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 atShahdara National Ambient Air Quality Monitoring stationin Delhi (India) in July 1999. The minimum pollutantconcentrations were observed during morning hours,whereas the highest concentrations were found during thelate night hours, which seem to be related with thevehicular emission. Pre-monsoon daily ambient airquality spatial pattern was compared with the spatialpattern during initial and subsequent rain shower ofmonsoon. These spatial patterns were found to beessentially the same before and during rain, however asignificant decrease in SO₂, NO₂ and TSPconcentrations (40-45%) was observed after initial andsubsequent rains of the monsoon, demonstrating theimportance of rainfall in the scavenging of thesecriteria air pollutants.