Variation in spatial pattern of criteria air pollutants before and during initial rain of monsoon.

Spatial patterns of various criteria air pollutants, like SO2, NO2, O3, and TSP were studied at Shahdara National Ambient Air Quality Monitoring station in Delhi (India) in July 1999. The minimum pollutant concentrations were observed during morning hours, whereas the highest concentrations were found during the late night hours, which seem to be related with the vehicular emission. Pre-monsoon daily ambient air quality spatial pattern was compared with the spatial pattern during initial and subsequent rain shower of monsoon. These spatial patterns were found to be essentially the same before and during rain, however a significant decrease in SO2, NO2 and TSP concentrations (40-45%) was observed after initial and subsequent rains of the monsoon, demonstrating the importance of rainfall in the scavenging of these criteria air pollutants.