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  1. The emergence of the novel corona virus and the resulting lockdowns over various parts of the world have substantially impacted air quality due to reduced anthropogenic activity. The objective of this study is to investigate the impact of COVID-19 lockdown and Spring Festival on air quality of four major cities of Yangtze River Delta (YRD) region, including Shanghai, Nanjing, Hefei, and Hangzhou. In situ measurements were taken for nitrogen dioxide (NO2), particulate matter (PM2.5) and ozone (O3). In situ measurements from 1 January to 25 April were taken two years prior to COVID-19 (2018–19), during COVID-19 lockdown (2020), and one year after the COVID-19 (2021). The results indicated that the concentration of NO2 and PM2.5 dropped considerably during the lockdown days compared to normal days while the O3 concentration showed an upsurge. The NO2 showed reduction of about 54% on average during lockdown level 1 in 2020 whereas, PM 2.5 showed reduction of about 36% through the YRD. A substantial drop was observed in concentration of NO2 during the Spring Festival holidays throughout the YRD from 2019 to 2021.
  2. Ambient carbonyls are critical precursors of ozone (O3) and secondary organic aerosols (SOA). To better understand the pollution characteristics of carbonyls in Taiyuan, field samplings were conducted, and 13 carbonyls were detected in an urban site of Taiyuan for the four seasons. The total concentration of carbonyls in the atmosphere was 19.67 ± 8.56 μg/m3. Formaldehyde (7.70 ± 4.78 μg/m3), acetaldehyde (2.95 ± 1.20 μg/m3) and acetone (5.57 ± 2.41 μg/m3) were the dominant carbonyl compounds, accounting for more than 85% of the total carbonyls. The highest values for formaldehyde and acetone occurred in summer and autumn, respectively, and the lowest occurred in winter. The variations for acetaldehyde were not distinct in the four seasons. Formaldehyde and acetone levels increased obviously in the daytime and decreased at night, while acetaldehyde did not show significant diurnal variations. Higher temperature and stronger sunlight intensity could facilitate the photochemical reaction of volatile organic compounds (VOCs) and enhance the O3 levels in summer. Formaldehyde and acetaldehyde contributed 70–95% of carbonyls’ ozone formation potential (OFP) caused by carbonyls with the highest totals of 268.62 μg/m3 and 38.14 μg/m3, respectively. The highest concentrations of carbonyls from south and southwest winds in summer suggest that the cokemore »industries in the southern Taiyuan Basin should be, firstly, controlled for the alleviation of ozone pollution.« less
  3. Recently, the occurrence of fog and haze over China has increased. The retrieval of trace gases from the multi-axis differential optical absorption spectroscopy (MAX-DOAS) is challenging under these conditions. In this study, various reported retrieval settings for formaldehyde (HCHO) and sulfur dioxide (SO2) are compared to evaluate the performance of these settings under different meteorological conditions (clear day, haze, and fog). The dataset from 1st December 2019 to 31st March 2020 over Nanjing, China, is used in this study. The results indicated that for HCHO, the optimal settings were in the 324.5–359 nm wavelength window with a polynomial order of five. At these settings, the fitting and root mean squared (RMS) errors for column density were considerably improved for haze and fog conditions, and the differential slant column densities (DSCDs) showed more accurate values compared to the DSCDs between 336.5 and 359 nm. For SO2, the optimal settings for retrieval were found to be at 307–328 nm with a polynomial order of five. Here, root mean square (RMS) and fitting errors were significantly lower under all conditions. The observed HCHO and SO2 vertical column densities were significantly lower on fog days compared to clear days, reflecting a decreased chemical productionmore »of HCHO and aqueous phase oxidation of SO2 in fog droplets.« less
  4. Abstract. Gas–particle partitioning of water-soluble organic compounds plays a significant role in influencing the formation, transport, and lifetime oforganic aerosols in the atmosphere, but is poorly characterized. In this work, gas- and particle-phase concentrations of isoprene oxidation products(C5-alkene triols and 2-methylterols), levoglucosan, and sugar polyols were measured simultaneously at a suburban site of the western Yangtze RiverDelta in east China. All target polyols were primarily distributed into the particle phase (85.9 %–99.8 %). Given the uncertainties inmeasurements and vapor pressure predictions, a dependence of particle-phase fractions on vapor pressures cannot be determined. To explore the impactof aerosol liquid water on gas–particle partitioning of polyol tracers, three partitioning schemes (Cases 1–3) were proposed based onequilibriums of gas vs. organic and aqueous phases in aerosols. If particulate organic matter (OM) is presumed as the only absorbing phase(Case 1), the measurement-based absorptive partitioning coefficients (Kp,OMm) of isoprene oxidation products and levoglucosan were more than 10 times greater than predicted values (Kp,OMt). The agreement betweenKp,OMm and Kp,OMt was substantially improved when solubility in a separate aqueous phase wasincluded, whenever water-soluble and water-insoluble OM partitioned into separate (Case 2) or single (Case 3) liquid phases,suggesting that the partitioning of polyol tracers into the aqueous phase in aerosols should not be ignored. Themore »measurement-based effective Henry'slaw coefficients (KH,em) of polyol tracers were orders of magnitude higher than their predicted values in pure water(KH,wt). Due to the moderate correlations between log⁡(KH,em/KH,wt) andmolality of sulfate ions, the gap between KH,em and KH,wt of polyol tracers could not be fullyparameterized by the equation defining “salting-in” effects and might be ascribed to mechanisms of reactive uptake, aqueous phase reaction,“like-dissolves-like” principle, etc. These study results also partly reveal the discrepancy between observation and modeling of organicaerosols.« less