F. Calzolari

High frequency new particle formation in the Himalayas

Rising air pollution levels in South Asia will have worldwide environmental consequences. Transport of pollutants from the densely populated regions of India, Pakistan, China, and Nepal to the Himalayas may lead to substantial radiative forcing in South Asia with potential effects on the monsoon circulation and, hence, on regional climate and hydrological cycles, as well as to dramatic impacts on glacier retreat. An improved description of particulate sources is needed to constrain the simulation of future regional climate changes. Here, the first evidence of very frequent new particle formation events occurring up to high altitudes is presented. A 16-month record of aerosol size distribution from the Nepal Climate Observatory at Pyramid (Nepal, 5,079 m above sea level), the highest atmospheric research station, is shown. Aerosol concentrations are driven by intense ultrafine particle events occurring on >35% of the days at the interface between clean tropospheric air and the more polluted air rising from the valleys. During a pilot study, we observed a significant increase of ion cluster concentrations with the onset of new particle formation events. The ion clusters rapidly grew to a 10-nm size within a few hours, confirming, thus, that in situ nucleation takes place up to high altitudes. The initiation of the new particle events coincides with the shift from free tropospheric downslope winds to thermal upslope winds from the valley in the morning hours. The new particle formation events represent a very significant additional source of particles possibly injected into the free troposphere by thermal winds.

Background ozone variations at Mt. Cimone Station

In order to evaluate the background ozone concentration at Mt. Cimone (2165 m a.s.l.), the contribution of air masses characterised by different provenances is analysed in this paper. The analysis method is based on identification of background air masses which travelled above the 780 hPa pressure level for at least 48 h before arriving at Mt. Cimone. Not being recently mixed with boundary layer air, these air masses are characterised by a chemical age greater than 2 days. This analysis has shown that under background conditions the yearly principal maximum of ozone is recorded in spring and a secondary maximum is recorded in summer. In contrast, if we consider non-background conditions, the principal maximum is found in late summer and a secondary one in spring. In addition, the analysis indicates the presence of a smooth latitudinal gradient of background ozone concentrations in air masses arriving at Mt. Cimone, with higher concentrations coming from the north and lower ones from the south.

A 6‐year analysis of stratospheric intrusions and their influence on ozone at Mt. Cimone (2165 m above sea level)

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.

Continuous CO and H2 measurements at Mt. Cimone (Italy): Preliminary results

Abstract Continuous measurements of atmospheric CO and H2 were carried out in situ at Mt. Cimone, northern Italy (44°12′N, 10°42′E, 2165 m a.s.l.), from May 1994 to date. The preliminary results (1994) are given in this article. Owing to the infrequent exposure to the influence of urban and industrial pollution and in spite of the continental location of the station, low CO and H2 concentrations (means of 164 for CO and of 582 ppbv for H2) were observed over most of the time. Their order of magnitude is comparable to what was found in other similar locations of the northern hemisphere, at similar latitudes. A few examples of different conditions, like pollution from the Po valley (CO increase) or transportation of air masses from the Sahara (CO decrease), are given. CO and H2 concentrations show different types of correlation with other chemical species (CO2, O3, used also as tracers) measured routinely at Mt. Cimone, as well as with meteorological and physical parameters.

Transport of short-lived climate forcers/ pollutants (SLCF/P) to the Himalayas during the South Asian summer monsoon onset

Over the course of six years (2006–2011), equivalent black carbon (eqBC), coarse aerosol mass (PM1–10), and surface ozone (O3), observed during the monsoon onset period at the Nepal Climate Observatory–Pyramid WMO/GAW Global Station (NCO-P, 5079 m a.s.l.), were analyzed to investigate events characterized by a significant increase in these short-lived climate forcers/pollutants (SLCF/P). These events occurred during periods characterized by low (or nearly absent) rain precipitation in the central Himalayas, and they appeared to be related to weakening stages (or ‘breaking’) of the South Asian summer monsoon system. As revealed by the combined analysis of atmospheric circulation, air-mass three-dimensional back trajectories, and satellite measurements of atmospheric aerosol loading, surface open fire, and tropospheric NOx, the large amount of SLCF/P reaching the NCO-P appeared to be related to natural (mineral dust) and anthropogenic emissions occurring within the PBL of central Pakistan (i.e., Thar Desert), the Northwestern Indo-Gangetic plain, and the Himalayan foothills. The systematic occurrence of these events appeared to represent the most important source of SLCF/P inputs into the central Himalayas during the summer monsoon onset period, with possible important implications for the regional climate and for hydrological cycles.

Influence of lower stratosphere/upper troposphere transport events on surface ozone at the Everest-Pyramid GAW Station (Nepal): first year of analysis

In this work, we present the first systematic identification of episodes of air mass transport from the lower stratosphere/upper troposphere (LS/UT) in the middle troposphere of the southern Himalayas. For this purpose, we developed an algorithm to detect LS/UT transport events on a daily basis at the Everest-Pyramid GAW station (EV-PYR, 5079 m a.s.l., Nepal). In particular, in situ surface ozone and atmospheric pressure variations as well as total ozone values from OMI satellite measurements have been analysed. Further insight is gained from three-dimensional backward trajectories and potential vorticity calculated with the LAGRANTO model. According to the algorithm outputs, 9.0% of the considered data set (365 days from March 2006 to February 2007) was influenced by this class of phenomena with a maximum of frequency during dry and pre-monsoon seasons. During 25 days of LS/UT transport events for which any influence of anthropogenic pollution was excluded, the daily ozone mixing ratio increased by 9.3% compared to the seasonal values. This indicates that under favourable conditions, downward air mass transport from the LS/UT can play a considerable role in determining the concentrations of surface ozone in the southern Himalayas.

Five-year analysis of background carbon dioxide and ozone variations during summer seasons at the Mario Zucchelli station (Antarctica)

Thework focuses on the analysis ofCO2 andO3 surface variations observed during five summer experimental campaigns carried out at the ‘Icaro Camp’ clean air facility (74.7◦S, 164.1◦E, 41 m a.s.l.) of the ‘Mario Zucchelli’ Italian coastal research station. This experimental activity allowed the definition of summer average background O3 values that ranged from 18.3 ± 4.7 ppbv (summer 2005–2006) to 21.3 ± 4.0 ppbv (summer 2003–2004). Background CO2 concentrations showed an average growth rate of 2.10 ppmv yr-1, with the highest CO2 increase between the summer campaigns 2002–2003 and 2001–2002 (+2.85 ppmv yr-1), probably reflecting the influence of the 2002/2003 ENSO event. A comparison with other Antarctic coastal sites suggested that the summer background CO2 and O3 at MZS-IC are well representative of the average conditions of the Ross Sea coastal regions. As shown by the analysis of local wind direction and by 3-D back-trajectory calculations, the highest CO2 and O3 values were recorded in correspondence to air masses flowing from the interior of the Antarctic continent. These results suggest that air mass transport from the interior of the continent exerts an important influence on air mass composition in Antarctic coastal areas.