Surface ozone in the Doon Valley of the Himalayan foothills during spring.

Elevated ozone (O) pollution is observed every spring over the Northern Indian region including the Himalayan foothills, with a maximum typically in the month of May. However, studies investigating influences of photochemistry and dynamics in the valleys of Central Himalaya are limited. Here, in situ surface O observations conducted at Dehradun (77.99° E, 30.27° N, 600 m above mean sea level) in the Doon Valley during April-July 2018 are presented. These O observations reveal the prevalence of an urban environment over Dehradun with enhanced levels during noontime (66.4 ppbv ± 11.0 ppbv in May) and lower levels during night (26.7 ppbv ± 11.5 ppbv). Morning time O enhancement rate at Dehradun (7.5 ppbv h) is found to be comparable to that at Bode (7.3 ppbv h) in another valley of Himalayan foothills (Kathmandu), indicating stronger anthropogenic emissions in the Doon Valley as well. Daily average O at Dehradun varied in the range of 13.7-71.3 ppbv with hourly values reaching up to 103.1 ppbv during the study period. Besides the in situ photochemical O production, the entrainment of O-rich air through boundary layer dynamics also contributes in noontime O enhancement in the Doon Valley. Monthly average O at Dehradun (49.3 ppbv ± 19.9 ppbv) is observed to be significantly higher than that over urban sites in Northern India (35-41 ppbv) and Bode (38.5 ppbv) in the Kathmandu Valley during May. O photochemical buildup, estimated to be 30.3 ppbv and 39.7 ppbv during April and May, respectively, is significantly lower in June (21.2 ppbv). Copernicus Atmosphere Monitoring Service (CAMS) model simulations successfully reproduce the observed variability in noontime O at Dehradun (r = 0.86); however, absolute O levels were typically overestimated. The positive relationship between CAMS O and CO (r = 0.65) together with an O/CO slope of 0.16 is attributed to the influences of biomass burning besides anthropogenic emissions on observed O variations in the Doon Valley. O observations show an enhancement by 35-56% at Dehradun during a high-fire activity period in May 2018 as compared to a low-fire activity period over the Northern Indian region in agreement with the enhancement found in CAMS O fields (10-65%) over the region in the vicinity of Dehradun.