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. The work presents a characterization of meteorological conditions and air-mass circulation at NCO-P during the first 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 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 of less than 10%. The lowest temperature and relative humidity seasonal 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 rain precipitation (seasonal mean: 237 mm), while wind was dominated by flows from the bottom of the valley (S&ndash;SW) and upper mountain (N&ndash;NE). <br><br> The atmospheric composition at NCO-P has been studied thanks to measurements of black carbon (BC), aerosol scattering coefficient, PM₁, coarse particles and ozone. The annual behaviour of the measured parameters shows the highest seasonal values during the pre-monsoon (BC: 316.9 ng m⁻³, PM₁: 3.9 μg m⁻³, scattering coefficient: 11.9 Mm⁻¹, coarse particles: 0.37 cm^&minus;3 and O₃: 60.9 ppbv), while the lowest concentrations occurred 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^&minus;3. At NCO-P, the synoptic-scale circulation regimes present three principal contributions: Westerly, South-Westerly and Regional, as shown by the analysis of in-situ meteorological parameters and 5-day LAGRANTO back-trajectories. <br><br> The influence of the brown cloud (AOD>0.4) extending over Indo–Gangetic Plains up to the Himalayan foothills has been evaluated by analysing the in-situ concentrations of the ABC constituents. This analysis revealed that brown cloud 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<sup>...

[1] In this paper we present a study on stratospheric intrusion (SI) events recorded at a high mountain station in the Italian northern Apennines. Six years (1998)(1999)(2000)(2001)(2002)(2003) of surface ozone and beryllium-7 concentration measurements as well as relative humidity values recorded at the GAW Mt. Cimone research station (44°11 0 N, 10°42 0 E; 2165 m asl) were analyzed. Moreover, three-dimensional backward trajectories calculated by the FLEXTRA model and potential vorticity values along these trajectories were used. In order to identify SI and evaluate their contribution to the tropospheric ozone at Mt. Cimone, a statistical methodology was developed. This methodology consists of different selection criteria based on observed and modeled stratospheric tracers as well as on tropopause height values recorded by radio soundings. On average, SI effects affected Mt. Cimone for about 36 days/year. The obtained 6-year SI climatology showed a clear seasonal cycle with a winter maximum and a spring-summer minimum. The seasonal cycle was also characterized by an interannual variation. In particular, during winter (autumn), SI frequency could be related to the intensity of the positive (negative) NAO phase. In order to separate direct SI from indirect SI, a restrictive selection criterion was set. This criterion, named Direct Intrusion Criterion (DIC), requested that all the analyzed tracers were characterized by stratospheric values. Direct SI affected Mt. Cimone for about 6 days/year, with frequency peaks in winter and early summer. At Mt. Cimone, SI contribution to background ozone concentrations was largest in winter. On average, an ozone increase of 8% (3%) with respect to the monthly running mean was found during direct (indirect) SI. Finally, the typical variations of stratospheric tracers during SI events were analyzed. The analysis of in situ atmospheric pressure values suggested that direct SI were connected with intense fronts affecting the region, while indirect SI were possibly connected with subsiding structures related with anticyclonic areas.

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.

Abstract. Its location in the Mediterranean region and its physical characteristics render Mt. Cimone (44 • 11 N, 10 • 42 E), the highest peak of the Italian northern Apennines (2165 m asl), particularly suitable to study the transport of air masses from the north African desert area to Europe. During these northward transports 12 dust events were registered in measurements of the aerosol concentration at the station during the period June-December 2000, allowing the study of the impact of mineral dust transports on free tropospheric ozone concentrations, which were also measured at Mt. Cimone. Three-dimensional backward trajectories were used to determine the air mass origin, while TOMS Aerosol Index data for the Mt. Cimone area were used to confirm the presence of absorbing aerosol over the measurement site.A trajectory statistical analysis allowed identifying the main source areas of ozone and aerosols. The analysis of these back trajectories showed that central Europe and north and central Italy are the major pollution source areas for ozone and fine aerosol, whereas the north African desert regions were the most important source areas for coarse aerosol and low ozone concentrations. During dust events, the Mt. Cimone mean volume concentration for coarse particles was 6.18 µm 3 /cm 3 compared to 0.63 µm 3 /cm 3 in dust-free conditions, while the ozone concentrations were 4% to 21% lower than the monthly mean background values. Our observations show that surface ozone concentrations were lower than the background values in air masses coming from north Africa, and when these air masses were also rich in coarse particles, the lowest ozone values were registered. Moreover, preliminary results on the possible impact of the dust events on PM 10 and ozone values measured in Italian urbanCorrespondence to: P. Bonasoni (p.bonasoni@isac.cnr.it) and rural areas showed that during the greater number of the considered dust events, significant PM 10 increases and ozone decreases have occurred in the Po valley.