Trace gases at a semi-arid urban site in western India: variability and inter-correlations

Abstract

Continuous measurements of O3, CO, NOx and SO2 were made in Ahmedabad, a semi-arid, urban region in western India, during May-October, 2011 to study their levels, variability and inter-relationships during this period of changing meteorological conditions. The levels of CO and SO2 observed in Ahmedabad were in general not very high compared to other major urban regions in India. On the other hand, NOx levels, impacted by vehicular emissions, were found to be substantially elevated. However, these values are still lower than levels in megacities like Beijing, Hong-Kong, Mexico etc. A sudden increase in trace gas levels was observed during post-monsoon months due to a concurrent change in prevailing winds. The levels, variability as well as diurnal amplitudes for all the gases were particularly high during October. While the primary pollutants were negatively correlated with wind speed, O3 interestingly showed a positive association. Low slope and relatively lower correlation between O3 and CO in Ahmedabad indicate incomplete photochemical processes in the ambient air. Despite an overall inverse relationship between O3 and NOx, daytime O3 was found to be positively correlated with night-time NOx as well as night-time CO. Levels of CO, NOx and SO2 showed unique relationships with wind direction depending on source locations. Low CO-NOx slope for Ahmedabad indicate relatively fresh emissions from local sources. SO2 values as well as SO2/NOx ratios were higher when wind was from the eastern sector of the city, which contains a thermal power station and a few industrial clusters. The SO2/NOx slope for point sources in Ahmedabad is estimated to be 0.4 while for mobile sources, it is 0.026.