Surface ozone and precursor gases at Gadanki (13.5°N, 79.2°E), a tropical rural site in India

Abstract

Ozone, nitrogen oxides (NO′x) (this includes NO, NO2 and some of their compounds converted by molybdenum converter; see section 2.2 for details), CO, and CH4 have been measured during the period 1993–1996 at Gadanki (13.5°N, 79.2°E), a rural site in the tropical Indian region. Observations show daytime photochemical ozone production that is initiated by the photooxidation of the precursor gases, with maximum noontime annual average O3 mixing ratios of only 34 ± 13 ppbv. Ozone levels were highest during the winter/spring period and lowest during summer, which differs from observations at sites in other regions. The rate of increase in O3 is greatest around 0900 hours local time, whereas the magnitude of the maximum rate of decrease during the evening is considerably smaller. This feature distinguishes the urban and rural sites since the magnitudes of the rates during morning and evening are more similar at urban sites. At Gadanki, annual averages of oxides of nitrogen (NO′x), CO, and CH4 are observed to be 2.1 ± 1.8 ppbv, 237 ± 64 ppbv, and 1.70 ± 0.11 ppmv, respectively. However, a poor correlation between ozone and NO′x was found, indicating that NO′x levels at this site are not controlled by fresh combustion or emissions and could be due to transport from the nearby major cities. Ozone production efficiency and estimated photochemical ozone production were also found to be lower at this site.