Headline: Ozone and some of the precursors at an urban site in the Kathmandu Valley, Nepal: Observations and modeling

An international field campaign SusKat-ABC (a Sustainable atmosphere for the Kathmandu Valley - AtmosphericBrown Clouds), was conducted during Dec 2012 – June 2013 in Kathmandu and surrounding regions to assess theinfluences of local and regional scale emissions in the Kathmandu Valley. Continuous surface based observationsof O3, CO and air sampling for light hydrocarbons were conducted at the supersite Bode (27.59N, 85.39E, 1326amsl), near the center of the valley. The diurnal variations at Bode were typical of a polluted urban site with sharpday time build-up in O3, and CO having higher levels during morning/evening hours. The average early morningCO levels were higher during winter (Jan-Feb, 1250 ppbv) than spring (Mar-May, 1000 ppbv) and reached ashigh as 3000 ppbv. However, daytime O3 levels were slightly higher during spring (62 ppbv) when comparedwith those during winter (54 ppbv). A distinct seasonal change in the diurnal cycle of O3 was observed and thelocal sources within the valley were often supplemented by regional scale pollution. The influence of northernIndian biomass burning was observed during the first week of May, during this period daily averaged O3 and COmixing ratios were about twice as high at Bode and at the Indian sites. WRF-Chem v3.6.1 simulations showed largeday to night variations in meteorological parameters at supersite Bode. The daytime differences in temperature andRH between model and AWS were 1oC and 10% respectively, however large variations (4-8oC and 40-60%respectively) were observed during night time. Model was able to capture winds well over the valley and < 10%differences were observed, however it failed to simulate fog during first few weeks of January over the Kathmanduvalley which might be the reason for large night-time variations in Temperature and RH. O3 and CO also showedlarge differences and showed little to no improvement with increasing model resolution. The day and night-timedifferences in O3 were 25 and 50ppb respectively. NO in the model was very low 0.2 ppb. Similar differenceswere also calculated for a mountain top Nagarkot (1898m amsl, east of Bode) and model performed very well therewith differences in temperature and RH during day and night were < 2 oC and < 15% respectively. Detailed resultswill be presented during the meeting.