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Ammonia (NH 3) plays an important role in the formation of atmospheric particulate matter, but the contribution of traffic-related emissions remains unclear, particularly in megacities with a large number of vehicles. Taking the opportunity of the stringent COVID-19 lockdowns implemented in Beijing and Shanghai in 2022, this study aims to estimate the traffic-related NH 3 emissions in these two megacities based on satellite observations. Differences between urban and suburban areas during the lockdown and non-lockdown periods are compared. It was found that despite different dominating sources, the overall NH 3 concentrations in urban and suburban areas were at a similar level, and the lockdown resulted in a more prominent decrease in urban areas, where traffic activities were most heavily affected. The traffic-related contribution to the total emission was estimated to be ∼30% in megacities, and ∼40% in urban areas, which are about 2–10 times higher than that in previous studies. The findings indicate that the traffic-related NH 3 emissions have been significantly underestimated in previous studies and may play a more critical role in the formation of air pollution in megacities, especially in winter, when agricultural emissions are relatively low. This study highlights the importance of traffic-related NH 3 emissions in Chinese megacities and the need to reassess the emissions and their impacts on air quality. [Display omitted] • A new way to estimate traffic-related NH 3 emissions using satellite observations was provided. • The COVID-19 lockdowns resulted in greater NH 3 reduction in urbans than in suburbs. • Traffic-related NH 3 emissions in megacities may have been remarkably underestimated. • Traffic-related NH 3 emissions in winter may play a more important role in PM 2.5 formation. [ABSTRACT FROM AUTHOR]

The knowledge of soil environmental quality and its changing trends is important for safe and sustainable land utilization. However, comprehensive information on soil environment carrying capacity, involving environmental, economic and social pressures, is relatively rare. In this study, a modified dynamic capacity model is developed to estimate soil environment carrying capacity in terms of a combined consideration of soil environment capacity, cumulative input/output rate and risk characteristics. Based on the method proposed, this paper demonstrates the current pollution status and remaining soil capacity of the Beijing urban area, and establishes a conceptual "early warning" model for soil environmental quality, to predict time-dependent changing patterns of soil pollutants under different accumulation scenarios. The results showed that for Beijing soil environmental carrying capacity varied with land use type and pollutant. Compared with Cu, Zn and Pb, Cd posed the greatest threat to soil environmental carrying capacity in both residential areas and green parks. Heavy metal carrying capacity in soils in built-up areas in Beijing was not overloaded currently, and will not deteriorate significantly over the short-to medium-term in a hypothetical "decreased input" scenario. The method proposed provides a simple, cost-effective, and quantitative tool for mapping soil quality level, and assessing the need for risk management measures, in China and elsewhere. • Natural attenuation capacity provides basis for soil environment carrying capacity. • Cumulative rate and risk characteristics are other dominant factors in model input. • An early warning system is necessary for effective risk management measures. • Soil environmental carrying capacity varied with land use type and pollutant. • The framework and model are transferable in other international urban settings. [ABSTRACT FROM AUTHOR]