Characteristics, formation, and sources of PM2.5 in 2020 in Suzhou, Yangtze River Delta, China.

Here we present seasonal chemical characteristics, formations, sources of PM2.5 in the year 2020 in Suzhou, Yangtze River Delta, China. Expectedly, organic matter (OM) found to be the most dominant component of PM2.5, with a year-average value of 10.3 ± 5.5 µg m-3, followed by NO3- (6.7 ± 6.5 µg m-3), SO42- (3.3 ± 2.5 µg m-3), NH4+ (3.2 ± 2.8 µg m-3), EC (1.1 ± 1.3 µg m-3), Cl- (0.57 ± 0.56 µg m-3), Ca2+ (0.55 ± 0.91 µg m-3), K+ (0.2 ± 1.0 µg m-3), Na+ (0.18 ± 0.45 µg m-3), and Mg2+ (0.09 ± 0.15 µg m-3). Seasonal variations of PM2.5 showed the highest average value in spring, followed by winter, fall, and summer. Meanwhile, the formation mechanisms of the major PM2.5 species (NO3-, SO42-, and OM) varied in seasons. Interestingly, NO2 may have the highest conversion rate to NO3- in spring, which might be linked with the nighttime chemistry due to the high relative humidity. Moreover, OM in summer was mainly produced by the daytime oxidation of volatile organic compounds, while local primary organic aerosols might play a significant role in other seasons. Source apportionment showed that the more-aged PM2.5 contributed significantly to the PM2.5 mass (42%), followed by the dust-related PM2.5 (38%) and the less-aged PM2.5 (21%). Potential contribution source function (PSCF) results indicated that aged PM2.5 were less affected by transportation than dust-related PM2.5.

Characterization of the aerosol chemical composition during the COVID-19 lockdown period in Suzhou in the Yangtze River Delta, China.

To control the spread of COVID-19, rigorous restrictions have been implemented in China, resulting in a great reduction in pollutant emissions. In this study, we evaluated the air quality in the Yangtze River Delta during the COVID-19 lockdown period using satellite and ground-based data, including particle matter (PM), trace gases, water-soluble ions (WSIs) and black carbon (BC). We found that the impacts of lockdown policy on air quality cannot be accurately assessed using MODIS aerosol optical depth (AOD) data, whereas the tropospheric nitrogen dioxide (NO2) vertical column density can well reflect the influences of these restrictions on human activities. Compared to the pre-COVID period, the PM2.5, PM10, NO2, carbon monoxide (CO), BC and WSIs during the lockdown in Suzhou were observed to decrease by 37.2%, 38.3%, 64.5%, 26.1%, 53.3% and 58.6%, respectively, while the sulfur dioxide (SO2) and ozone (O3) increased by 1.5% and 104.7%. The WSIs ranked in the order of NO3- > NH4+ > SO42- > Cl- > Ca2+ > K+ > Mg2+ > Na+ during the lockdown period. By comparisons with the ion concentrations during the pre-COVID period, we found that the ions NO3-, NH4+, SO42-, Cl-, Ca2+, K+ and Na+ decreased by 66.3%, 48.8%, 52.9%, 56.9%, 57.9% and 76.3%, respectively, during the lockdown, in contrast to Mg2+, which increased by 30.2%. The lockdown policy was found to have great impacts on the diurnal variations of Cl-, SO42-, Na+ and Ca2+.

[Ozone Pollution in Suzhou During Early Summertime: Formation Mechanism and Interannual Variation].

This study investigated the concentrations of atmospheric pollutants in the urban area of Suzhou from May to June, 2017-2021. The variation characteristics and annual changes of ozone (O3), nitrogen oxide (NOx), total oxidant (Ox), carbon monoxide (CO), and volatile organic compounds (VOCs) were analyzed. The O3 formation mechanism and its annual changes were studied using an Observation-Based Model (OBM), and VOCs source apportionments and their trends were discussed. The results indicated that ① The volume fractions of Ox and the concentrations of NOx and CO have decreased in the urban area of Suzhou in recent years, while the volume fractions of VOCs have increased, and sufficient photochemical conditions for O3 formation still existed during polluted days. ② The O3-NOx-VOCs sensitivity in Suzhou was in the VOCs-limited regime. The long-term reduction ratio between VOCs and NOx should not be less than 5:1, and aromatics and alkenes were the critical VOCs for mitigating O3 pollution. ③ The results of VOCs source apportionment revealed that industrial emissions, gasoline vehicle exhaust, and diesel engine exhaust were the major sources of VOC emissions in Suzhou. Industrial emissions and solvent usage declined from 2017 to 2021; however, gasoline vehicle exhaust and gasoline evaporation, which possess higher O3 formation potential(OFP), increased significantly. ④ The OFP source apportionments results indicated that controlling VOC emissions from solvent usage and gasoline vehicle exhaust is crucial for O3 pollution control in Suzhou.

[Concentration Variation and Source Analysis of Metal Elements in PM2.5 During COVID-19 Control in Suzhou].

To study the variation in concentration and source analysis of metal elements during COVID-19 control in Suzhou, a multi-metal online monitor was used to determine hourly online data of 14 metal elements from December 1, 2019 to March 31, 2020. This study analyzed variation in concentration and source analysis of metal elements using a PMF model before, during, and after shutdown during COVID-19 control. The results showed that the concentrations of Cr, Mn, Zn, and Fe during shutdown decreased the most, by 87.6%, 85.6%, 78.3%, and 72.2%, respectively, compared with those before shutdown. The concentrations of Mn, Cr, Zn, and Fe after shutdown increased the most, by 227.0%, 215.4%, 147.4%, and 113.4%, respectively, compared with those of the previous stage. The diurnal variation in K differed at three stages. Zn showed a single peak shape at three stages, but the peak width and peak time were different. Unlike the concentrations, the diurnal variations in Fe, Mn, Pb, Se, and Hg were not significantly changed. The daily variation characteristics of Ca, Ba, Cu, As, Cr, and Ni during and after shutdown were significantly different from those before shutdown. The results of source analysis by the PMF model showed that metal elements mainly came from dust, motor vehicle, coal burning, industrial smelting, and mixed-combustion sources. Among them, the concentration of industrial smelting sources changed greatly, with the concentration decreasing by 89.0% during shutdown and increasing by 358.0% after shutdown.

Air pollutant variations in Suzhou during the 2019 novel coronavirus (COVID-19) lockdown of 2020: High time-resolution measurements of aerosol chemical compositions and source apportionment.

To control the spread of the 2019 novel coronavirus (COVID-19), China imposed rigorous restrictions, which resulted in great reductions in pollutant emissions. This study examines the characteristics of air pollutants, including PM2.5 (particles with aerodynamic diameters < 2.5 µm), gas pollutants, water-soluble ions (WSIs), black carbon (BC) and elements, as well as the source apportionment of PM2.5 in Suzhou before, during and after the Chinese New Year (CNY) holiday of 2020 (when China was under an unprecedented state of lockdown to restrict the COVID-19 outbreak). Compared to those before CNY, PM2.5, BC, SNA (sulfate, nitrate and ammonium), other ions, elements, and NO2 and CO mass concentrations decreased by 9.9%-64.0% during CNY. The lockdown policy had strong (weak) effects on the diurnal variations in aerosol chemical compositions (gas pollutants). Compared to those before CNY, source concentrations and contributions of vehicle exhaust during CNY decreased by 72.9% and 21.7%, respectively. In contrast, increased contributions from coal combustion and industry were observed during CNY, which were recorded to be 2.9 and 1.7 times higher than those before CNY, respectively. This study highlights that the lockdown policy that was imposed in Suzhou during CNY not only reduced the mass concentrations of air pollutants but also modified their diurnal variations and the source contributions of PM2.5, which revealed the complex responses of PM2.5 sources to the rare, low emissions of anthropogenic pollutants that occurred during the COVID-19 lockdown.

[Distribution Characteristics and Source Analysis of Atmospheric Mercury Speciation in Suzhou].

To explore the variation of mercury in the atmosphere in Suzhou, continuous monitoring of gaseous element mercury (GEM), gaseous oxidized mercury (GOM), and particulate bound mercury (PBM) was conducted from January 1 to December 31, 2018, in Suzhou. The weights trajectory analysis method (CWT) and concentration rose were used to analyze the atmospheric mercury sources and concentration variation. The results showed that during the monitoring period, the concentration ranges of GEM, GOM, and PBM in Suzhou were 0-53.3 ng·m-3, 0-256 pg·m-3, and 0-5208 pg·m-3, respectively. The corresponding annual average concentrations of the three mercury species were (2.57±2.09) ng·m-3, (5.27±15.7) pg·m-3, and (16.0±157) pg·m-3, respectively. GEM was the main component of atmospheric mercury in Suzhou. During the monitoring period, the average concentration of GEM in Suzhou was highest in winter, higher in spring than in autumn, and lowest in summer. According to the CWT, the mercury-containing air mass in spring and winter predominantly originated from inland; in summer, it mainly originated from the local area, the Yellow Sea, and the East China Sea, and in autumn from inland, the Yellow Sea, and the Bohai Sea. The wind and mercury rose charts showed that atmospheric mercury concentrations were higher from inland and lower from the ocean. During the monitoring period, the average concentrations of GEM and PBM in Suzhou were lower during the day than the night. The diurnal variation of GEM and PBM was significantly and strongly correlated with solar radiation, humidity, and air temperature. The average concentration of GOM showed multiple peaks and valleys in one day. Some peaks were caused by fuel oil combustion emissions, and some by O3 oxidation with GEM.