Evaluation of urban ozone in the Brahmaputra River Valley

Ozone (O₃), carbon monoxide (CO), nitrogen oxides (NOx = NO + NO₂), methane (CH₄) and non-methane hydrocarbons (NMHCs) were measured in Guwahati in the Brahmaputra River Valley (BRV) of northeastern India from January 1, 2013 to June 30, 2014. The current analysis shows that the annual mean concentrations of O_3, CO, CO_2, NOx, CH_4, and NMHC in Guwahati were 10.5 (±6.30 ppb), 0.5 (±0.2 ppm), 411.7 (±37.5 ppm), 19.6 (±11.6 ppb), 2045(±417 ppb) and 173(±93 ppb), respectively. The pattern of O₃concentrations in Guwahati indicated it was strongly influenced by local NOx sources with an adjacent national highway being the likely major source. Thus, the measured O₃ values were reduced by local titration. Diel, weekly, and seasonal patterns for each of these gaseous species showed the strong influence of local sources. The mean ventilation coefficient was greater than >6000 m²s during the day in the pre-monsoon season indicating pollutant dispersion. The seasonally average solar radiation (SR) fluxes decreased in the order: pre-monsoon (180 W m⁻²), monsoon (176 W m⁻²), post-monsoon (148 W m⁻²) and winter (110 W m⁻²).The corresponding mean O₃ concentrations were 14.8, 7.5, 6.4, and 10.1 ppb, respectively. In the pre-monsoon season, an impact of SR on the photochemical formation of O₃was observed. High O₃ winter concentrations were observed likely driven by local biomass burning providing reactive VOCs that contributed to ozone formation.