Investigation into relationships among NO, NO2, NOx, O3, and CO at an urban background site in Delhi, India

• Hourly averaged data of NO, NO2, NOx, O3 & OX is analyzed for New Delhi, India.
• Relationships between oxidants (OX) and NOx are elucidated.
• Local and regional contributions of OX are determined.
• Possible sources of OX are elucidated using meteorology.

High resolution concentrations of nitric oxide (NO), nitrogen dioxide (NO2), oxides of nitrogen (NOx), and Ozone (O3) were measured at a urban site (urban background) in New Delhi, India for a period of two years from September 2010 to August 2012. During the study period, the mean concentrations of NO, NO2, NOx, O3 (ppb), and CO (ppm) were observed to be 17.2, 12.5, 29.3, 23.6, and 1.97, respectively. This data was then employed to investigate the relationships between NO, NO2, and O3 as a function of NOx. The highest mean concentrations of NO were observed from midnight 00:00 to 05:00 h local time (LT) in the morning as a result of an increase in traffic emissions and a reduction in boundary layer height during the night. The total levels of oxidant [OX], which are considered to be the sum of O3 and NO2, were determined. A study of variation of [OX] and NOx identified two distinct contributions to ambient OX concentrations, i.e., NOx independent and NOx dependent. The NOx-dependent contribution corresponds to the local production of ozone, and the NOx-independent contribution corresponds to regional concentrations, which at this site is the background level of ozone. The monthly and diurnal variations of [OX] are discussed. Wind directions were used to identify possible regional sources of [OX]. The analysis suggests that [OX] concentrations were about six times higher with winds originating from the Northwest direction (NW) compared to those from the East.