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This paper reports on daytime total vertical column abundances of formic acid (HCOOH) above the Northern mid-latitude, high altitude Jungfraujoch station (Switzerland; 46.5° N, 8.0° E, 3580 m alt.). The columns were derived from the analysis of infrared solar observations regularly performed with high spectral resolution Fourier transform spectrometers during over 1500 days between September 1985 and September 2007. The investigation was based on the spectrometric fitting of five spectral intervals, one encompassing the HCOOH ν<sub>6</sub> band Q branch at 1105 cm<sup>−1</sup>, and four additional ones allowing to optimally account for critical temperature-sensitive or time-evolving interferences by other atmospheric gases, in particular HDO, CCl<sub>2</sub>F<sub>2</sub> and CHClF<sub>2</sub>. The main results derived from the 22 years long database indicate that the free tropospheric burden of HCOOH above the Jungfraujoch undergoes important short-term daytime variability, diurnal and seasonal modulations, inter-annual anomalies, but no significant long-term background change. <br><br> A major progress in the remote determination of the atmospheric HCOOH columns reported here has resulted from the adoption of new, improved absolute spectral line intensities for the infrared ν<sub>6</sub> band of trans-formic acid, resulting in retrieved free tropospheric loadings being about a factor two smaller than if derived with previous spectroscopic parameters. Implications of this significant change with regard to earlier remote measurements of atmospheric formic acid and comparison with relevant Northern mid-latitude findings, both in situ and remote, will be assessed critically. Sparse HCOOH model predictions will also be evoked and assessed with respect to findings reported here.
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In this study ideas from extreme value theory are for the first time applied in the field of stratospheric ozone research, because statistical analysis showed that previously used concepts assuming a Gaussian distribution (e.g. fixed deviations from mean values) of total ozone data do not adequately address the structure of the extremes. We show that statistical extreme value methods are appropriate to identify ozone extremes and to describe the tails of the Arosa (Switzerland) total ozone time series. In order to accommodate the seasonal cycle in total ozone, a daily moving threshold was determined and used, with tools from extreme value theory, to analyse the frequency of days with extreme low (termed ELOs) and high (termed EHOs) total ozone at Arosa. The analysis shows that the Generalized Pareto Distribution (GPD) provides an appropriate model for the frequency distribution of total ozone above or below a mathematically well-defined threshold, thus providing a statistical description of ELOs and EHOs. The results show an increase in ELOs and a decrease in EHOs during the last decades. The fitted model represents the tails of the total ozone data set with high accuracy over the entire range (including absolute monthly minima and maxima), and enables a precise computation of the frequency distribution of ozone mini-holes (using constant thresholds). Analyzing the tails instead of a small fraction of days below constant thresholds provides deeper insight into the time series properties. Fingerprints of dynamical (e.g. ENSO, NAO) and chemical features (e.g. strong polar vortex ozone loss), and major volcanic eruptions, can be identified in the observed frequency of extreme events throughout the time series. Overall the new approach to analysis of extremes provides more information on time series properties and variability than previous approaches that use only monthly averages and/or mini-holes and mini-highs.
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Climate models contain closure parameters to which the model climate is sensitive. These parameters appear in physical parameterization schemes where some unresolved variables are expressed by predefined parameters rather than being explicitly modeled. Currently, best expert knowledge is used to define the optimal closure parameter values, based on observations, process studies, large eddy simulations, etc. Here, parameter estimation, based on the adaptive Markov chain Monte Carlo (MCMC) method, is applied for estimation of joint posterior probability density of a small number (<i>n</i>=4) of closure parameters appearing in the ECHAM5 climate model. The parameters considered are related to clouds and precipitation and they are sampled by an adaptive random walk process of the MCMC. The parameter probability densities are estimated simultaneously for all parameters, subject to an objective function. Five alternative formulations of the objective function are tested, all related to the net radiative flux at the top of the atmosphere. Conclusions of the closure parameter estimation tests with a low-resolution ECHAM5 climate model indicate that (i) adaptive MCMC is a viable option for parameter estimation in large-scale computational models, and (ii) choice of the objective function is crucial for the identifiability of the parameter distributions.
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Volcanic emissions from the Eyjafjallajökull volcano eruption on the Southern fringe of Iceland in April 2010 were detected at the Global Atmosphere Watch (GAW) station Zugspitze/Hohenpeissenberg (Germany) by means of in-situ measurements, ozone sondes and ceilometers. Information from the German Meteorological Service (DWD) ceilometer network (Flentje et al., 2010) aided identifying the air mass origin. We discuss ground level in-situ measurements of sulphur dioxide (SO<sub>2</sub>), sulphuric acid (H<sub>2</sub>SO<sub>4</sub>) and particulate matter as well as ozone sonde profiles and column measurements of SO<sub>2</sub> by a Brewer spectrometer. At Hohenpeissenberg, a number of reactive gases, e.g. carbon monoxide and nitrogen oxides, and particle properties, e.g. size distribution and ionic composition, were additionally measured during this period. Our results describe the arrival of the volcanic plume at Zugspitze and Hohenpeissenberg during 16 and 17 April 2010 and its residence in the planetary boundary layer (PBL) for several days thereafter. The ash plume was first seen in the ceilometer backscatter profiles at Hohenpeissenberg in about 6–7 km altitude. After entrainment into the PBL at noon of 17 April, largely enhanced values of sulphur dioxide, sulphuric acid and super-micron-particle number concentration were recorded at Zugspitze/Hohenpeissenberg till 21 April.
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For the first time a high mass resolution thermal desorption proton transfer reaction mass spectrometer (hr-TD-PTR-MS) was deployed in the field to analyze the composition of the organic fraction of aerosols. We report on measurements from the remote Mt. Sonnblick observatory in the Austrian alps (3108 m a.s.l.) during a 7 week period in summer 2009. A total of 638 mass peaks in the range 18–392 Da were detected and quantified in aerosols. An empirical formula was tentatively attributed to 464 of these compounds by custom-made data analysis routines which consider compounds containing C, H, O, N, and S atoms. Most of the other (unidentified) compounds must contain other elements – most likely halogenated compounds. The mean total concentration of all detected compounds was 1.1 μg m<sup>−3</sup>. Oxygenated hydrocarbons constitute the bulk of the aerosol mass (75%) followed by organic nitrogen compounds (9%), inorganic compounds (mostly NH<sub>3</sub>, 8%), unidentified/halogenated (3.8%), hydrocarbons (2.7%), and organic sulfur compounds (0.8%). The measured O/C ratios are lower than expected and suggest a significant effect from charring. Organic carbon concentrations measured with TD-PTR-MS were about 25% lower than measurements on high volume filter samples.
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The concept of Ozone Depletion Potentials (ODPs) is extensively used in policy considerations related to concerns about the effects of various halocarbons and other gases on stratospheric ozone. Many of the recent candidate replacement compounds have atmospheric lifetimes shorter than one year in order to limit their environmental effects, especially on stratospheric ozone. Using a three-dimensional global chemistry-transport model (CTM) of the troposphere and the stratosphere, the purpose of this study is to evaluate the potential effects of several very short-lived iodinated substances, namely iodotrifluoromethane (CF<sub>3</sub>I) and methyl iodide (CH<sub>3</sub>I), on atmospheric ozone. Like other chemicals with extremely short lifetimes, the stratospheric halogen loading and resulting ozone effects from these compounds are strongly dependent on the location of emissions. For CF<sub>3</sub>I, a possible replacement candidate for bromotrifluoromethane (CF<sub>3</sub>Br), ODPs derived by the three-dimensional model are 0.008 with chemical lifetime of 5.03 days and 0.016 with a lifetime of 1.13 days for emissions assumed to be evenly distributed over land surfaces at mid-latitudes and the tropics, respectively. While this is the first time the ODPs have been evaluated with a three-dimensional model, these values are in good agreement with those derived previously. The model calculations suggest that tropical convection could deliver a larger portion of the gas and their breakdown products to the upper troposphere and lower stratosphere if emission source is located in the tropics. The resulting ODP for CH<sub>3</sub>I, emitted from mid-latitudes, is 0.017 with lifetime of 13.59 days. Valid simulations of convective transport, vertical mixing and degradation chemistry of CH<sub>3</sub>I are shown that have good qualitative agreement between the model derived distribution of background CH<sub>3</sub>I, based on global source emission fluxes from previous studies, and available observations especially in vertical profiles.
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Collocated data sets of glyoxal (CHO.CHO) and formaldehyde (HCHO) were retrieved for the first time from measurements of the Global Ozone Monitoring Experiment-2 (GOME-2) during the first two years of operation in 2007 and 2008. Both oxygenated Volatile Organic Compounds, OVOC, are key intermediate species produced during the oxidation of precursor hydrocarbons. Their short lifetime of a few hours in the lower troposphere links them to emission sources and makes them useful tracers of photochemical activity. The global composite maps of GOME-2 HCHO and CHO.CHO have strong similarities confirming their common atmospheric and/or surface sources. The highest column amounts of these OVOCs are recorded over regions with enhance biogenic emissions (e.g. tropical forests in South America, Africa and Indonesia). Enhanced OVOC values are also present over areas of anthropogenic activity and biomass burning (e.g. over China, N. America, Europe and Australia). The ratio of CHO.CHO to HCHO, <i>R</i><sub>GF</sub>, has been used, for the first time on a global scale, to classify the sources according to biogenic and/or anthropogenic emissions of the precursors; <i>R</i><sub>GF</sub> between 0.040 to 0.060 point to the existence of biogenic emissions with the highest values being observed at the highest Enhanced Vegetation Index, EVI. <i>R</i><sub>GF</sub>s below 0.040 are indicative of anthropogenic emissions and associated with high levels of NO<sub>2</sub>. This decreasing tendency of <i>R</i><sub>GF</sub> with increasing NO<sub>2</sub> is also observed when analyzing data for individual large cities, indicating that it is a common feature. The results obtained for <i>R</i><sub>GF</sub> from GOME-2 data are compared with the findings based on regional SCIAMACHY observations showing good agreement. This is explained by the excellent correlation of the global retrieved column amounts of CHO.CHO and HCHO from the GOME-2 and SCIAMACHY instruments for the period 2007–2008.
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Surface measurements of aerosol and ice nuclei (IN) at a Central European mountain site during an episode of dust transport from the Sahara are presented. Ice nuclei were sampled by electrostatic precipitation on silicon wafers and were analyzed in an isothermal static vapor diffusion chamber. The transport of mineral dust is simulated by the Eulerian regional dust model DREAM. Ice nuclei and mineral dust are significantly correlated, in particular IN number concentration and aerosol surface area. The ice nucleating characteristics of the aerosol as analyzed with respect to temperature and supersaturation are similar during the dust episode than during the course of the year. This suggests that dust may be a main constituent of ice nucleating aerosols in Central Europe.
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A modelling study of radical chemistry in the coastal Antarctic boundary layer, based upon observations performed in the course of the CHABLIS (Chemistry of the Antarctic Boundary Layer and the Interface with Snow) campaign at Halley Research Station in coastal Antarctica during the austral summer 2004/2005, is described: a detailed zero-dimensional photochemical box model was used, employing inorganic and organic reaction schemes drawn from the Master Chemical Mechanism, with additional halogen (iodine and bromine) reactions added. The model was constrained to observations of long-lived chemical species, measured photolysis frequencies and meteorological parameters, and the simulated levels of HO<sub>x</sub>, NO<sub>x</sub> and XO compared with those observed. The model was able to replicate the mean levels and diurnal variation in the halogen oxides IO and BrO, and to reproduce NO<sub>x</sub> levels and speciation very well. The NO<sub>x</sub> source term implemented compared well with that directly measured in the course of the CHABLIS experiments. The model systematically overestimated OH and HO<sub>2</sub> levels, likely a consequence of the combined effects of (a) estimated physical parameters and (b) uncertainties within the halogen, particularly iodine, chemical scheme. The principal sources of HO<sub>x</sub> radicals were the photolysis and bromine-initiated oxidation of HCHO, together with O(<sup>1</sup>D) + H<sub>2</sub>O. The main sinks for HO<sub>x</sub> were peroxy radical self- and cross-reactions, with the sum of all halogen-mediated HO<sub>x</sub> loss processes accounting for 40% of the total sink. Reactions with the halogen monoxides dominated CH<sub>3</sub>O<sub>2</sub>-HO<sub>2</sub>-OH interconversion, with associated local chemical ozone destruction in place of the ozone production which is associated with radical cycling driven by the analogous NO reactions. The analysis highlights the need for observations of physical parameters such as aerosol surface area and boundary layer structure to constrain such calculations, and the dependence of simulated radical levels and ozone loss rates upon a number of uncertain kinetic and photochemical parameters for iodine species.
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The influence of halogen oxidation on the variabilities of ozone (O<sub>3</sub>) and volatile organic compounds (VOCs) within the Arctic and sub-Arctic atmospheric boundary layer was investigated using field measurements from multiple campaigns conducted in March and April 2008 as part of the POLARCAT project. For the ship-based measurements, a high degree of correlation (<i>r</i> = 0.98 for 544 data points collected north of 68° N) was observed between the acetylene to benzene ratio, used as a marker for chlorine and bromine oxidation, and O<sub>3</sub> signifying the vast influence of halogen oxidation throughout the ice-free regions of the North Atlantic. Concurrent airborne and ground-based measurements in the Alaskan Arctic substantiated this correlation and were used to demonstrate that halogen oxidation influenced O<sub>3</sub> variability throughout the Arctic boundary layer during these springtime studies. Measurements aboard the R/V <i>Knorr</i> in the North Atlantic and Arctic Oceans provided a unique view of the transport of O<sub>3</sub>-poor air masses from the Arctic Basin to latitudes as far south as 52° N. FLEXPART, a Lagrangian transport model, was used to quantitatively determine the exposure of air masses encountered by the ship to first-year ice (FYI), multi-year ice (MYI), and total ICE (FYI+MYI). O<sub>3</sub> anti-correlated with the modeled total ICE tracer (<i>r</i> = −0.86) indicating that up to 73% of the O<sub>3</sub> variability measured in the Arctic marine boundary layer could be related to sea ice exposure.
