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Abstract. An Analysis of Covariance (ANCOVA) was used to derive the influence of the meteorological variability on the daily maximum ozone concentrations at 12 low-elevation sites north of the Alps in Switzerland during the four seasons in the 1992-2002 period. The afternoon temperature and the morning global radiation were the variables that accounted for most of the meteorological variability in summer and spring, while other variables that can be related to vertical mixing and dilution of primary pollutants (afternoon global radiation, wind speed, stability or day of the week) were more significant in winter. In addition, the number of days after a frontal passage was important to account for ozone build-up in summer and ozone destruction in winter. The statistical model proved to be a robust tool for reducing the impact of the meteorological variability on the ozone concentrations. The explained variance of the model, averaged over all stations, ranged from 60.2% in winter to 71.9% in autumn. The year-to-year variability of the seasonal medians of daily ozone maxima was reduced by 85% in winter, 60% in summer, and 50% in autumn and spring after the meteorological adjustment. For most stations, no significantly negative trends (at the 95% confidence level) of the summer medians of daily O 3 or O x (O 3 +NO 2 ) maxima were found despite the significant reduction in the precursor emissions in Central Europe. However, significant downward trends in the summer 90th percentiles of daily O x maxima were observed at 6 sites in the region around Zürich (on average −0.73 ppb yr −1 for those sites). The lower effect of the titration by NO as a consequence of the reduced emissions could partially explain the significantly positive O 3 trends in the cold seasons (on average 0.69 ppb yr −1 in winter and 0.58 ppb yr −1 in autumn). The increase of O x found for most stations in autumn (on average 0.23 ppb yr −1 ) and winter (on average 0.39 ppb yr −1 ) could be due to increasing European background ozone levCorrespondence to: A. S. H. Prévôt (andre.prevot@psi.ch) els, in agreement with other studies. The statistical model was also able to explain the very high ozone concentrations in summer 2003, the warmest summer in Switzerland for at least ∼150 years. On average, the measured daily ozone maximum was 15 ppb (nearly 29%) higher than in the reference period summer 1992-2002, corresponding to an excess of 5 standard deviations of the summer means of daily ozone maxima in that period.

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Abstract. Net vertical air mass export by thermally driven flows from the atmospheric boundary layer (ABL) to the free troposphere (FT) above deep Alpine valleys was investigated. The vertical export of pollutants above mountainous terrain is presently poorly represented in global chemistry transport models (GCTMs) and needs to be quantified. Air mass budgets were calculated using aircraft observations obtained in deep Alpine valleys. The results show that on average 3 times the valley air mass is exported vertically per day under fair weather conditions. During daytime the type of valleys investigated in this study can act as an efficient "air pump" that transports pollutants upward. The slope wind system within the valley plays an important role in redistributing pollutants. Nitrogen oxide emissions in mountainous regions are efficiently injected into the FT. This could enhance their ozone (O 3 ) production efficiency and thus influences tropospheric pollution budgets. Once lifted to the FT above the Alps pollutants are transported horizontally by the synoptic flow and are subject to European pollution export. Forward trajectory studies show that under fair weather conditions two major pathways for air masses above the Alps dominate. Air masses moving north are mixed throughout the whole tropospheric column and further transported eastward towards Asia. Air masses moving south descend within the subtropical high pressure system above the Mediterranean.

Atmospheric transport processes, relevant to high Alpine sites, were deduced from 2 sets of aerosol records: a 9‐year record from the Jungfraujoch (3454 m) on the northern side of the Swiss Alps and a 2.5‐year record from Colle Gnifetti (4452 m) on the southern side. A classification scheme for synoptic weather types was applied to separate the aerosol data into groups corresponding to different atmospheric transport conditions. For both sites, vertical aerosol transport by thermally driven convection, acting between late spring and late summer, was found to be the dominant transport process. In summer, the thermally driven aerosol transport to both sites caused an increase of the seasonally averaged aerosol concentration between 0800 and 1800 local standard time by a factor of two. Under anticyclonic conditions, when subsidence on a synoptic scale is present, the thermally driven aerosol transport is most pronounced. Therefore, the aerosol determining thermal transport takes place within a synoptic scale vertical motion of opposite direction. Under cyclonic conditions, when lifting on a synoptic scale is present, the thermally driven aerosol transport is nearly absent. In winter, thermally driven convection does not contribute to the aerosol concentrations at both sites. Nevertheless, also in winter statistically significant differences in aerosol concentration were found between cyclonic and anticyclonic weather conditions, which can be attributed to the vertical transport acting on the synoptic scale. These differences in aerosol concentration were small compared to the corresponding differences in summer. Within the weather types, which are dominated by horizontal advection in the Alpine region, the aerosol concentrations are more diffcult to interpret with respect to the effective transport process.

The first nitryl chloride (ClNO 2 ) measurements in the UK were made during the summer 2012 ClearfLo campaign with a chemical ionization mass spectrometer, utilizing an I À ionization scheme.Concentrations of ClNO 2 exceeded detectable limits (11 ppt) every night with a maximum concentration of 724 ppt. A diurnal profile of ClNO 2 peaking between 4 and 5 A.M., decreasing directly after sunrise, was observed. Concentrations of ClNO 2 above the detection limit are generally observed between 8 P.M. and 11 A.M. Different ratios of the production of ClNO 2 :N 2 O 5 were observed throughout with both positive and negative correlations between the two species being reported. The photolysis of ClNO 2 and a box model utilizing the Master Chemical Mechanism modified to include chlorine chemistry was used to calculate Cl atom concentrations. Simultaneous measurements of hydroxyl radicals (OH) using low pressure laser-induced fluorescence and ozone enabled the relative importance of the oxidation of three groups of measured VOCs (alkanes, alkenes, and alkynes) by OH radicals, Cl atoms, and O 3 to be compared. For the day with the maximum calculated Cl atom concentration, Cl atoms in the early morning were the dominant oxidant for alkanes and, over the entire day, contributed 15%, 3%, and 26% toward the oxidation of alkanes, alkenes, and alkynes, respectively.

The Clean Air for London (ClearfLo) project provides integrated measurements of the meteorology, composition, and particulate loading of the urban atmosphere in London, United Kingdom, to improve predictive capability for air quality. METEOROLOGY, AIR QUALITY, AND HEALTH IN LONDONThe ClearfLo Project Economic and Social Affairs 2013). Urban populations are exposed to stressful environmental conditions, such as local and nonlocal pollutants, that cause poor air quality and microclimates that exacerbate heat stress during heat waves. These are projected to increase in a warming climate. Our cities are therefore nexus points for several environmental health stresses that we currently face (Rydin et al. 2012) and the interacting issues around sustainability and human health.The purpose of this paper is to introduce the Clean Air for London (ClearfLo) project, which investigates the atmospheric science that underpins these health stresses, with a particular focus on the urban increment in atmospheric drivers. We focused on three atmospheric drivers of environmental health stress in cities, namely, heat, gas-phase pollutants, and particulate matter (PM). Health stresses from the urban atmospheric environment.Heat waves have an impact on human health. Populations typically display an optimal temperature range at which the (daily or weekly) mortality rate is lowest. Mortality rates rise as temperatures exceed this optimal range (e.g., Rydin et al. 2012). The 2003 European heat wave (Stedman 2004) in combination with air pollution was responsible for more than 2000 excess deaths in the United Kingdom (Johnson et al. 2005). Under a warming climate, the risks posed by heat stress are predicted to increase (Hacker et al. 2005). People living in urban environments are exposed to higher temperatures than in nonurban regions. Thus, heat-related deaths could be higher within urban areas (Mavrogianni et al. 2011). Hence, ClearfLo is concerned with measuring the factors controlling the urban atmospheric boundary layer, that is, the surface energy balance.The World Health Organization (WHO) reported (WHO 2006) that the strongest effects of air quality 779MAY 2015 AMERICAN METEOROLOGICAL SOCIETY | on health are attributable to PM, followed by ozone (O 3 ) and nitrogen dioxide (NO 2 ). A recent report (Guerreiro et al. 2013) indicates that in 2011 up to 88% of the urban population in Europe was exposed to concentrations exceeding the WHO air quality guidelines for PM 10 (defined as particles that pass through a size-selective inlet with a 50% efficiency cutoff at 10-µm aerodynamic diameter, representative of the inhalable fraction). It is estimated that a reduction of PM 10 to the WHO annual-mean guideline of 20 µg m −3 would reduce attributable deaths per year in Europe by 22,000. Further, this would lead to a substantial improvement in the quality of life for millions with a preexisting respiratory or cardiovascular disease (COMEAP 2010).Epidemiological studies consistently demonstrate an association between the PM mass concentr...

Abstract. Fourteen years of meteorological parameters, aerosol variables (absorption and scattering coefficients, aerosol number concentration) and trace gases (CO, NO x , SO 2 ) measured at the Jungfraujoch (JFJ, 3580 m a.s.l.) have been analyzed as a function of different synoptic weather types. The Schüepp synoptic weather type of the Alps (SYNALP) classification from the Alpine Weather Statistics (AWS) was used to define the synoptic meteorology over the whole Swiss region. The seasonal contribution of each synoptic weather type to the aerosol concentration was deduced from the aerosol annual cycles while the planetary boundary layer (PBL) influence was estimated by means of the diurnal cycles. Since aerosols are scavenged by precipitation, the diurnal cycle of the CO concentration was also used to identify polluted air masses. SO 2 and NO x concentrations were used as precursor tracers for new particle formation and growth, respectively. The aerosol optical parameters and number concentration show elevated loadings during advective weather types during the December-March period and for the convective anticyclonic and convective indifferent weather types during the April-September period. This study confirms the consensus view that the JFJ is mainly influenced by the free troposphere during winter and by injection of air parcels from the PBL during summer. A more detailed picture is, however, drawn where the JFJ is completely influenced by free tropospheric air masses in winter during advective weather types and largely influenced by the Correspondence to: M. Collaud Coen (martine.collaud@meteoswiss.ch) PBL also during the night in summer during the subsidence weather type. Between these two extreme situations, the PBL influence at the JFJ depends on both the time of year and the synoptic weather type. The fraction of PBL air transported to the JFJ was estimated by the relative increase of the specific humidity and CO.