Distributions of C2–C5 NMHCs and related trace gases at a tropical urban site in India

Simultaneous surface measurements of C2–C5 non-methane hydrocarbons (NMHCs), O3, CO and CH4 were made during the year 2002 at a tropical urban site, Ahmedabad. This is the first time that NMHCs levels have been characterized in detail in India. The diurnal distributions of these species show pronounced variations in the winter months and less during the summer months. The seasonal variations of all these species show substantially higher levels during the winter and lowest during the summer season. The strength (winter to summer ratios) of seasonal variations in NMHCs are observed to be higher than other reported measurements elsewhere. The seasonal changes in transport patterns, boundary layer height and OH concentrations, all contribute in the seasonal variations of these trace gases. The correlation studies of various NMHCs and CO indicate dominant role of local emissions in the observed distributions of trace gases. The natural gas emission and leakage of liquid petroleum gas contribute to elevated levels of ethane and propane. While emissions from vehicular exhaust are found to be dominant sources of ethene, propene and acetylene. The higher C2H2/CO ratio of about 6.4 pptv/ppbv indicates influences of fresh emissions at Ahmedabad.