Light non-methane hydrocarbons at two sites in the Indo-Gangetic Plain

Abstract

Measurements of light (C₂–C₅) non-methane hydrocarbons (NMHCs) were made along with ozone (O₃), oxides of nitrogen (NO_x), carbon monoxide (CO) and methane (CH₄) at Hissar and Kanpur in the Indo-Gangetic Plain (IGP) in India during the month of December, 2004. Air samplings during noon and evening hours provided an opportunity to study the emission characteristics and changes during this period at these sites. The mixing ratio of O₃ was higher during noon hours due to photochemical formation, while the levels of precursor gases showed elevated values during the evening hours on a clear sky day. On foggy days there is no such variation. The lower mixing ratios of O₃ observed on foggy days could be due to the slower rate of photochemical formation caused by a reduction in solar flux and surface deposition caused by the presence of a stable planetary boundary layer. Propene and ethene show the highest evening to noon ratio due to their faster reactivities with OH radicals. Correlations among different species of the measured gases indicate contributions of emissions from biomass and biofuel burning as well as fossil fuel combustion. Although qualitatively in relation to O₃, the propylene (propene) equivalents of NMHCs have been calculated to investigate their roles in O₃ photochemistry and compared with the data from Ahmedabad, an urban site in western India. The important result, which has emerged from the analysis of the observed data, is that while the total amount of these NMHCs is least at Hissar and highest at Ahmedabad, the total propylene-equivalent is highest at Hissar and lowest at Ahmedabad. Further, these two sites in the IGP show significant contributions, almost 72–77%, by propene and ethene while the contribution by these two gases at Ahmedabad is only about 47%. The surface level mixing ratios of O₃ could be treated as representative for the chemical characterization of air mass at a regional scale over the IGP as the month long trends of O₃ show significant similarity compared to the trends in precursors at the two sites.