Paolo Bonasoni

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.

Saharan dust and daily mortality in Emilia-Romagna (Italy)

Objective To investigate the association between Saharan dust outbreaks and natural, cardiovascular and respiratory mortality. Methods A case–crossover design was adopted to assess the effects of Saharan dust days (SDD) on mortality in the Emilia-Romagna region of Italy. The population under study consisted of residents in the six main towns of the central-western part of the region who died between August 2002 and December 2006. The association of Saharan dust outbreaks and PM10 concentration with mortality was estimated using conditional logistic regression, adjusted for apparent temperature, holidays, summer population decrease, flu epidemic weeks and heat wave days. The role of the interaction term between PM10 and SDD was analysed to test for effect modification induced by SDD on the PM10-mortality concentration–response function. Separate estimates were undertaken for hot and cold seasons. Results We found some evidence of increased respiratory mortality for people aged 75 or older on SDD. Respiratory mortality increased by 22.0% (95% CI 4.0% to 43.1%) on the SDD in the whole year model and by 33.9% (8.4% to 65.4%) in the hot season model. Effects substantially attenuated for natural and cardiovascular mortality with ORs of 1.042 (95% CI 0.992 to 1.095) and 1.043 (95% CI 0.969 to 1.122), respectively. Conclusions Our findings suggest an association between respiratory mortality in the elderly and Saharan dust outbreaks. We found no evidence of an effect modification of dust events on the concentration–response relationship between PM10 and daily deaths. Further work should be carried out to clarify the mechanism of action.

First comparison between ground‐based and satellite‐borne measurements of tropospheric nitrogen dioxide in the Po basin

in the Mount Cimone area is good (R 2 = 0.9) with the mixing properties of the atmosphere being the most important parameter for a valid comparison of the measurements. However, even when the atmospheric mixing properties are optimal for comparison, the ratio between GOME and ground-based tropospheric column data may not be unity. It is demonstrated that the values obtained (less than 1) are related to the fraction of the satellite ground pixel occupied by the NO2 hot spot. INDEX TERMS: 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; KEYWORDS: tropospheric NO2, satellite validation, Po basin

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.

Intercomparison of UV/visible spectrometers for measurements of stratospheric NO2 for the Network for the Detection of Stratospheric Change

During the period May 12–23, 1992, seven groups from seven countries met in Lauder, New Zealand, to intercompare their remote sensing instruments for the measurement of atmospheric column NO2 from the surface. The purpose of the intercomparison was to determine the degree of intercomparability and to qualify instruments for use in the Network for the Detection of Stratospheric Change (NDSC). Three of the instruments which took part in the intercomparison are slated for deployment at primary NDSC sites. All instruments were successful in obtaining slant column NO2 amounts at sunrise and sunset on most of the 12 days of the intercomparison. The group as a whole was able to make measurements of the 90° solar zenith angle slant path NO2 column amount that agreed to about ±10% most of the time; however, the sensitivity of the individual measurements varied considerably. Part of the sensitivity problem for these measurements is the result of instrumentation, and part is related to the data analysis algorithms used. All groups learned a great deal from the intercomparison and improved their results considerably as a result of this exercise.

Background ozone in the southern Europe and Mediterranean area: Influence of the transport processes

The troposphere is subject to continuous inputs, production and removal processes of ozone and its precursors from natural processes and human activities acting together within a very complex system. In order to assess the behaviour of background ozone in the Mediterranean area, a description of trends, seasonal and diurnal behaviours of free tropospheric ozone is provided. In the Mediterranean area and southern Europe the background tropospheric ozone concentration appears significantly affected by three main air mass transport processes: (i) transport of polluted air masses on regional and long-range scales, (ii) downward transport of stratospheric air masses, and (iii) transport of mineral dust from the Sahara desert. In this review of the literature of the last two decades, we present an overview of these phenomena, mainly monitored at high baseline mountain stations representative of background atmospheric conditions.

Ten years of continuous observations of stratospheric ozone depleting gases at Monte Cimone (Italy)--comments on the effectiveness of the Montreal Protocol from a regional perspective.

Halogenated gases potentially harmful to the stratospheric ozone layer are monitored worldwide in order to assess compliance with the Montreal Protocol requiring a phase out of these compounds on a global scale. We present the results of long term (2002-2011) continuous observation conducted at the Mt. Cimone GAW Global Station located on the highest peak of the Italian Northern Apennines, at the border of two important regions: the Po Valley (and the Alps) to the North and the Mediterranean Basin to the South. Bi-hourly air samples of CFC-12, CFC-11, CFC-114, CFC-115, H-1211, H-1301, methyl chloroform, carbon tetrachloride, HCFC-22, HCFC-142b, HCFC-124 and methyl bromide are collected and analysed using a gas chromatograph-mass spectrometer, providing multi annual time series. In order to appreciate the effectiveness of the Montreal Protocol from a regional perspective, trends and annual growth rates of halogenated species have been calculated after identification of their baseline values. A comparison with results from other international observation programmes is also presented. Our data show that the peak in the atmospheric mixing ratios of four chlorofluorocarbons, two halons and two chlorocarbons has been reached and all these species now show a negative atmospheric trend. Pollution episodes are still occurring for species like halon-1211, methyl chloroform and carbon tetrachloride, indicating fresh emissions from the site domain which could be ascribed both to fugitive un-reported uses of the compounds and/or emissions from banks. For the hydrofluorocarbons changes in the baseline are affected by emissions from fast developing Countries in East Asia. Fresh emissions from the site domain are clearly declining. Methyl bromide, for which the Mediterranean area is an important source region, shows, in a generally decreasing trend, an emission pattern that is not consistent with the phase-out schedule of this compound, with a renewed increase in the last two years of pollution episodes.

Transport of Stratospheric Air Masses to the Nepal Climate Observatory–Pyramid (Himalaya; 5079 m MSL): A Synoptic-Scale Investigation

AbstractThis work analyzes and classifies stratospheric airmass transport events (ST) detected at the Nepal Climate Observatory–Pyramid (NCO-P; 27°57′N, 86°48′E, 5079 m MSL) Global Atmospheric Watch–World Meteorological Organization station from March 2006 to February 2008. For this purpose, in situ ozone (O3), meteorological parameters (atmospheric pressure and relative humidity), and black carbon (BC) are analyzed. The paper describes the synoptic-scale meteorological scenarios that are able to favor the development of ST over the southern Himalaya, by analyzing the meteorological fields provided by the ECMWF model (geopotential height, wind speed, and potential vorticity), satellite Ozone Monitoring Instrument data (total column ozone), and three-dimensional back trajectories calculated with the Lagrangian Analysis Tool (LAGRANTO) model. The study, which represents the first “continuous” classification of ST in the southern Himalaya, permitted classification of 94% of ST days within four synoptic-scale...

Light absorption properties of brown carbon in the high Himalayas

The light-absorbing properties of water-soluble brown carbon (WS-BrC) and methanol-soluble brown carbon (MeS-BrC) were studied in PM10 aerosols collected at the “Nepal Climate Observatory-Pyramid” (NCO-P) station (5079 m above sea level) during the period 2013–2014. The light absorption coefficients of WS-BrC and MeS-BrC were the highest during the premonsoon season and the lowest during monsoon. MeS-BrC absorbs about 2 times higher at 365 nm and about 3 times more at 550 nm compared to WS-BrC. The mass absorption cross section (MAC) of WS-BrC measured at 365 nm is similar to that observed previously at South Asian low-altitude sites. Fractional solar radiation absorption by BrC compared to BC considering the full solar spectrum showed that WS-BrC absorbs 4 ± 1% and MeS-BrC absorbs 9 ± 2% compared to BC at NCO-P. Such ratios become 8 ± 1% (for WS-BrC respect to BC) and 17 ± 5% (for MeS-BrC respect to BC) when accounting for correction factors proposed by previous studies to convert absorption coefficients in bulk solutions into light absorption by accumulation mode aerosol particles. These results confirm the importance of BrC in contributing to light-absorbing aerosols in this region of the world. However, the BrC absorption at 550 nm appears small compared to that of BC (1–5%, or 3–9% with conversion factors), and it is lower compared to global model estimates constrained by Aerosol Robotic Network observations. Finally, our study provides no clear evidence of a change in the fractional contribution of BrC with respect to BC to light absorption in the middle troposphere respect to the Indo-Gangetic plain boundary layer.