Variations in surface ozone and carbon monoxide in the Kathmandu Valley and surrounding broader regions during SusKat-ABC field campaign: Role of local and regional sources
Authors: Bhardwaj Piyush , Naja Manish , Rupakheti Maheswar , Panday Arnico K. , Kumar Rajesh , Mahata Khadak , Lal Shyam , Chandola Harish C. , Lawrence Mark G. 
Published: July 2017

Abstract. Air pollutants emitted from rapid urbanization and associated human activities in the Kathmandu Valley of Nepal over the past two decades are causing serious air quality and health concerns. These concerns led to a multinational field campaign SusKat-ABC (Sustainable atmosphere for the Kathmandu Valley-Atmospheric Brown Clouds) that measured different trace gases, aerosols and meteorological parameters in the Kathmandu Valley and surrounding regions during December 2012 to June 2013 to understand the factors influencing air quality of the Kathmandu Valley. This study provides information about the regional distribution of ozone and some precursor gases using simultaneous in situ measurements from a SusKat-ABC supersite at Bode, Nepal and two Indian sites: a high-altitude site Nainital located in the central Himalayan region and a low altitude site Pantnagar located in the Indo-Gangetic Plain (IGP). The diurnal variations at Bode showed a daytime buildup in O3 while CO shows morning and evening peaks. Similar variations (with lower levels) were also observed at Pantnagar but not at Nainital. Several events of hourly ozone levels exceeding 80 ppbv were also observed at Bode. The CO levels showed a decrease from their peak level of above 2000 ppbv in January to about 680 ppbv in June at Bode. The hourly mean ozone and CO levels showed a strong negative correlation during winter (r2 = 0.82 in January and r2 = 0.71 in February), but this negative correlation gradually becomes weaker, with the lowest value in May (r2 = 0.12). The background O3 and CO mixing ratios at Bode were estimated to be about 14 ppbv and 325 ppbv respectively. The rate of change of ozone at Bode showed a more rapid increase (~ 17 ppbv/hour) during morning than the decrease in the evening (5–6 ppbv/hour), suggesting a semi urban kind of environment at Bode. The slower evening time ozone decrease rates and lower CO levels during spring suggests an important contribution of regional transport than the contribution of local sources to the springtime ozone enhancement in the Kathmandu Valley. The winter time higher ozone levels at Bode are largely due to local sources with relatively less contributions from regional sources. We show that regional pollution resulting from agricultural crop residue burning in north-western IGP led to large (~ 2-fold) increase in O3 and CO levels simultaneously at all three sites, i.e., Bode, Pantnagar and Nainital during first week of May 2013. Biomass burning induced increase in ozone and related gases was also confirmed by a global model and balloon borne observations over Nainital. A comparison of surface ozone variations and composition of light non-methane hydrocarbons among different sites indicated the differences in emission sources of the Kathmandu Valley and the IGP. These results highlight the contribution of regional sources to air pollution in the Kathmandu Valley versus the local sources in the Valley.