Spatial variability of concentrations of gaseous pollutants across the National Capital Region of Delhi, India

This study presents a systematic evaluation of a year-long, continuous, real-time measurements of gaseous pollutants including oxides of nitrogen (NOx = NO + NO2), carbon monoxide (CO), and ozone (O3) in the National Capital Region (NCR) of Delhi. Data are available from seven air quality monitoring stations (AMS) and meteorological parameters were measured at three meteorological observatories (MO). The daily mean concentrations of NOx and CO across NCR of Delhi (average of all seven AMSs) were 37.1 ± 11.6 ppb (NO: 18.3 and NO2: 18.8 ppb), and 2.3 ± 0.6 ppm, respectively. The highest diurnal CO concentrations were observed during midnight (21:00 to 01:00 h LT) due to nocturnal boundary conditions. The daily mean O3 concentration was 37.5 ± 11.0 ppb. The diurnal variations of O3 concentrations were characterized by the high concentrations during the daytime (07:00 to 17:00 h LT: 43.6 ppb) and low concentrations during the late evening and early morning hours (18:00 to 06:00 h LT: 31.1 ppb). From 07:00 h onwards, the O3 concentrations start increasing gradually after sunrise coinciding with the increasing solar radiation. The highest concentrations (51.7 ppb) were observed around 15:00 h. Thereafter, it is decreased. Local sources of gaseous pollutants were identified by analyzing the surface wind patterns. The annual resultant wind vector were 338° (57%) and 288° (27%) at the Hindon and Palam MOs, respectively. Local NO emissions substantially reduced the urban ozone concentrations as seen by the observed untitrated NO concentrations.