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The coronavirus (COVID-19) pandemic is disrupting the world from many aspects. In this study, the impact of emission variations on PM 2.5 -bound elemental species and health risks associated to inhalation exposure has been analyzed based on real-time measurements at a remote coastal site in Shanghai during the pandemic. Most trace elemental species decreased significantly and displayed almost no diel peaks during the lockdown. After the lockdown, they rebounded rapidly, of which V and Ni even exceeded the levels before the lockdown, suggesting the recovery of both inland and shipping activities. Five sources were identified based on receptor modeling. Coal combustion accounted for more than 70% of the measured elemental concentrations before and during the lockdown. Shipping emissions, fugitive/mineral dust, and waste incineration all showed elevated contributions after the lockdown. The total non-carcinogenic risk (HQ) for the target elements exceeded the risk threshold for both children and adults with chloride as the predominant species contributing to HQ. Whereas, the total carcinogenic risk (TR) for adults was above the acceptable level and much higher than that for children. Waste incineration was the largest contributor to HQ, while manufacture processing and coal combustion were the main sources of TR. Lockdown control measures were beneficial for lowering the carcinogenic risk while unexpectedly increased the non-carcinogenic risk. From the perspective of health effects, priorities of control measures should be given to waste incineration, manufacture processing, and coal combustion. A balanced way should be reached between both lowering the levels of air pollutants and their health risks. [Display omitted]. [ABSTRACT FROM AUTHOR]

• NO 2 concentrations were 77.3% lower during the closure of WUH. • Steeper reduction in NO than in BTEX levels at SHA. • NO 2 concentrations at WUH were dropped to near zero during airport closure. • Increase in O 3 levels at both WUH and SHA due to reduced NO x emissions. Wuhan Tianhe International Airport (WUH) was suspended to contain the spread of COVID-19, while Shanghai Hongqiao International Airport (SHA) saw a tremendous flight reduction. Closure of a major international airport is extremely rare and thus represents a unique opportunity to straightforwardly observe the impact of airport emissions on local air quality. In this study, a series of statistical tools were applied to analyze the variations in air pollutant levels in the vicinity of WUH and SHA. The results of bivariate polar plots show that airport SHA and WUH are a major source of nitrogen oxides. NO x , NO 2 and NO diminished by 55.8%, 44.1%, 76.9%, and 40.4%, 33.3% and 59.4% during the COVID-19 lockdown compared to those in the same period of 2018 and 2019, under a reduction in aircraft activities by 58.6% and 61.4%. The concentration of NO 2 , SO 2 and PM 2.5 decreased by 77.3%, 8.2%, 29.5%, right after the closure of airport WUH on 23 January 2020. The average concentrations of NO, NO 2 and NO x scatter plots at downwind of SHA after the lockdown were 78.0%, 47.9%, 57.4% and 62.3%, 34.8%, 41.8% lower than those during the same period in 2018 and 2019. However, a significant increase in O 3 levels by 50.0% and 25.9% at WUH and SHA was observed, respectively. These results evidently show decreased nitrogen oxides concentrations in the airport vicinity due to reduced aircraft activities, while amplified O 3 pollution due to a lower titration by NO under strong reduction in NO x emissions. Significantly reduced NO x concentrations and amplified O 3 pollution at the airfield boundary of Shanghai Hongqiao International Airport due to reduced aircraft activities. [Display omitted] [ABSTRACT FROM AUTHOR]

• Reductions of PM, NO 2 , SO 2 and CO under lockdown differed in urban agglomerations. • The mitigation of PM2.5 concentrations due to lockdown was considerable. • Although the lockdown did not reverse uptrend of O 3 , it retarded increment of O 3. • BTH, YRD and PRD suffered from more polluted air masses of regional transport. To investigate the air quality change during the COVID-19 pandemic, we analyzed spatiotemporal variations of six criteria pollutants in nine typical urban agglomerations in China using ground-based data and examined meteorological influences through correlation analysis and backward trajectory analysis under different responses. Concentrations of PM 2.5 , PM 10 , NO 2 , SO 2 and CO in urban agglomerations respectively decreased by 18%–45% (30%–62%), 17%–53% (22%–39%), 47%-64% (14%–41%), 9%–34% (0%–53%) and 16%-52% (23%–56%) during Lockdown (Post-lockdown) period relative to Pre-lockdown period. PM 2.5 pollution events occurred during Lockdown in Beijing-Tianjin-Hebe (BTH) and Middle and South Liaoning (MSL), and daily O 3 concentration rose to grade Ⅱ standard in Post-lockdown period. Distinct from the nationwide slump of NO 2 during Lockdown period, a rebound (∼40%) in Post-lockdown period was observed in Cheng-Yu (CY), Yangtze River Middle-Reach (YRMR), Yangtze River Delta (YRD) and Pearl River Delta (PRD). With slightly higher wind speed compared with 2019, the reduction of PM 2.5 (51%–62%) in Post-lockdown period is more than 2019 (15%–46%) in HC (Harbin-Changchun), MSL, BTH, CP (Central Plain) and SP (Shandong-Peninsula), suggesting lockdown measures are effective to PM 2.5 alleviation. Although O 3 concentrations generally increased during the lockdown, its increment rate declined compared with 2019 under similar sunlight duration and temperature. Additionally, unlike HC, MSL and BTH, which suffered from additional (> 30%) air masses from surrounding areas after the lockdown, the polluted air masses reaching YRD and PRD mostly originated from the long-distance transport, highlighting the importance of joint regional governance. [Display omitted] [ABSTRACT FROM AUTHOR]

• The ratio between OH production from HONO and from (HONO+O3) lager than 90% before 12:00. • The source of HONO from combustion emission is ~26%. • NO 2 being converted to HONO under the conditions with high aerosol extinction coefficient and high relative humidity was a dominant factor for the secondary produce of HONO. • HONO mainly distributed near the surface and localized, and NO 2 mainly transmitted from north at ~ 0.6 km. Ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) observations were operated from 02 to 21 December 2018 in Leshan, southwest China, to measure HONO, NO 2 and aerosol extinction vertical distributions, and these were the first MAX-DOAS measurement results in Sichuan Basin. During the measurement period, characteristic ranges for surface concentration were found to be 0.26–4.58 km−1 and averaged at 0.93 km−1 for aerosol extinction, 0.49 to 35.2 ppb and averaged at 4.57 ppb for NO 2 and 0.03 to 7.38 ppb and averaged at 1.05 ppb for HONO. Moreover, vertical profiles of aerosol, NO 2 and HONO were retrieved from MAX-DOAS measurements using the Heidelberg Profile (HEIPRO) algorithm. By analysing the vertical gradients of pollutants and meteorological information, we found that aerosol and HONO are strongly localised, while NO 2 is mainly transmitted from the north direction (city center direction). Nitrogen oxides such as HONO and NO 2 are important for the production of hydroxyl radical (OH) and oxidative capacity in the troposphere. In this study, the averaged value of OH production rate from HONO is about 0.63 ppb/hr and maximum value of ratio between OH production from HONO and from (HONO+O 3) is > 93% before12:00 in Leshan. In addition, combustion emission contributes to 26% for the source of HONO in Leshan, and we found that more NO 2 being converted to HONO under the conditions with high aerosol extinction coefficient and high relative humidity is also a dominant factor for the secondary produce of HONO. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract To investigate formation mechanisms of secondary organic carbon (SOC) in Eastern China, measurements were conducted in an urban site in Shanghai in the summer of 2015. A period of high O 3 concentrations (daily peak > 120 ppb) was observed, during which daily maximum SOC concentrations exceeding 9.0 μg/(C·m3). Diurnal variations of SOC concentration and SOC/organic carbon (OC) ratio exhibited both daytime and nighttime peaks. The SOC concentrations correlated well with O x (= O 3 + NO 2) and relative humidity in the daytime and nighttime, respectively, suggesting that secondary organic aerosol formation in Shanghai is driven by both photochemical production and aqueous phase reactions. Single particle mass spectrometry was used to examine the formation pathways of SOC. Along with the daytime increase of SOC, the number fraction of elemental carbon (EC) particles coated with OC quickly increased from 38.1% to 61.9% in the size range of 250–2000 nm, which was likely due to gas-to-particle partitioning of photochemically generated semi-volatile organic compounds onto EC particles. In the nighttime, particles rich in OC components were highly hygroscopic, and number fraction of these particles correlated well with relative humidity and SOC/OC nocturnal peaks. Meanwhile, as an aqueous-phase SOC tracer, particles that contained oxalate-Fe(III) complex also peaked at night. These observations suggested that aqueous-phase processes had an important contribution to the SOC nighttime formation. The influence of aerosol acidity on SOC formation was studied by both bulk and single particle level measurements, suggesting that the aqueous-phase formation of SOC was enhanced by particle acidity. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]