Abstract: The results of an analysis of changes in the atmospheric air quality in Moscow during the lockdown period and the decline in business activity caused by the COVID-19 coronavirus pandemic are presented. The observed changes in urban air pollution represent a unique experiment to assess the impact of various anthropogenic influences on the composition of atmospheric air. The influence of weather factors and transport activity on the level of pollution in the spring of 2020 is considered. The level of the main pollutants in the atmosphere decreased by 30–50% during the period of lockdown, and a different type of changes was revealed near highways and in residential areas. This article discusses the possibilities and difficulties of using a “unique experiment” on a sharp decrease in traffic intensity in large cities to determine the contribution of various branches of the urban economy (transport, energy, industry, waste processing) to the total air pollution in large cities.