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 the System of Air quality Forecasting And Research (SAFAR) project. The objective of this network was to enable better understanding of air quality in terms of atmospheric chemistry, emissions and forecasting in 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 (Φ) during a short period in 2012 (1–31 December). Large variations were observed in the NO (<1 ppbv to a peak of 295 ppbv), NO2 (<2 ppbv–47 ppbv) and O3 (4 ppbv–95 ppbv) mixing ratios, all of which showed strong diurnal variation. The Φ values showed large deviations from unity over the measurement period, with mostly negative deviations (Φ < 1), showing that the air masses were dominated by local sources of NOx and that the PSS was not achieved. Positive deviations (Φ > 1) were also observed occasionally, and these data were used to estimate the total peroxy radical (PO2) mixing ratios. This is the first estimate of PO2 reported for the city of Delhi and compares well with the results in the literature.