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

摘要

This paper provides a detailed description of the atmospheric conditionscharacterizing the high Himalayas, thanks to continuous observations begunin March 2006 at the Nepal Climate Observatory-Pyramid (NCO-P) located at5079 m a.s.l. on the southern foothills of Mt. Everest, in the framework ofABC-UNEP and SHARE-Ev-K2-CNR projects. The work presents a characterizationof meteorological conditions and air-mass circulation at NCO-P during thefirst two years of activity. 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 themonsoon season, which was also characterized by thick clouds, present inabout 80% of the afternoon hours, and by a frequency of cloud-free sky ofless than 10%. The lowest temperature and relative humidity seasonalvalues were registered during winter, ?6.3 °C and 22%, respectively,the season being characterised by mainly cloud-free sky conditions and rarethick clouds. The summer monsoon influenced rain precipitation (seasonalmean: 237 mm), while wind was dominated by flows from the bottom of thevalley (S–SW) and upper mountain (N–NE).The atmospheric composition at NCO-P has been studied thanks to measurementsof black carbon (BC), aerosol scattering coefficient, PM₁, coarse particlesand ozone. The annual behaviour of the measured parameters shows the highestseasonal values during the pre-monsoon (BC: 316.9 ng m?3, PM₁: 3.9 μg m?3, scattering coefficient: 11.9 Mm?1, coarseparticles: 0.37 cm−3 and O₃: 60.9 ppbv), while the lowest concentrationsoccurred during the monsoon (BC: 49.6 ng m?3, PM₁: 0.6 μg m?3, scattering coefficient: 2.2 Mm?1, and O₃: 38.9 ppbv)and, for coarse particles, during the post-monsoon (0.07 cm−3. AtNCO-P, the synoptic-scale circulation regimes present three principalcontributions: Westerly, South-Westerly and Regional, as shown by theanalysis of in-situ meteorological parameters and 5-day LAGRANTO back-trajectories.The influence of the brown cloud (AOD>0.4) extending over Indo–GangeticPlains up to the Himalayan foothills has been evaluated by analysing thein-situ concentrations of the ABC constituents. This analysis revealed that browncloud hot spots mainly influence the South Himalayas during the pre-monsoon,in the presence of very high levels of atmospheric compounds (BC: 1974.1 ng m?3, PM₁: 23.5 μg m?3,scattering coefficient: 57.7 Mm?1, coarse particles: 0.64 cm?3, O₃: 69.2 ppbv,respectively). During this season 20% of the days were characterised by astrong brown cloud influence during the afternoon, leading to a 5-foldincreased in the BC and PM₁ values, in comparison with seasonal means. Ourinvestigations provide clear evidence that, especially during thepre-monsoon, the southern side of the high Himalayan valleys represent a"direct channel" able to transport brown cloud pollutants up to 5000 m a.s.l., where the pristine atmospheric composition can be stronglyinfluenced.