Enhanced VOC emission with increased temperature contributes to the shift of O3-precursors relationship and optimal control strategy.

Assessing the impact of anthropogenic volatile organic compounds (VOCs) on ozone (O3) formation is vital for the management of emission reduction and pollution control. Continuous measurement of O3 and the major precursors was conducted in a typical light industrial city in the YRD region from 1 May to 25 July in 2021. Alkanes were the most abundant VOC group, contributing to 55.0% of TVOCs concentration (56.43 ± 21.10 ppb). OVOCs, aromatics, halides, alkenes, and alkynes contributed 18.7%, 9.6%, 9.3%, 5.2% and 1.9%, respectively. The observational site shifted from a typical VOC control regime to a mixed regime from May to July, which can be explained by the significant increase of ROx production, resulting in the transition of environment from NOx saturation to radical saturation with respect to O3 production. The optimal O3 control strategy should be dynamically changed depending on the transition of control regime. Under NOx saturation condition, minimizing the proportion of NOx in reduction could lead to better achievement of O3 alleviation. Under mixed control regime, the cut percentage gets the top priority for the effectiveness of O3 control. Five VOCs sources were identified: temperature dependent source (28.1%), vehicular exhausts (19.9%), petrochemical industries (7.2%), solvent & gasoline usage (32.3%) and manufacturing industries (12.6%). The increase of temperature and radiation would enhance the evaporation related VOC emissions, resulting in the increase of VOC concentration and the change of ROx circulation. Our results highlight determination of the optimal control strategies for O3 pollution in a typical YRD industrial city.

Aerosol liquid water in PM2.5 and its roles in secondary aerosol formation at a regional site of Yangtze River Delta.

Aerosol liquid water content (ALWC) plays an important role in secondary aerosol formation. In this study, a whole year field campaign was conducted at Shanxi in north Zhejiang Province during 2021. ALWC estimated by ISORROPIA-II was then investigated to explore its characteristics and relationship with secondary aerosols. ALWC exhibited a highest value in spring (66.38 µg/m3), followed by winter (45.08 µg/m3), summer (41.64 µg/m3), and autumn (35.01 µg/m3), respectively. It was supposed that the secondary inorganic aerosols (SIA) were facilitated under higher ALWC conditions (RH > 80%), while the secondary organic species tended to form under lower ALWC levels. Higher RH (> 80%) promoted the NO3- formation via gas-particle partitioning, while SO42- was generated at a relative lower RH (> 50%). The ALWC was more sensitive to NO3- (R = 0.94) than SO42- (R = 0.90). Thus, the self-amplifying processes between the ALWC and SIA enhanced the particle mass growth. The sensitivity of ALWC and OX (NO2 + O3) to secondary organic carbon (SOC) varied in different seasons at Shanxi, more sensitive to aqueous-phase reactions (daytime R = 0.84; nighttime R = 0.54) than photochemical oxidation (daytime R = 0.23; nighttime R = 0.41) in wintertime with a high level of OX (daytime: 130-140 µg/m3; nighttime: 100-140 µg/m3). The self-amplifying process of ALWC and SIA and the aqueous-phase formation of SOC will enhance aerosol formation, contributing to air pollution and reduction of visibility.

Characteristics and sources of ambient Volatile Organic Compounds (VOCs) at a regional background site, YRD region, China: Significant influence of solvent evaporation during hot months.

Continuous measurement of 98 volatile organic compounds (VOCs) was conducted during 2017-2019 at a regional background site (Shanxi) located at northeast of Zhejiang Province, YRD region, China. The average concentration of total VOCs (TVOCs) was 25.4 ± 18.4 ppbv, and an increasing trend (+12.2 %) was observed. Alkanes were the most abundant VOC group among all seasons, accounting for 43.5 % of TVOCs. Oxygenated VOCs (OVOCs), aromatics, halides and alkenes contributed 15.9 %, 15.7 %, 11.7 % and 10.3 % of TVOCs concentration, respectively. Biogenic VOCs (BVOCs) and OVOCs showed distinguished diurnal cycle from primary anthropogenic VOCs. Photochemical reactivity analysis based on ozone formation potential (OFP) and OH loss rate (LOH) indicated that aromatics and alkenes were the most significant contributor, respectively. Toluene, xylene (m/p- and o-), ethene and propene were the largest contributor of annual OFP, with the mean OFP being 33.8 ± 44.3 µg·m-3, 31.9 ± 32.1 µg·m-3, 9.29 ± 11.4 µg·m-3, 22.1 ± 21.3 µg·m-3 and 12.8 ± 19.5 µg·m-3, respectively. Seven sources were identified with positive matrix factorization (PMF): petrochemical industry (13.8 %), biogenic emission (1.0 %), solvent usage-toluene (16.9 %), vehicular exhaust (43.8 %), Integrated circuits industry (3.8 %), solvent usage-C8 aromatics (10.9 %), and gasoline evaporation (9.8 %). Vehicular exhaust was the most significant source (43.8 %) during the whole measurement period. Solvent usage, petrochemical industry, and gasoline evaporation showed high temperature dependency. The integrated contribution of solvent usage and industrial processes were higher than vehicular exhaust during hot months. These sources also have higher chemical reactivities and can contribute more on O3 formation. Our results are helpful on determining the control strategies aiming at alleviating O3 pollution.

Chemical characterization, formation mechanisms and source apportionment of PM2.5 in north Zhejiang Province: The importance of secondary formation and vehicle emission.

PM2.5 affects air quality, therefore, chemical evolution, formation mechanism and source identification of PM2.5 are essential to help figure out mitigation measures. PM2.5 and its constituents were comprehensively characterized with highly time-resolved measurements from 2019 to 2020 in north Zhejiang Province (Shanxi, SX) for the first time, with an emphasis on the contribution of secondary formation and vehicle emission to PM2.5. Secondary inorganic ions (sulfate: 3.86 µg/m3, nitrate: 7.82 µg/m3 and ammonium: 4.59 µg/m3, SNA) were found to be the major components (54%) in PM2.5 (29.70 µg/m3). The highly consistence of nitrate, sulfate and secondary organic compounds (SOC) with Ox (NO2 + O3) or RH indicated the importance of photochemical oxidation and heterogeneous reaction in different scenarios. Higher atmospheric oxidative potential facilitated the SOC formation in spring. The PM2.5 mass was apportioned to eight sources resolved by positive matrix factorization (PMF): secondary nitrate (9.63 µg/m3), secondary sulfate (5.14 µg/m3), vehicle emission (7.26 µg/m3), coal combustion (2.39 µg/m3), biomass burning (1.38 µg/m3), soil dust (0.86 µg/m3), industry emission (0.50 µg/m3), and ship emission (0.32 µg/m3). Secondary nitrate (35%) and sulfate (19%) formation and vehicle emission (26%) were the main factors contributing to the PM2.5. Furthermore, the contribution of secondary nitrate formation increased with elevating PM2.5 concentration. Regional transport was synthetically studied by chemical and backward trajectory analysis, reflecting that secondary nitrate contributed severely to the air quality at SX, while vehicle emission contribution enhanced when atmosphere was stagnant. This study first provides long-term comprehensive chemical characterization and source apportionments of PM2.5 pollution in north Zhejiang, which may provide some guidance for the air pollution control.

Critical Role of Simultaneous Reduction of Atmospheric Odd Oxygen for Winter Haze Mitigation.

The lockdown due to COVID-19 created a rare opportunity to examine the nonlinear responses of secondary aerosols, which are formed through atmospheric oxidation of gaseous precursors, to intensive precursor emission reductions. Based on unique observational data sets from six supersites in eastern China during 2019-2021, we found that the lockdown caused considerable decreases (32-61%) in different secondary aerosol components in the study region because of similar-degree precursor reductions. However, due to insufficient combustion-related volatile organic compound (VOC) reduction, odd oxygen (Ox = O3 + NO2) concentration, an indicator of the extent of photochemical processing, showed little change and did not promote more decreases in secondary aerosols. We also found that the Chinese provinces and international cities that experienced reduced Ox during the lockdown usually gained a greater simultaneous PM2.5 decrease than other provinces and cities with an increased Ox. Therefore, we argue that strict VOC control in winter, which has been largely ignored so far, is critical in future policies to mitigate winter haze more efficiently by reducing Ox simultaneously.