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  1. null (Ed.)
    Abstract. In response to the coronavirus disease of 2019 (COVID-19),California issued statewide stay-at-home orders, bringing about abrupt anddramatic reductions in air pollutant emissions. This crisis offers us anunprecedented opportunity to evaluate the effectiveness of emissionreductions in terms of air quality. Here we use the Weather Research and Forecastingmodel with Chemistry (WRF-Chem) in combination with surface observations tostudy the impact of the COVID-19 lockdown measures on air quality insouthern California. Based on activity level statistics and satelliteobservations, we estimate the sectoral emission changes during the lockdown.Due to the reduced emissions, the population-weighted concentrations of fineparticulate matter (PM2.5) decrease by 15 % in southernCalifornia. The emission reductions contribute 68 % of the PM2.5concentration decrease before and after the lockdown, while meteorologyvariations contribute the remaining 32 %. Among all chemical compositions,the PM2.5 concentration decrease due to emission reductions isdominated by nitrate and primary components. For O3 concentrations, theemission reductions cause a decrease in rural areas but an increase in urbanareas; the increase can be offset by a 70 % emission reduction inanthropogenic volatile organic compounds (VOCs). These findings suggest thata strengthened control on primary PM2.5 emissions and a well-balancedcontrol on nitrogen oxides and VOC emissions are needed to effectively andsustainably alleviate PM2.5 and O3 pollution in southernCalifornia. 
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  2. Abstract. The interactions between aerosols and ice clouds represent one of the largest uncertainties in global radiative forcing from pre-industrial time to the present. In particular, the impact of aerosols on ice crystal effective radius (Rei), which is a key parameter determining ice clouds' net radiative effect, is highly uncertain due to limited and conflicting observational evidence. Here we investigate the effects of aerosols on Rei under different meteorological conditions using 9-year satellite observations. We find that the responses of Rei to aerosol loadings are modulated by water vapor amount in conjunction with several other meteorological parameters. While there is a significant negative correlation between Rei and aerosol loading in moist conditions, consistent with the "Twomey effect" for liquid clouds, a strong positive correlation between the two occurs in dry conditions. Simulations based on a cloud parcel model suggest that water vapor modulates the relative importance of different ice nucleation modes, leading to the opposite aerosol impacts between moist and dry conditions. When ice clouds are decomposed into those generated from deep convection and formed in situ, the water vapor modulation remains in effect for both ice cloud types, although the sensitivities of Rei to aerosols differ noticeably between them due to distinct formation mechanisms. The water vapor modulation can largely explain the difference in the responses of Rei to aerosol loadings in various seasons. A proper representation of the water vapor modulation is essential for an accurate estimate of aerosol–cloud radiative forcing produced by ice clouds.

     
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  3. To tackle the severe fine particle (PM2.5) pollution in China, the government has implemented stringent control policies mainly on power plants, industry, and transportation since 2005, but estimates of the effectiveness of the policy and the temporal trends in health impacts are subject to large uncertainties. By adopting an integrated approach that combines chemical transport simulation, ambient/household exposure evaluation, and health-impact assessment, we find that the integrated population-weighted exposure to PM2.5(IPWE) decreased by 47% (95% confidence interval, 37–55%) from 2005 [180 (146–219) μg/m3] to 2015 [96 (83–111) μg/m3]. Unexpectedly, 90% (86–93%) of such reduction is attributed to reduced household solid-fuel use, primarily resulting from rapid urbanization and improved incomes rather than specific control policies. The IPWE due to household fuels for both cooking and heating decreased, but the impact of cooking is significantly larger. The reduced household-related IPWE is estimated to avoid 0.40 (0.25–0.57) million premature deaths annually, accounting for 33% of the PM2.5-induced mortality in 2015. The IPWE would be further reduced by 63% (57–68%) if the remaining household solid fuels were replaced by clean fuels, which would avoid an additional 0.51 (0.40–0.64) million premature deaths. Such a transition to clean fuels, especially for heating, requires technology innovation and policy support to overcome the barriers of high cost of distribution systems, as is recently being attempted in the Beijing–Tianjin–Hebei area. We suggest that household-fuel use be more highly prioritized in national control policies, considering its effects on PM2.5exposures.

     
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