Analysis of wintertime ozone at two urban centres in western India.

Western India is a region which is exposed to pollution contributed by both natural and anthropogenic sources that may be local, regional or remote sources as reported in limited observational studies that have been conducted in the region. In the present study, temporal variation of surface ozone is studied at two urban centres (Jodhpur and Ajmer) in western India during a winter season (December 2018 – February 2019) by using ground based observation data from Central Pollution Control Board (CPCB). Jodhpur is found to have the higher seasonal average ozone (~48 µg/m³) with the daily mean values reaching to as high as 87.5 µg/m³. Ajmer closely followed with seasonal average ozone as ~44 µg/m³ and the daily mean values reaching upto 80 µg/m³. Diurnal trend (seasonally averaged) for ozone at Ajmer reveals comparatively higher afternoon values (even exceeding 100 µg/m³) but lower night time levels (close to 20 µg/m³) thus suggesting a stronger ozone production rate (~9.2 µg/m³/hr) at Ajmer during daytime (0600 – 1500 hrs local time). The ozone production rate at Jodhpur is lower (5.4 µg/m³/hr) during the same time due to lower afternoon values (~80 µg/m³) and higher nighttime levels (> 30 µg/m³). Subsequently, a detailed analysis is performed for the monthly peak ozone days in each winter month for both urban centres using ground based observation data, a back trajectory model  and a widely used reanalysis dataset. The analysis reveals contribution of both regional as well as long range transport from different directions in different months to higher levels of surface ozone, in addition to enhanced photochemistry and/or weak ozone titration on some days.  The ozone levels even breached the Indian National Ambient Air Quality Standards (NAAQS) limit of 100 µg/m³ set by CPCB for daily maximum 8 hour ozone concentration on some of the monthly peak ozone days at both urban centres. The study highlights the importance of transported ozone in enhancing ozone levels over the western India region during winter which may have severe impacts on vegetation, human health and climate change.