Nitrogen dioxide (NO₂) is a major air pollutant with strong spatial variability near urban sources. We use a regional chemical transport model to simulate NO₂ levels over Bucharest and compare the results with in situ, aircraft, and satellite measurements. We find that emissions of nitrogen oxides are likely underestimated and that the accuracy of the satellite instrument varies with pollution levels. Our results improve the interpretation of satellite data for air quality monitoring.

We ask how the amount of aerosol particles shapes cloud structure. Using computer simulations of a laboratory cloud chamber, we varied aerosol levels and tracked droplet growth. When aerosols are few, cloud water increases with height; when many, it becomes almost uniform because vapor is used up near the bottom. These findings clarify when upward motions matter and guide chamber design and better cloud treatment in weather and climate models.