Seasonal variation of short-lived climate forcers/pollutants at Paknajol, an urban area in the city of Kathmandu, Nepal

Several regions of the world are defined as "hot spots" in terms of air pollution; among these, the Kathmandu Valley in South Asia is currently facing severe air quality problems, due to rapid urbanization processes, dramatic land use changes, socioeconomic transformation and high population growth. In this work, we analysed one full year (February 2013 - February 2014) of short-lived climate forcers/pollutants (SLCF/P) measurements, like ozone and equivalent black carbon, at Paknajol (27°43'4" N, 85°18'32" E, 1380 m), a site located in the city of Kathmandu. These observations were carried out in the framework of the SusKat-ABC (A Sustainable Atmosphere for the Kathmandu Valley) field campaign. Equivalent BC (eqBC), surface ozone (O3), aerosol number concentration and mass presented seasonal cycles with maximum values in winter (18.3 ± 14.1 ?g/m3 for eqBC) and pre-monsoon seasons (668.1 ± 382.9 cm-3 and 4.2 ± 2.5 cm-3, for accumulation and coarse fractions; 38.0 ± 25.6 nmol/mol for O3), while the lowest concentrations and mixing ratios have been found during the monsoon season (winter for O3). The diurnal behavior of eqBC and aerosol number concentration indicated that local pollution sources represent the major contribution to air pollution in Kathmandu. Concerning O3, the analysis of seasonal diurnal cycle and correlation with meteorological parameters and aerosol properties suggested a significant role of photochemistry, planetary boundary layer (PBL) dynamic and wind breeze in determining its variability. Especially during pre-monsoon, high O3 values were observed during the afternoon. We suggest that mixing and vertical entrainment processes between upper layers (residual layers of free troposphere) and PBL could partially explain the occurrence of these high values. The possible impact on SLCF/P variability by open major fire emissions occurring at regional scale has been assessed by analysing MODIS fire distribution. The analysis of large-scale atmospheric circulation demonstrated a significant impact of the "background" synoptic-scale scenarios on diurnal cycles of eqBc and O3 at Kathmandu. In particular, atmospheric circulation related to westerly and regional circulations appeared to be especially conducive for the occurrence of the high eqBC and O3 values; all of this information may be useful for implementing control measures to mitigate occurrence of acute pollution level in the Kathmandu municipality.