The distributions and direct radiative effects of marine aerosols over East Asia in springtime

Radiative effects and feedbacks of anthropogenic aerosols on boundary layer meteorology and fine particulate matter during the COVID-19 lockdown over China

The COVID-19 epidemic has exerted significant impacts on human health, social and economic activities, air quality and atmospheric chemistry, and potentially on climate change. In this study, an online coupled regional climate-chemistry-aerosol model (RIEMS-Chem) was applied to explore the direct, indirect, and feedback effects of anthropogenic aerosols on radiation, boundary layer meteorology, and fine particulate matter during the COVID-19 lockdown period from 23 January to 8 April 2020 over China. Model performance was validated against a variety of observations for meteorological variables, PM_2.5 and its chemical components, aerosol optical properties, as well as shortwave radiation flux, which demonstrated that RIEMS-Chem was able to reproduce the spatial distribution and temporal variation of the above variables reasonably well. During the study period, direct radiative effect (DRE) of anthropogenic aerosols was stronger than indirect radiative effect (IRE) in most regions north of the Yangtze River, whereas IRE dominated over DRE in the Yangtze River regions and South China. In North China, DRE induced larger changes in meteorology and PM_2.5 than those induced by IRE, whereas in South China, the changes by IRE were remarkably larger than those by DRE. Emission reduction alone during the COVID-19 lockdown reduced PM_2.5 concentration by approximately 32 % on average over East China. As a result, DRE at the surface was weakened by 15 %, whereas IRE changed little over East China, leading to a decrease in total radiative effect (TRE) by approximately 7 % in terms of domain average. The DRE-induced changes in meteorology and PM_2.5 were weakened due to emission reduction, whereas the IRE-induced changes were almost the same between the cases with and without emission reductions. By aerosol radiative and feedback effects, the COVID-19 emission reductions resulted in 0.06 °C and 0.04 °C surface warming, 1.6 and 4.0 μg m^-3 PM_2.5 decrease, 0.4 and 1.3 mm precipitation increase during the lockdown period in 2020 in terms of domain average over North China and South China, respectively, whereas the lockdown caused negligible changes on average over East Asia.

Direct and indirect effects and feedbacks of biomass burning aerosols over Mainland Southeast Asia and South China in springtime

Southeast Asia is one of the largest biomass burning (BB) source regions in the world. In order to promote our understanding of BB aerosol characteristics and environmental impacts, this study investigated the emission, composition, evolution, radiative effects, and feedbacks of BB aerosols from Mainland Southeast Asia during 15 March to 15 April 2019 by using an online-coupled regional chemistry/aerosol-climate model RIEMS-Chem. Model results are compared against a variety of ground and vertical observations, indicating a generally good model performance for meteorology, aerosol chemical compositions, and aerosol optical properties. It is found that BB aerosols contributed significantly to regional particulate matter (PM), accounting for up to 90 % of the near-surface PM_2.5, BC, and OC concentrations over the BB source regions of north Mainland Southeast Asia and for approximately 30-70 % over wide downwind areas including most areas of southwest China and portions of south China. At the top of atmosphere (TOA), BB aerosols exerted a positive all-sky radiative effect (DRE_BB) up to 25 W/m² over north Vietnam and south China, a negative DRE_BB up to -10 W/m² over Myanmar, western Thailand, and southwest China. Meanwhile, the indirect radiative effect (IRE_BB) was consistently negative, with the maximum of -10 W/m² over downwind areas with cloud coverage, e.g., from north Vietnam to most of south China. The subregional (95-125°E and 10-30°N) and period mean DRE_BB and IRE_BB at TOA were estimated to be 0.69 W/m² and - 0.63 W/m², respectively, leading a total radiative effect (TRE_BB) of 0.06 W/m² at TOA. The radiative effects of BB aerosols led to decreases in sensible and latent heat fluxes, near-surface temperature, PBL height, and wind speed of 6.0 Wm^-2, 9.0 Wm^-2, 0.26 °C, 38.7 m, and 0.1 m/s, respectively, accompanied with an increase in RH of 1.9 %, averaged over the subregion and the study period. The accumulated precipitation during the study period was apparently reduced by BB aerosols from east Thailand to south China, with the maximum reduction up to 14 cm (exceeding 40 %) over north Vietnam and south China. TRE_BB tended to increase mean near-surface PM_2.5 and its component concentrations, with the maximum percentage increase up to 24 % over the BB source regions of north Mainland Southeast Asia, resulting from the combined effects of dynamic and chemical feedbacks. DRE_BB generally dominated over IRE_BB in the feedback-induced PM_2.5 concentration changes.

Effects of natural and anthropogenic emissions on the composition and toxicity of aerosols in the marine atmosphere

The impacts of natural dimethyl sulfide (DMS) and ship emissions on marine environments and particulate matter (PM) over the western and southern sea areas around South Korea were studied based on field campaigns from August-September 2017 and May-June 2018 using the Community Multi-scale Air Quality v5.3.2 modeling system. DMS oxidation enhanced the concentrations of both sulfur dioxide (SO₂) and sulfate (SO₄^2-) in PM_2.5 by 6.2-6.4% and 2.9-3.6%, respectively, in the marine atmosphere during the study period, whereas it slightly decreased nitrate (NO₃^-) concentrations (by -1.3%), compared to the simulation without DMS oxidation chemistry. Furthermore, ship emissions increased the concentrations of SO₄^2-, NO₃^-, and NH₄⁺ by 4.5%, 23%, and 7.3%, respectively. Methane sulfonic acid concentration was 0.17 μg m^-3, suggesting the importance of the addition channel in the DMS oxidation pathway. The model simulation indicated that ship emissions in the target area contributed dominantly to non-sea-salt SO₄^2-, and the marine DMS emission source was non-negligible. The geographical distribution of PM toxicity (aerosol oxidative potential) was assessed in the marine atmosphere during the study period.

A pH dependent sulfate formation mechanism caused by hypochlorous acid in the marine atmosphere

Secondary sulfate plays a crucial role in forming marine aerosol, which in turn is an important source of natural aerosol at a global level. Recent experimental studies suggest that oxidation of S(IV) compounds, in practice dissolved sulfur dioxide, to sulfate (S(VI)) by hypochloric acid could be one of the most significant pathways for sulfate formation in marine areas. However, the exact mechanism responsible for this process remains unknown. Using high-level quantum chemical calculations, we studied the reaction between dissolved sulfur dioxide and hypochloric acid. We account for the dominant protonation states of reactants in the pH range 3.0-9.0. We also consider possible catalytic effects of species such as H₂O. Our results show that sulfate formation in HOCl+HOSO₂^- and HOCl+SO₃^2- reactions relevant to acidic and nearly neutral conditions can occur either through previously proposed Cl⁺ transfer or through a novel HO⁺ transfer mechanism. In alkaline conditions, where the dominant reactants are OCl^- and SO₃^2-, an O atom transfer mechanism proposed in previous experimental studies may be more important than Cl⁺ transfer. Catalysis by common cloud-water species is found to lower barriers of Cl⁺ transfer mechanisms substantially. Nevertheless, we find that the dominant S(IV) + HOCl reaction mechanism for the full studied pH range is HO⁺ transfer from HOCl to SO₃^2-, which leads directly to sulfate formation without ClSO₃^- intermediates. The rate-limiting barrier of this reaction is low, leading to an essentially diffusion-controlled reaction rate. S(IV) lifetimes due to this reaction decrease with increasing pH due to the increasing fractional population of SO₃^2-. Especially in neutral and alkaline conditions, depletion of HOCl by the reaction is so rapid that S(IV) oxidation will be controlled mainly by mass transfer of gas-phase HOCl to the liquid phase. The mechanism proposed here may help to explain marine sulfate sources missing from current atmospheric models.

Enhanced PM2.5 Decreases and O3 Increases in China During COVID-19 Lockdown by Aerosol-Radiation Feedback

We apply an online-coupled meteorology-chemistry model (WRF-Chem) embedded with an improved process analysis to examine aerosol-radiation feedback (ARF) impacts on effectiveness of emission control due to Coronavirus Disease 2019 (COVID-19) lockdown over North China Plain. Emission reduction alone induces PM2.5 decrease by 16.3 μg m-3 and O3 increase by 10.2 ppbv during COVID-19 lockdown. The ARF enhances PM2.5 decrease by 2.7 μg m-3 (16.6%) and O3 increase by 0.8 ppbv (7.8%). The ARF-induced enhancement of PM2.5 decline is mostly attributed to aerosol chemistry process, while enhancement of O3 rise is ascribed to physical advection and vertical mixing processes. A set of sensitivity experiments with emission reductions in different degrees indicate that the ARF-induced enhancements of PM2.5 declines (O3 rises) follow a robust linear relationship with the emission-reduction-induced PM2.5 decreases. The fitted relationship has an important implication for assessing the effectiveness of emission abatement at any extent.