Variabilities in ozone at a semi‐urban site in the Indo‐Gangetic Plain region: Association with the meteorology and regional processes

Abstract

The Indo‐Gangetic Plain (IGP) region is one of the most densely populated regions in the World, but ground‐based observations of air pollutants are highly limited in this region. Here, surface ozone observations made during March 2009–June 2011 at a semi‐urban site (Pantnagar; 29.0°N, 79.5°E, 231 m amsl) in the IGP region are presented. Ozone mixing ratios show a daytime photochemical buildup with ozone levels sometimes as high as 100 ppbv. Seasonal variation in 24‐h average ozone shows a distinct spring maximum (39.3 ± 18.9 ppbv in May) while daytime (1130–1630 h) average ozone shows an additional peak during autumn (48.7 ± 13.8 ppbv in November). The daytime, but not daily average, observed ozone seasonality is in agreement with the space‐borne observations of OMI tropospheric column NO2, TES CO (681 hPa), surface ozone observations at a nearby high altitude site (Nainital) in the central Himalayas and to an extent with results from a global chemistry transport model (MATCH‐MPIC). It is suggested that spring and autumn ozone maximum are mainly due to photochemistry, involving local pollutants and small‐scale dynamical processes. Biomass burning activity over the northern Indian region could act as an additional source of ozone precursors during spring. The seasonal ozone photochemical buildup is estimated to be 32–41 ppbv during spring and autumn and 9–14 ppbv during August–September. A correlation analysis between ozone levels at Pantnagar and Nainital along with the mixing depth data suggests that emissions and photochemical processes in the IGP region influence the air quality of pristine Himalayan region, particularly during midday hours of spring. The evening rate of change (8.5 ppbv hr−1) is higher than the morning rate of change, which is dissimilar to those at other urban or rural sites. Ozone seasonality over the IGP region is different than that over southern India. Results from the MATCH‐MPIC model capture observed ozone seasonality but overestimate ozone levels. Model simulated daytime ratios of H2O2/HNO3 are higher and suggesting that this region is in a NOx‐limited regime. A chemical box model (NACR Master Mechanism) is used to further corroborate this using a set of sensitivity simulations, and to estimate the integrated net ozone production in a day (72.9 ppbv) at this site.

WRF-Chem simulated surface ozone over south Asia during the pre-monsoon: effects of emission inventories and chemical mechanisms

“…The accuracy of these measurements is reported to be about 5 % (Kleinmann et al, 1994). The response time of such instruments is about 20 s and instruments have a lower detection limit of 1 ppbv (Ojha et al, 2012). Here, we have used the hourly and monthly average data for the model evaluation.…”

“…It was shown that the anthropogenic emissions and their subsequent photochemical degradation over south Asia influence air quality over the Himalayas (e.g. Ojha et al, 2012;Sarangi et al, 2014) and the Tibetan Plateau (Lüthi et al, 2015) as well as the marine environment downwind of India (e.g. Lawrence and Lelieveld, 2010).…”

“…However, an evaluation of modelled ozone based on data from a network of stations across south Asia is imperative considering very large spatiotemporal heterogeneity in the distribution of ozone over this region (e.g. Kumar et al, 2010;Ojha et al, 2012;Kumar et al, 2012b) mainly resulting from heterogeneous precursor sources and population distribution. WRF-Chem simulated ozone distributions have also been utilized to assess the losses in crop yields, and it was suggested that the estimated crop losses would be sufficient to feed about 94 million people living below the poverty line in this region .…”

“…These results indicate the need for evaluating the effects of emission inventories and chemical mechanisms on the model performance using a network of stations across south Asia, which has not been carried out thus far. We focus on the pre-monsoon season (March-May) for the study, as O 3 mixing ratios at the surface are generally the highest over most of south Asia during this period (Jain et al, 2005;Debaje et al, 2006;Reddy et al, 2010;Ojha et al, 2012;Gaur et al, 2014;Renuka et al, 2014;Bhuyan et al, 2014;Sarangi et al, 2014;Yadav et al, 2014;Sarkar et al, 2015). This is because photochemistry over south Asia is most intense during this season due to the combined effects of high pollution loading, biomass-burning emissions and a lack of precipitation.…”

Science at High-Altitude Sites of ARIES - Astrophysics and Atmospheric Sciences

“…A correlation analysis between ozone levels over a site in the IGP region and Nainital along with the mixing depth data suggests that emissions and photochemical processes in the IGP region influence the air quality of pristine Himalayan region, particularly during midday hours of spring. Ozone seasonality over the IGP region is different than that over southern India (Ojha et al, 2012).…”