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Volatile organic compounds (VOCs) play a crucial role in the formation of tropospheric ozone (O 3) and secondary organic aerosols. VOC emissions are generally considered to have larger uncertainties compared to other pollutants, such as sulfur dioxide and fine particulate matter (PM 2.5). Although predictions of O 3 and PM 2.5 have been extensively evaluated in air quality modeling studies, there has been limited reporting on the evaluation of VOCs, mainly due to a lack of routine VOC measurements at multiple sites. In this study, we utilized VOC measurements from the "Towards an Air Toxic Management System in China" (ATMSYC) project at 28 sites across China and assessed the predicted VOC concentrations using the Community Multiscale Air Quality (CMAQ) model with the widely used Multi-resolution Emission Inventory for China (MEIC). The ratio of predicted to observed total VOCs was found to be 0.74 ± 0.40, with underpredictions ranging from 2.05 to 50.61 ppbv (5.77 % to 85.40 %) at 24 sites. A greater bias in VOC predictions was observed in industrial cities in the north and southwest, such as Jinan, Shijiazhuang, Lanzhou, Chengdu, and Guiyang. In terms of different VOC components, alkanes, alkenes, non-naphthalene aromatics (ARO2MN), alkynes, and formaldehyde (HCHO) had prediction-to-observation ratios of 0.53 ± 0.38, 0.51 ± 0.48, 0.31 ± 0.38, 0.41 ± 0.47, and 1.21 ± 1.61, respectively. Sensitivity experiments were conducted to assess the impact of the VOC prediction bias on O 3 predictions. While emission adjustments improved the model performance for VOCs, resulting in a change in the ratio of total VOCs to 0.86 ± 0.47, they also exacerbated O 3 overprediction relative to the base case by 0.62 % to 6.27 % across the sites. This study demonstrates that current modeling setups and emission inventories are likely to underpredict VOC concentrations, and this underprediction of VOCs contributes to lower O 3 predictions in China. [ABSTRACT FROM AUTHOR]

In 2021, the World Health Organization ranked Nigeria among the most polluted nations in the world, an indication of a deteriorating air quality, especially in the major urban areas of the country, which might pose adverse human health impacts. In this study, the Integrated Source Apportionment Method (ISAM) tool in the Community Multiscale Air Quality (CMAQ) model (CMAQ‐ISAM) was employed to quantify the contributions of eight emissions sectors to fine particulate matter (PM2.5) and its major components in Lagos during a prolonged severe atmospheric pollution episode (APE) in January 2021. The influence of meteorological conditions on the formation and dispersion of PM2.5 during the APE was also elucidated. Spatially, elevated PM2.5 concentrations were found in the northwestern region of Lagos, an urban area with larger anthropogenic emissions. Residential and industry were the two major sources of PM2.5. Residential contributed the most to total PM2.5 (∼40 μg/m3), followed by industry (∼20 μg/m3). High concentrations of secondary inorganic aerosols (SIA) at the northwest and upper northern areas of Lagos were majorly attributed to residential and industry sectors. In addition, sulfate accounted for the largest fraction of PM2.5, with residential, industry, and energy being its major sources. Residential, industry, and on‐road sectors dominated the contributions to nitrate, while residential and industry were the major contributors to ammonium. Furthermore, the elevated PM2.5 concentrations during the APE were greatly enhanced by unfavorable meteorological conditions. This study provides insights for designing effective emissions control strategies to mitigate future severe PM2.5 pollution episode in Lagos. Plain Language Summary: With a population of about 23.5 million as at 2018, Lagos is the largest metropolitan city in Africa, and also the most economically and industrially‐developed city in Nigeria. As a result, the city has been suffering from severe fine particulate matter (PM2.5) pollution for more than two decades. In January 2021, a prolonged severe atmospheric pollution episode (APE) occurred in the city. In this study, the contributions of eight emissions sectors to PM2.5 and its major components, as well as the meteorological influences on PM2.5 pollution during the APE were elucidated. The results provide insights into the emissions sectors dominating PM2.5 pollution in Lagos as well as the meteorological influences, and could be used to better understand the causal factors of PM2.5 pollution episode in other areas of the world that can then be applied to develop more effective control strategies that would reduce the magnitude of the episode and better protect public health. Key Points: Lagos experienced a prolonged severe PM2.5 pollution episode in January 2021PM2.5 pollution was enhanced by unfavorable meteorological conditionsEffective emissions control strategies to reduce PM2.5 concentrations are urgently required in Lagos [ABSTRACT FROM AUTHOR]