Air Masses Of Different Origin Research Articles

Airborne observations of peroxy radicals during the EMeRGe campaign in Europe

Abstract. In this study, airborne measurements of the sum of hydroperoxyl (HO2) and organic peroxy (RO2) radicals that react with nitrogen monoxide (NO) to produce nitrogen dioxide (NO2), coupled with actinometry and other key trace gases measurements, have been used to test the current understanding of the fast photochemistry in the outflow of major population centres. The measurements were made during the airborne campaign of the EMeRGe (Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales) project in Europe on board the High Altitude and Long Range Research Aircraft (HALO). The measurements of RO2∗ on HALO were made using the in situ instrument Peroxy Radical Chemical Enhancement and Absorption Spectrometer (PeRCEAS). RO2∗ is to a good approximation the sum of peroxy radicals reacting with NO to produce NO2. RO2∗ mixing ratios up to 120 pptv were observed in air masses of different origins and composition under different local actinometric conditions during seven HALO research flights in July 2017 over Europe. Radical production rates were estimated using knowledge of the photolysis frequencies and the RO2∗ precursor concentrations measured on board, as well as the relevant rate coefficients. Generally, high RO2∗ concentrations were measured in air masses with high production rates. In the air masses investigated, RO2∗ is primarily produced by the reaction of O1D with water vapour and the photolysis of nitrous acid (HONO) and of the oxygenated volatile organic compounds (OVOCs, e.g. formaldehyde (HCHO) and glyoxal (CHOCHO)). Due to their short lifetime in most environments, the RO2∗ concentrations are expected to be in a photostationary steady state (PSS), i.e. a balance between production and loss rates is assumed. The RO2∗ production and loss rates and the suitability of PSS assumptions to estimate the RO2∗ mixing ratios and variability during the airborne observations are discussed. The PSS assumption for RO2∗ is considered robust enough to calculate RO2∗ mixing ratios for most conditions encountered in the air masses measured. The similarities and discrepancies between measured and PSS calculated RO2∗ mixing ratios are discussed. The dominant terminating processes for RO2∗ in the pollution plumes measured up to 2000 m are the formation of nitrous acid, nitric acid, and organic nitrates. Above 2000 m, HO2–HO2 and HO2–RO2 reactions dominate the RO2∗ removal. RO2∗ calculations by the PSS analytical expression inside the pollution plumes probed often underestimated the measurements. The underestimation is attributed to the limitations of the PSS equation used for the analysis. In particular, this expression does not account for the yields of RO2∗ from the oxidation and photolysis of volatile organic compounds, VOCs, and OVOCs other than those measured during the EMeRGe research flights in Europe. In air masses with NO mixing ratios ≤50 pptv and low VOC/NO ratios, the RO2∗ measured is overestimated by the analytical expression. This may be caused by the formation of H2O and O2 from OH and HO2, being about 4 times faster than the rate of the OH oxidation reaction of the dominant OVOCs considered.

Aerial transport of bacteria by dust plumes in the Eastern Mediterranean revealed by complementary rRNA/rRNA-gene sequencing

Processes influencing the transport of airborne bacterial communities in the atmosphere are poorly understood. Here, we report comprehensive and quantitative evidence of the key factors influencing the transport of airborne bacterial communities by dust plumes in the Eastern Mediterranean. We extracted DNA and RNA from size-resolved aerosols sampled from air masses of different origins, followed by qPCR and high-throughput amplicon sequencing of 16 S ribosomal RNA gene and transcripts. We find that airborne bacterial community composition varied with air mass origin and particle size. Bacterial abundance, alpha diversity and species richness were higher in terrestrially influenced air masses than in marine-influenced air masses and higher in the coarse particle fraction (3.0 to 10.0 µm) than in the fine fraction (0.49 to 1.5 µm). This suggests that airborne bacteria mainly were associated with dust particles or transported as cell aggregates. High abundances of rRNA from human, animal and plant pathogen taxa indicate potential ecological impacts of atmospheric bacterial transport.

Air quality modelling over the Eastern Mediterranean: Seasonal sensitivity to anthropogenic emissions

We employ the online coupled WRF/Chem model to study air pollution over the eastern Mediterranean during winter and summer. We utilize three nested domains with horizontal resolutions down to 2 km, over the area of interest. Dust, sea-salt, and biogenic emissions are calculated online, while anthropogenic emissions are based on the EDGAR-HTAP global emission inventory. A new, up-to-date and high spatiotemporal resolution anthropogenic emission inventory for the high-resolution domain has been implemented. Its impact on the model skill to simulate the concentrations of atmospheric pollutants at background and urban sites over Cyprus, being strongly affected by polluted air masses of different origin and composition over the year, is examined. The model output from three simulations over the innermost domain using the EDGAR emission inventory, the new emission inventory, and zero emissions is compared with measurements from background and urban ground stations in Cyprus. The implementation of the updated emission inventory results in about 5% reduction in the normalized mean bias between the modelled and observed CO mixing ratios. Underestimation in wintertime CO mixing ratios and PM2.5 concentrations is attributed to missing residential heating sources from the emission inventory, while the absence of a PM2.5 re-suspension mechanism leads to underestimation in PM2.5 concentrations during summer. The normalized mean bias between the modelled and observed NOx mixing ratios at the urban sites is reduced from −67% to −29% and from −51% to −10% for the winter and summer, respectively. In line with this, the overestimation in O3 mixing ratios was reduced from 45% to 28% during the winter and from 25% to 19% during summer. Taking into account the diurnal variability in the emission inventory is found to be crucial for the simulation of the daily profiles of NOx and O3 at urban sites, which is important both for policy making and air quality modelling.

Composite analysis of the tropopause inversion layer in extratropical baroclinic waves

Abstract. The evolution of the tropopause inversion layer (TIL) during cyclogenesis in the North Atlantic storm track is investigated using operational meteorological analysis data (Integrated Forecast System from the European Centre for Medium-Range Weather Forecasts). For this a total of 130 cyclones have been analysed during the months August through October between 2010 and 2014 over the North Atlantic. Their paths of migration along with associated flow features in the upper troposphere and lower stratosphere (UTLS) have been tracked based on the mean sea level pressure field. Subsets of the 130 cyclones have been used for composite analysis using minimum sea level pressure to filter the cyclones based on their strength. The composite structure of the TIL strength distribution in connection with the overall UTLS flow strongly resembles the structure of the individual cyclones. Key results are that a strong dipole in TIL strength forms in regions of cyclonic wrap-up of UTLS air masses of different origin and isentropic potential vorticity. These air masses are associated with the cyclonic rotation of the underlying cyclones. The maximum values of enhanced static stability above the tropopause occur north and northeast of the cyclone centre, vertically aligned with outflow regions of strong updraft and cloud formation up to the tropopause, which are situated in anticyclonic flow patterns in the upper troposphere. These regions are co-located with a maximum of vertical shear of the horizontal wind. The strong wind shear within the TIL results in a local minimum of Richardson numbers, representing the possibility for turbulent instability and potential mixing (or air mass exchange) within regions of enhanced static stability in the lowermost stratosphere.

Climatology of aerosol properties and clear‐sky shortwave radiative effects using Lidar and Sun photometer observations in the Dakar site

AbstractThis paper presents the analysis of nearly a decade of continuous aerosol observations performed at the Mbour site (Senegal) with Sun photometer, Lidar, and Tapered Electromagnetic Oscillating Microbalance. This site is influenced all year‐round by desert dust and sporadically, in wintertime, by biomass burning particles. Different patterns are revealed for winter and summer, seasons associated to air masses of different origin. The summer (wet season) is characterized by a high aerosol loading (optical thickness, AOT, around 0.57 at 532 nm) composed of large and weakly absorbing particles (Angstrom exponent, α, of 0.23 and single‐scattering albedo, ϖ0, of 0.94 at 532 nm). A lower aerosol loading (AOT = 0.32) is observed during winter (dry season) for finer and absorbing particles (α = 0.48 and ϖ0 = 0.87) revealing the presence of biomass burning aerosols and a greater proportion of local emissions. This latter anthropogenic contribution is visible at weekly and daily scales through AOT cycles. A decrease of about 30% in AOT has been featured in autumn since 2003. The derivation of the extinction profiles highlights a dust transport close to the ground during winter and in an aloft layer (up to 5 km) during summer. Accurate calculations of the daily aerosol radiative effect in clear‐sky conditions are finally addressed. From spring to winter, seasonal shortwave radiative forcing averages of 14.15, 11.15, 8.92, and 12.06 W m−2 have been found respectively. Up to 38% of the solar clear‐sky atmospheric heating can be attributed to the aerosols in this site.

Submicron NE Atlantic marine aerosol chemical composition and abundance: Seasonal trends and air mass categorization

Three years of continuous Aerosol Mass Spectrometry measurements at the Mace Head Global Atmosphere Watch research station revealed seasonal patterns in the chemical composition of submicron NE Atlantic marine aerosol as well as distinct chemical signatures associated with marine air masses of different origin (i.e., polar, Arctic, or tropical). Concentrations of secondary inorganic aerosol species and both primary and secondary organic compounds were closely related to oceanic biological activity and ranged from low median mass concentrations during winter to high median values during summer as follows: 0.025–0.9 µg m−3 for nonsea-salt sulfate (nss-sulfate), 0.025–0.4 µg m−3 for organic matter, 0–0.09 µg m−3 for methanesulfonic acid (MSA). Sea-salt concentrations illustrated an opposite pattern with the highest median value being observed during winter (0.74 µg m−3) and lowest during summer (0.08 µg m−3). Maritime polar air masses typically featured the highest concentrations of sea salt and marine organics, particularly enhanced under primary organic plumes during periods of high biological activity. MSA and nss-sulfate were more prominent in tropical air masses. The oxidation of organic matter increased with increasing ozone concentration and wintertime (low biological activity) organic matter displayed a different fragmentation pattern from that of summertime organic compounds.

Mercury in precipitation at an urbanized coastal zone of the Baltic Sea (Poland).

Wet deposition is an important source of metals to the sea. The temporal variability of Hg concentrations in precipitation, and the impact of air masses of different origins over the Polish coastal zone were assessed. Samples of precipitation were collected (August 2008–May 2009) at an urbanized coastal station in Poland. Hg analyses were conducted using CVAFS. These were the first measurements of Hg concentration in precipitation obtained in the Polish coastal zone. Since Poland was identified as the biggest emitter of Hg to the Baltic, these data are very important. In the heating and non-heating season, Hg concentrations in precipitation were similar. Hg wet deposition flux dominated in summer, when the production of biomass in the aquatic system was able to actively adsorb Hg. Input of metal to the sea was attributed to regional and distant sources. Maritime air masses, through transformation of Hg(0), were an essential vector of mercury in precipitation.

Chemical composition and hygroscopic properties of aerosol particles over the Aegean Sea

Abstract. The chemical composition and water uptake characteristics of sub-micrometre atmospheric particles over the region of the Aegean Sea were measured between 25 August and 11 September 2011 within the framework of the Aegean-Game campaign. High temporal-resolution measurements of the chemical composition of the particles were conducted using an airborne compact time-of-flight aerosol mass spectrometer (cToF-AMS). These measurements were performed during two flights from the island of Crete to the island of Lemnos and back. A hygroscopic tandem differential mobility analyser (HTDMA) located on the island of Lemnos was used to measure the ability of the particles to take up water. The HTDMA measurements showed that the particles in the dominant mode were internally mixed, having hygroscopic growth factors that ranged from 1.00 to 1.59 when exposed to 85% relative humidity. When the aircraft flew near the ground station on Lemnos, the cToF-AMS measurements showed that the organic volume fraction of the particles ranged from 43 to 56%. These measurements corroborate the range of hygroscopic growth factors measured by the HTDMA during that time. Good closure between HTDMA and cToF-AMS measurements was achieved when assuming that the organic species were less hygroscopic and had an average density that corresponds to aged organic species. Using the results from the closure study, the cToF-AMS measurements were employed to determine vertical profiles of a representative aerosol hygroscopic parameter κmix. Calculated κmix values ranged from 0.19 to 0.84 during the first flight and from 0.22 to 0.80 during the second flight. Air masses of different origin as determined by back trajectory calculations can explain the spatial variation in chemical composition and κmix values of the particles observed in the region.

Application of urea adduction technique to polluted urban aerosols for the determination of hydrogen isotopic composition of n-alkanes

We examined an applicability of an improved urea adduction technique for the determination of hydrogen isotopic composition (δD) of homologous series of n-alkanes present in polluted urban aerosols using GC/TC/IRMS. Unresolved complex mixture (UCM) of hydrocarbons that interferes with accurate isotope measurements of n-alkanes was removed from n-alkane fraction by a urea adduction method. Recoveries of C20 to C30 n-alkanes during the urea adduction procedure were greater than 90% when the concentrations of total n-alkanes exceed 6.1 µg mL−1. Our compound-specific D/H ratios confirm the absence of significant hydrogen isotope fractionation in n-alkanes during urea adduction and recovery of the purified n-alkane fraction. We applied this technique to the urban aerosols that contain a large quantity of UCM to measure δD of C20 to C35 n-alkanes in urban aerosols from Tokyo and Sapporo with an accuracy less than 10‰. We found that the δD values widely ranged from −38 to −179‰. Based on the δD values of individual n-alkanes in aerosol samples, we can obtain further information on the sources of aerosol n-alkanes and their source regions, and the atmospheric processes such as long-range transport and atmospheric mixing of air masses of different origin.

Isotopic composition of precipitation in Portorož (Slovenia)

The stable isotopic composition of hydrogen and oxygen (d2H and d18O) and tritium activity (3H) have been monitored in monthly precipitation at Portorož airport meteorological station since October 2000. Here we present a complete set of numerical data and a statistical analysis for the period 2000–2006. Seasonal variations of d2H and d18O were observed but are much less pronounced than for continental stations of the Northern Hemisphere. The weighted mean d2H and d18O values are -43 ‰ and -6.6 ‰, respectively. The relation between hydrogen and oxygen isotopic composition is expressed by the orthogonal Local Meteoric Water Line as d2H = (8.05 ± 0.22) d18O + (9.35 ± 1.55), and the temperature coefficient of d18O is 0.13 ‰/°C. The deuterium excess weighted mean value is 9.8 ‰ and shows some seasonal variations that reflect the variable influence of air masses of different origin (either Atlantic or Mediterranean). Tritium activity in monthly precipitation also showed seasonal variations, with a weighted mean value of 6.9 TU.

Two extreme types of mixing of dust with urban aerosols observed in Kosa particles: ‘After’ mixing and ‘on-the-way’ mixing

Besides well-known episodic Kosa during spring, high concentrations of Ca 2+ in aerosols were observed early in summer as well as in the semi-continuous data of the aerosols at the summit of Mt. Fuji. We further analysed the data to study the chemical characteristics of the high calcium event during early summer. The back trajectory analyses of the event indicated that Ca was transported from arid and semi-arid regions (e.g. the Taklamakan desert) through the westerly-dominated troposphere higher than the height of the summit of Fuji. The amount of SO 4 2− was always equivalent to that of NH 4 + unlike the case of the normal Kosa period where SO 4 2− is in excess with respect to NH 4 +. This shows the ‘after’ mixing of unreacted CaCO 3 of Kosa origin with (NH 4) 2SO 4, which was only realized by the downward injection of Kosa particles from higher altitudes to the air masses of different origin. In the case of normal Kosa, the air bearing Kosa particles passed through the polluted area to absorb unneutralized acids (‘on-the-way’ mixing), whereas in the case of the Kosa-like phenomena in summer, the acids from the polluted area have been neutralized by NH 4 + and become inactive before mixing with CaCO 3 (“after” mixing). We have simplified the chemistry of aerosols using their three major components, Ca 2+, SO 4 2− and NH 4 +, and introduced a new triangle diagram with the three assumed end-members of CaCO 3, CaSO 4 and (NH 4) 2SO 4 to quantify the contribution of the ‘after’ mixing to the aerosols (AMI; ‘after’ mixing index). Based on the back trajectories of some high AMI cases, CaCO 3 in Kosa particles was transported through the middle troposphere (5000–7000 m) and descended to meet another air mass where SO 4 2− had been already neutralized by NH 3.

Sulfur dioxide measurements in the lower, middle and upper troposphere: Deployment of an aircraft-based chemical ionization mass spectrometer with permanent in-flight calibration

Measurements of atmospheric SO 2 have been made at altitudes between ground level and 12 km in the lower, middle and upper troposphere. The measurements were carried out within the framework of the ITOP (Intercontinental Transport of Ozone and Precursors) campaign in summer 2004 above Europe and the Eastern Atlantic. They were made using a novel very sensitive and fast-response aircraft-based ion trap CIMS instrument (ITCIMS; CIMS=chemical ionization mass spectrometry), which was continuously calibrated using isotopically labelled SO 2. During a total of eight flights of the research aircraft FALCON (DLR) air masses of different origin and different degree of pollution, indicated by measured elevated atmospheric SO 2 mole fractions, were intercepted. Often elevated concentrations of SO 2, which stemmed from North America were observed over Europe and the eastern Atlantic.

Evaluation of the Impact of Long-range Transport and Aerosol Concentration Temporal Variations at the Eastern Coast of the Baltic Sea

Ambient particles vary greatly in their ability to affect visibility, climate and human health. The fine fraction of aerosol is responsible for greater and wider effects on human health; thus, investigation of this fraction is very important. Continuous measurements of PM2.5 (particulate matter below 2.5 microm in size) concentrations at the Preila monitoring station started in 2003. During a period of 2 years, the episodes of high daily and semi-hourly concentrations of PM2.5 were measured. These episodes did not depend on the season or time of day. The substantial role of long-range transport of pollutants to these increases in concentration was shown using chemical and statistical analysis. It was found that most of the severe episodes occurred when air masses came from a specific site besides it was established that air masses of different origin were characterized by different mixing layer depth. Lower mixing depth was observed in air masses characterized by higher observed concentrations at the measuring site and vice versa. PM2.5 concentrations showed diurnal and seasonal variations whose pattern reflected the regional origin of the aerosol. The regional pollution level was evaluated by the statistical analysis of PM2.5 concentrations. The background annual average of PM2.5 mass concentration for the eastern coast of the Baltic Sea was 15.1 +/- 0.8 microg m(-3).

The contribution of sulphuric acid to atmospheric particle formation and growth: a comparison between boundary layers in Northern and Central Europe

Abstract. Atmospheric gaseous sulphuric acid was measured and its influence on particle formation and growth was investigated building on aerosol data. The measurements were part of the EU-project QUEST and took place at two different measurement sites in Northern and Central Europe (Hyytiälä, Finland, March-April 2003 and Heidelberg, Germany, March-April 2004). From a comprehensive data set including sulphuric acid, particle number size distributions and meteorological data, particle growth rates, particle formation rates and source rates of condensable vapors were inferred. Growth rates were determined in two different ways, from particle size distributions as well as from a so-called timeshift analysis. Moreover, correlations between sulphuric acid and particle number concentration between 3 and 6 nm were examined and the influence of air masses of different origin was investigated. Measured maximum concentrations of sulphuric acid were in the range from 1x106 to 16x106cm-3. The gaseous sulphuric acid lifetime with respect to condensation on aerosol particles ranged from 2 to 33min in Hyytiälä and from 0.5 to 8 min in Heidelberg. Most calculated values (growth rates, formation rates, vapor source rates) were considerably higher in Central Europe (Heidelberg), due to the more polluted air and higher preexistent aerosol concentrations. Close correlations between H2SO4 and nucleation mode particles (size range: 3-6 nm) were found on most days at both sites. The percentage contribution of sulphuric acid to particle growth was below 10% at both places and to initial growth below 20%. An air mass analysis indicated that at Heidelberg new particles were formed predominantly in air advected from southwesterly directions.

Ozone perturbations in the Arctic summer lower stratosphere as a reflection of NOX chemistry and planetary scale wave activity

Ozone concentration perturbations in the high‐latitude lower stratosphere in the Northern Hemisphere were observed by Improved Limb Atmospheric Spectrometer (ILAS) after the polar vortex breakdown at the beginning of May 1997 and until the end of June of that same year. Simulations and a passive tracer experiment using the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) nudging chemical transport model (CTM) show that the low‐ozone perturbations observed in May were caused by the Arctic polar vortex debris, while those after the end of May resulted from a dynamical elongation due to zonal wave number 2 planetary waves of the low‐ozone region in the summer polar stratosphere, which had been developed by the catalytic ozone destruction cycle of NOX. These low‐O3 air masses of different origin were advected or elongated from the polar region to the ILAS measurement points. An episodic event of a dynamical O3 perturbation in June 1997 on a chemically induced meridional O3 gradient is described. These results show that a timing of the polar vortex breakdown and activity of planetary waves after the breakdown may affect the O3 background gradient in the summer lower stratosphere at middle and high latitudes.

Synoptic tracer gradients in the upper troposphere over central Canada during the Stratosphere‐Troposphere Experiments by Aircraft Measurements 1998 summer campaign

During the July 1998 Stratosphere‐Troposphere Experiments by Aircraft Measurements (STREAM) intensive campaign, eight measurement flights were conducted from Timmins airport (Ontario, Canada, 48.2°N, 79.3°W). In situ measurements of ozone, carbon monoxide, carbon dioxide, and nonmethane hydrocarbons, as well as three‐dimensional back trajectories based on European Centre for Medium‐Range Weather Forecasts wind field analyses, are used to characterize upper tropospheric air masses with respect to their origin. In the upper troposphere between 8 km and the local tropospause, about 40% of the air masses originated from the subtropics/tropics south of 30°N, about 50% originated from midlatitudes (between 30° and 50°N), and 12% originated from the polar region north of 50°N. In general, highest trace gas levels were observed in air masses originating from midlatitudes (e.g., CO, 107 ± 25 ppbv; ethane, 983 ± 385 pptv; acetylene, 119 ± 43 pptv), with the exception of CO, which had highest mixing ratios (121 ± 11 ppbv) in air masses of polar origin. The lowest concentrations were generally observed in air masses of (sub)tropical origin (e.g., CO, 64 ± 2 ppbv; ethane, 424 ± 86 pptv; acetylene, 39 ± 14 pptv). For CO and most of the alkanes a significant positive latitudinal gradient was observed, while CO2 and CH3Cl exhibited an inverse gradient with lowest mixing ratios at high northern latitudes. This reflects the different source and sink distributions of the investigated species. Ozone mixing ratios significantly increased in the upper troposphere above 8 km altitude, at least partially because of stratosphere‐troposphere exchange. Approximately 10% of the upper tropospheric air was directly influenced by recent cross‐tropopause exchange. A case study indicates that the convergence of air masses of different origin along a cold front creates sharp gradients not only in temperature and humidity but also in trace gas concentrations. It is shown that these mesoscale gradients result from the organized flows of tropical, arctic, and stratospheric air masses.

Time study of trace elements and major ions during two cloud events at the Mt. Brocken

Cloud water investigations have been performed at the highest elevation of Central Germany in 1997. Results of extensive trace element measurements are presented. Besides conductivity, pH, liquid water content and major ions the data set includes 49 minor and trace elements. Estimation of crustal enrichment factors (EFs) provides an indication of the anthropogenic contributions to the cloud water concentrations. The variation of cloud composition with time has been illustrated for two selected events with different air mass origins. The chemical composition of the cloud condensation nuclei on which the droplets grow mainly determines the cloud water chemistry. For a cloud event in June 1997 the concentrations of the crustally derived elements Si, Al, Fe, Ti, Ce, La and Nd follow each other closely. The fact that SO 4 2−, NO 3 − and NH 4 + are only moderately correlated with the particular pollutants with high enrichment factors such as Cd, Sb, Pb, Zn, Cu, As, Bi, Sn, Mo, Ni, Tl and V indicates that their source regions are more widespread. During an event in October 1997 the time trends for most minor and trace elements follow rather closely those for the major ions NH 4 +, SO 4 2− and NO 3 −. Back trajectories show that the transport from continental and marine European sources was the likely cause of the sample concentrations. EFs of trace elements in cloud water samples during the June and October event show a strong correlation with those obtained for urban particulate matter. Although both events are influenced by air masses of different origin, there is a good agreement between the EF signatures.

Effects of subgrid segregation on ozone production efficiency in a chemical model

Ozone production efficiency is a parameter used for evaluating the effect of anthropogenic NO x on tropospheric ozone. For this work, zero-dimensional simulations were run to compare the differences in ozone production efficiency when air masses of different origin were separated and when they were merged. The purpose of the simulations was to estimate whether coarsely resolved models might under or overpredict ozone production due to their blending of air masses of different origin. Cases were run for several combinations of air mass origin, different latitudes, times of year, air mass dilution ratios and initial gas concentrations. The main result of this study is that integrated ozone production may be overpredicted by as much as 60% in coarse-model grid cells exposed to different air masses. Under certain conditions, such as in rather finely resolved urban airshed models, ozone production may actually be underpredicted by about 20% in mid-latitudes during summer. The results imply that large-scale global models may have a difficult time correctly predicting ozone concentrations near, for instance, urban/free tropospheric boundaries, a conclusion supported by other studies examining parameters other than ozone production efficiency.

Determination of trace metals in size fractionated particles from arctic air by electrothermal vaporization inductively coupled plasma mass spectrometry

Studies of element composition in small atmospheric particles aid the clarification of processes such as long-range transport, deposition and transformation of particles and quantification of emission from natural and anthropogenic sources. For this purpose, a highly sensitive method was developed for the trace analysis of atmospheric particles. The particles were sampled and separated according to size, directly on separate small graphite discs arranged behind the jet-nozzles of an eight-stage cascade impactor. To determine the elemental composition of the particles, the ETV-ICP-MS technique was applied. In an appropriately sealed electrothermal vaporizer, linked to an inductively coupled plasma mass spectrometer, the targets were heated and the sample vapour was swept by argon into the plasma. The system described was used for the analysis of long-range transported particles from Arctic air sampled at the German Arctic research station at Spitsbergen, Norway, in spring 1998. For the elements Mn, Fe, Co, Ni, Ag, Cd, Sn, Sb and Pb the trace element content per cubic metre of air was measured as a function of the aerodynamic particle diameter. Air masses of different origin cause characteristic particle distributions at low changes in total dust burden. The relative detection limits for the elements measured in an air volume of 0.275 m3 were determined to be within 0.3–10 pg m–3; the overall analytical precision was around 20% for all trace metals.

Microphysical and radiative properties of stratocumulus

During the Cloudy Column project of the Aerosol Characterisation Experiment (ACE2) stratocumulus clouds formed in air masses of different origins have been sampled. The paper presents results from in-situ measurements and the comparison of the derived radiative properties with those remotely sensed. A simple parametrisation of the optical thickness of the adiabatic cloud regions is presented. The cloud optical thickness is much more sensitive to the cloud geometrical thickness (H53) than to the droplet concentration (N13).