Recent studies both on a modeling basis and on observed data have shown that air quality improved following the adoption of measures to contain the spread of the COVID-19 epidemic. The objective of this work is to analyze the atmospheric concentration of NO₂ in Lombardy during the spring lockdown. The study involved the Lombardy Region as it is characterized by high levels of air pollution and is also the first region in Europe to impose a lockdown. The modeling suite composed of CAMx (Comprehensive Air Quality Model with Extensions) and WRF (Weather Research and Forecasting model) provides the tool to simulate the concentration of NO₂ actually found in the atmosphere from mid-February to the end of March and to compare it to a similar setting in conditions of usual emissive activities. In particular, this work analyzed the atmospheric concentration of NO₂ in the face of an increasingly incisive reduction in emissions from road traffic. The situation during the lockdown allows us to simulate, for the first time, a scenario of reduced mobility and compare it with measured air quality data. As regards the city of Milan, the decrease in the concentration of NO₂ goes hand in hand with the reduction in traffic: a reduction in traffic of 71% corresponds to the level of nitrogen dioxide concentration being reduced by one third . The results also show that industrial activities have a significant impact on the atmospheric concentration of NO₂, especially in the provinces of Brescia and Bergamo. The analysis of air quality in Lombardy during the lockdown shows the degree of incisiveness of mobility policies required to achieve the set environmental targets effectively and consequently protect human health.