Seasonal variations in surface ozone and its precursors over an urban site in India

Abstract

Surface measurements of ozone and its precursor gases (NO_x, CO and CH₄) have been made over an urban site, Ahmedabad (23°N, 72.6°E) in tropical India. Measurements of ozone are made for the period 1991-1995 whereas measurements of NO_x, CO and CH₄ are made for the period 1993-1996. Ozone, NO_x and CO show diurnal variations typical to an urban site. However, high levels of ozone, exceeding 80 ppbv, are rarely observed. The day time ozone production is basically due to the photoxidation of the precursor gases. Boundary layer processes and meteorology also play an important role in its variabilities. During the day, the average amount of ozone increases from the nighttime base value of about 14±2 to 34±9 ppbv. However, its diurnal amplitude varies during different months. Diurnal variations in NO_x and CO are a manifestation of combined effects of local emissions, boundary layer processes, chemistry and local wind pattern. The annual variation in average ozone concentrations ranges from a minimum value of about 12±2 ppbv during August to a high value of about 30±3 ppbv during November. Ozone concentrations are observed to be maximum during autumn and winter months due to higher amounts of precursor gases, in spite of lower solar radiation. Higher levels of precursors during autumn and winter are due to large scale transportation from the continents and lower boundary layer heights. Contrary to the seasonal variation, precursor gases show anti-correlation with ozone in the diurnal variations. These seasonal variations observed in ozone are found to be different than the variations observed at other sites in India as well as in the northern mid-latitudes.