High black carbon and ozone concentrations during pollution transport in the Himalayas: five years of continuous observations at NCO-P global GAW station

To study the influence of polluted air-mass transport carrying ozone (O3) and black carbon (BC) in the high Himalayas, since March
2006 the Nepal Climate Observatory at Pyramid (NCO-P) GAW-WMO global station (Nepal, 5079 m a.s.l.) is operative. During the
first 5-year measurements, the O3 and BC concentrations have shown a mean value of 48 ± 12 ppb (± standard deviation) and 208 ±
374 ng/m3, respectively. Both O3 and BC showed well defined seasonal cycles with maxima during pre-monsoon (O3: 61.3 ± 7.7 ppbV;
BC: 444 ± 433 ng/m3) and minima during the summer monsoon (O3: 40.1 ± 12.4 ppbV; BC: 64 ± 101 ng/m3). The analysis of the
days characterised by the presence of a significant BC increase with respect to the typical seasonal cycle identified 156 days affected
by "acute" pollution events, corresponding to 9.1% of the entire data-set. Such events mostly occur in the pre-monsoon period, when
the O3 diurnal variability is strongly related to the transport of polluted air-mass rich on BC. On average, these "acute" pollution events
were characterised by dramatic increases of BC (352%) and O3 (29%) levels compared with the remaining days.