Abstract. Sulfur dioxide (SO2) is a major atmospheric pollutant and precursor of sulfate aerosols, which influences air quality, cloud microphysics, and climate. Therefore, better understanding the conversion of SO2 to sulfate is essential to simulate and predict sulfur compounds more accurately. This study evaluates the effects of in-cloud aqueous-phase chemistry on SO2 oxidation in the Community Earth System Model version 2 (CESM2). We replaced the default parameterized SO2 aqueous-phase reactions with detailed HOx, Fe, N, and carbonate chemistry in cloud droplets and performed a global simulation for 2014–2015. Compared with the observations, the results incorporating detailed cloud aqueous-phase chemistry greatly reduced SO2 overestimation. This overestimation was reduced by 0.1–10 ppbv (parts per billion by volume) in most of Europe, North America, and Asia and more than 10 ppbv in parts of China. The biases in annual simulated SO2 mixing ratios decreased by 46 %, 41 %, and 22 % in Europe, the USA, and China, respectively. Fe chemistry and HOx chemistry contributed more to SO2 oxidation than N chemistry. Higher concentrations of soluble Fe and higher pH values could further enhance the oxidation capacity. This study emphasizes the importance of detailed in-cloud aqueous-phase chemistry for the oxidation of SO2. These mechanisms can improve SO2 simulation in CESM2 and deepen understanding of SO2 oxidation and sulfate formation.

Sulfate is one of the major species of aerosol. Both natural sources (such as oxidation of dimethyl sulfur [DMS]) over the ocean and anthropogenic emissions of sulfur dioxide (SO 2 ) are important sources of sulfate (Hussain & Lun, 2019;Sanchez et al., 2018;Yan & Xu, 2021). Human industrial activities have increased significantly since the industrial era, and the gas-phase, liquid-phase and surface heterogeneous oxidations of SO 2 have gradually accounted for the main sources of sulfate (B. Liu et al., 2017;Xue et al., 2016). As an important component of atmospheric particulate matter (PM), sulfate at high concentrations will aggravate the formation of haze, thus causing serious air pollution and influencing human health (Sha et al., 2019;B. Zheng et al., 2015;Zhou et al., 2020). Moreover, sulfate is also one of the main species of acid deposition, reducing the pH values of rain droplets and aggravating acid rain (M. Liu et al., 2020;Lu et al., 2015;. At the same time, sulfate is also the main component of cloud condensation nuclei, which affects microphysical processes, including the formation of clouds and precipitation, thus affecting solar radiation and climate (Cziczo et al., 2013). Furthermore, sulfate itself is also one of the key short-lived species that has negative radiative forcing (RF),

Sulfate is a major atmospheric pollutant and radiative forcing (RF) factor that influences air quality, cloud microphysics and climate. Therefore, a better evaluation of sulfate concentrations and RF patterns is essential for policy-making and the management of air pollution and climate change. This study comprehensively estimates the global distribution of sulfate concentrations and RFs and analyzes the sources of uncertainty in the Community Earth System Model version 2 (CESM2) and the Parallel Offline Radiative Transfer (PORT) model. Compared with the observations, the incorporation of detailed in-cloud aqueous-phase chemistry and the enhanced wet deposition flux of sulfate significantly improved the simulations of sulfur species both near the ground and at high altitudes, which is beneficial for a more accurate estimation of the global sulfate RF. The improved simulated RF of sulfate from 1850 to 2015 is -0.382 W.m -2 . This study finds that wet deposition is the key process governing both the horizontal and vertical distributions of sulfate concentrations. The overestimation of surface sulfate and the underestimation of high-altitude sulfate made by the model are essential uncertainty factors of the sulfate RF estimation. This study emphasizes the importance of improving the simulation of global sulfate distribution as well as its RF, which may strongly pressure the near-future warming potential when witnessing a rapid transition to a carbon neutral world that is phasing out fossil fuel. A more accurate assessment of sulfate levels and radiation effects will play a remarkable guiding role in the formulation of global emission reduction-related policies in the future.

Urban traffic pollution, which is strongly influenced by the complex urban morphology, has posed a great threat to human health. In this study, we performed a high-resolution simulation of traffic pollution in a typical city block in Baoding, China, based on the Parallelized Large-eddy simulation Model (PALM), to examine the distribution patterns of traffic-related pollutants and explore their relationship with urban morphology. Based on the model results, we conducted a multi-linear regression (MLR) analysis and found that the distribution of air pollutants inside the city block was dominated by both traffic emissions and urban morphology, which explained about 70% of the total variance in spatial distribution of air pollutants. Excluding the contribution of emissions, over 50% of the total variance can still be explained by the urban morphology. Among these urban morphological factors, the key factors determining the spatial distribution of air pollution are “Distance from the road” (DR), “Building Coverage Ratio” (BCR) and “Aspect Ratio” (H/W) of the street canyon. Specifically, urban areas with lower Aspect Ratio, lower BCR and larger DR are less affected by traffic pollution. Compiling these individual factors, we developed a complex Urban Morphology Pollution Index (UMPI). Each unit increase in UMPI is associated with a one percent increase of nearby traffic pollution contribution. This index can help urban planners to semi-quantitatively evaluate building groups which tend to trap or ventilate traffic pollution and thus help to reduce human exposure to street canyon level pollution through either traffic emission control or urban morphology amelioration.

Carbon, the human’s most reliable fuel type in the past, must be neutralized in this century towards the Paris Agreement temperature goals. Solar power is widely believed a key fossil fuel substitute, but suffers from the needs of large space occupation and huge energy storage for peak shaving. Here we propose a solar network circumnavigating the globe to connecting large-scale desert photovoltaics among continents. By evaluating the generation potential of desert photovoltaic plants on each continent (taking dust accumulation into account) and the hourly maximum transmission potential that each inhabited continent can receive (taking transmission loss into account), we find that the current total annual human demand for electricity will be more than met by this solar network. The local imbalanced diurnal generation of photovoltaic energy can be made up by transcontinental power transmission from other power stations in the network to meet the hourly electricity demand. We also find that laying solar panels over a large space may darken the earth’s surface, but this albedo warming effect is orders of magnitude lower than that of CO2 released from thermal power plants. From practical needs and ecological effects, this powerful and stable power network with lower climate perturbability could potentially help to phase out global carbon emissions in the 21st century.