Deviations from the O3-NO-NO2 photo-stationary state in Delhi, India

A  network of air quality and weather monitoring stations was set-up across Delhi, India, under theSystem of Air quality Forecasting And Research (SAFAR) project. The objective of this network was toenable better understanding of air quality in terms of atmospheric chemistry, emissions and forecastingin Delhi, one of the largest metropolises in the world. In this study, we focus on the O3-NO-NO2-triad. Photo Stationary State (PSS), and investigate site-specific deviations in the Leighton Ratio (F) during ashort period in 2012 (1e31 December). Large variations were observed in the NO (<1 ppbv to a peak of295 ppbv), NO2(<2 ppbve47 ppbv) and O3(4 ppbve95 ppbv) mixing ratios, all of which showed strongdiurnal variation. TheFvalues showed large deviations from unity over the measurement period, withmostly negative deviations (F< 1), showing that the air masses were dominated by local sources of NOxand that the PSS was not achieved. Positive deviations (F> 1) were also observed occasionally, and thesedata were used to estimate the total peroxy radical (PO2) mixing ratios. This is the first estimate of PO2reported for the city of Delhi and compares well with the results in the literature.