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

Elevated ozone (O₃) 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 O₃ 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 O₃ 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 O₃ enhancement rate at Dehradun (7.5 ppbv h^-1) is found to be comparable to that at Bode (7.3 ppbv h^-1) in another valley of Himalayan foothills (Kathmandu), indicating stronger anthropogenic emissions in the Doon Valley as well. Daily average O₃ 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 O₃ production, the entrainment of O₃-rich air through boundary layer dynamics also contributes in noontime O₃ enhancement in the Doon Valley. Monthly average O₃ 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. O₃ 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 O₃ at Dehradun (r = 0.86); however, absolute O₃ levels were typically overestimated. The positive relationship between CAMS O₃ and CO (r = 0.65) together with an O₃/CO slope of 0.16 is attributed to the influences of biomass burning besides anthropogenic emissions on observed O₃ variations in the Doon Valley. O₃ 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 O₃ fields (10-65%) over the region in the vicinity of Dehradun.