Atmospheric Brown Clouds in the Himalayas: first two years of continuous observations at the Nepal-Climate Observatory at Pyramid (5079 m)

Abstract

Abstract. South Asia is strongly influenced by the so-called Atmospheric Brown Cloud (ABC), a wide polluted layer extending from the Indian Ocean to the Himalayas during the winter and pre-monsoon seasons (November to April). This thick, grey-brown haze blanket substantially interacts with the incoming solar radiation, causing a cooling of the Earth's surface and a warming of the atmosphere, thus influencing the monsoon system and climate. In this area, the Himalayan region, particularly sensitive to climate change, offers a unique opportunity to detect global change processes and to analyse the influence of anthropogenic pollution on background atmospheric conditions through continuous monitoring activities. This paper provides a detailed description of the atmospheric conditions characterizing the high Himalayas, thanks to continuous observations begun in March 2006 at the Nepal Climate Observatory – Pyramid (NCO-P) located at 5079 m a.s.l. on the southern foothills of Mt. Everest, in the framework of ABC-UNEP and SHARE-Ev-K2-CNR projects. Besides giving an overview of the measurement site and experimental activities, the work presents an in-depth characterization of meteorological conditions and air-mass circulation at NCO-P during the first two years of activity (March 2006–February 2008). The mean values of atmospheric pressure, temperature and wind speed recorded at the site were: 551 hPa, −3.0 °C, 4.7 m s−1, respectively. The highest seasonal values of temperature (1.7 °C) and relative humidity (94%) were registered during the monsoon season, which was also characterized by thick clouds present in about 80% of the afternoon hours and by a frequency of cloud-free sky less than 10%. The lowest temperature and relative humidity values were registered during winter, −6.3 °C and 22%, respectively, the season being characterised by mainly cloud-free sky conditions and rare thick clouds. The summer monsoon influenced the rain precipitation (seasonal mean 237 mm), while wind was dominated by flows from the bottom of the valley (S-SW) and upper mountain (N-NE). In relation to seasonal weather conditions, the time series variability of black carbon and dust particles (optical active aerosols) and ozone (regional greenhouse gas) were analysed, as they are significant constituents of the Atmospheric Brown Cloud and strongly influence the atmospheric radiative forcing. The highest seasonal values of black carbon (BC), ozone (O3) and dust particles were observed during the pre-monsoon season (316.9 ng m−3, 60.9 ppbv, 0.37 cm−3, respectively), while the lowest concentrations occurred during the monsoon for BC and O3 (49.6 ng m−3 and 33.6 ppbv, respectively) and post-monsoon for dust particles (0.07 cm−3). The seasonal cycles of these compounds are influenced both by the local mountain wind system and by the three principal large-scale circulation regimes: Westerly, South-Westerly and Regional, as shown by the analysis of in-situ meteorological parameters and 5-day LAGRANTO back-trajectories. In particular, the analysis of data representative of synoptic-scale circulation showed that the highest median values (O3: 68 ppbv, BC: 124 ng m−3, dust particles: 0.44 cm−3, respectively) were related with air-masses from polluted and arid regions in the Indian subcontinent, as well as the Arabian Peninsula and Persian Gulf. Furthermore, it was documented that in 90% of pre-monsoon days the Khumbu valley represents a "direct channel" able to transport polluted air-masses from the Asian Brown Cloud up to NCO-P and to higher altitudes. On such days the average day-time BC concentration (625 ng m−3) was at least double that recorded on the remaining days, even if during some pollution hot spots BC daily values increased up to 1000 ng m−3. In this study, two years of Himalayan observation activities carried out at NCO-P, in conjunction with model circulation analyses, provide some of the first evidence that polluted air-masses linked to the Atmospheric Brown Cloud can reach the high Himalayas, in particular during the pre-monsoon season, influencing the pristine atmospheric composition.