger particles exhibited considerably higher hygroscopicity than the smaller particles. The urban aerosol was characterised by substantially smaller kappa values than for regional or remote locations. All these could be virtually linked to specific source composition in cities. The relatively large variability in the hygroscopicity parameter sets for a given S emphasized that their individual values represented the CCN population in the ambient air, while the averages stood mainly for the particles with a size close to the effective critical dry particle diameters.

Сhanges in air quality and aerosol pollution in Moscow megacity and its direct and indirect impact on radiative and meteorological properties of the atmosphere due to COVID-19 pandemic lockdown in spring 2020 according to modelling and measurements.

<p>Atmospheric aerosol has a noticeable effect on the microphysical and optical properties of the atmosphere, solar radiation, temperature and humidity conditions, thereby determining the quality of the forecast of important meteorological elements and affecting the regional climate and the dynamics of geochemical processes. Using the results of the spring AeroRadCity experiment at the MSU Meteorological Observatory in 2018-2019, and numerical calculations on the base of modern COSMO and COSMO-ART mesoscale models using Russian (-Ru) configurations we determined the level and main features of urban air/aerosol pollution, and assessed its magnitude and its impact on the radiative and meteorological characteristics of the atmosphere in typical conditions (Chubarova et al., 2020). In the context of the coronavirus pandemic in 2020, especially during the period of lockdown in the spring, there was a significant decrease in emissions of pollutants in many countries, including Russia. The aim of this study is to show the consequences of decrease in emissions of pollutants on the air quality and on urban aerosol pollution. A special attention is paid to the division between the effects of meteorological factors and the influence of pollution emission on aerosol and gas concentration. The effects of the air pollution decrease on solar radiation and air temperature during this period have been analyzed using COSMO-Ru-ART model.  For a more detailed study of the observed spatial aerosol distribution on solar radiation and air temperature, we have developed a methodology of the implementation of the satellite aerosol optical thickness (AOT) data in the COSMO-Ru model. Using this approach we evaluated the radiative and temperature effects observed due to aerosol in typical conditions during the spring of 2018-2019 and during the period of lockdown in the spring of 2020 under various meteorological conditions. To do this, the satellite AOT data from the MAIAC/MODIS algorithm and aerosol measurements from Cimel sun photometers data were used for characterising the urban aerosol in typical and lockdown conditions. We also discuss the aerosol indirect effects on cloud properties using an experimental scheme of COSMO-Ru model and their influence on solar radiation and surface temperature during this period. The aerosol study has been partially supported by the RSF grant number 18-17-00149; the analysis of gas species has been partially funded by the megagrant number 2020-220-08-5835.</p><p>Reference:</p><p>Chubarova N.Ye., Ye.Yu. Zhdanova., Ye.Ye. Androsova, A.A. Kirsanov, M.V. Shatunova, Yu.O. Khlestova, Ye.V. Volpert, A.A. Poliukhov, I.D. Eremina, D.V. Vlasov, O.B. Popovicheva, A.S. Ivanov, Ye.V. Gorbarenko, Ye.I. Nezval, D.V. Blinov, G.S. Rivin. The aerosol urban pollution and its effects on weather, regional climate and geochemical processes: Monograph / Edited by N.Ye. Chubarova – Moscow, MAKS Press, 2020. 339 pp.  ISBN 978-5-317-06464-8</p>