Atmospheric Environment (v.34, #25)

Formation and cycling of aerosols in the global troposphere by Frank Raes; Rita Van Dingenen; Elisabetta Vignati; Julian Wilson; Jean-Philippe Putaud; John H. Seinfeld; Peter Adams (4215-4240).
Aerosols are formed, evolve, and are eventually removed within the general circulation of the atmosphere. The characteristic time of many of the microphysical aerosol processes is days up to several weeks, hence longer than the residence time of the aerosol within a typical atmospheric compartment (e.g. the marine boundary layer, the free troposphere, etc.). Hence, to understand aerosol properties, one cannot confine the discussion to such compartments, but one needs to view aerosol microphysical phenomena within the context of atmospheric dynamics that connects those compartments. This paper attempts to present an integrated microphysical and dynamical picture of the global tropospheric aerosol system. It does so by reviewing the microphysical processes and those elements of the general circulation that determine the size distribution and chemical composition of the aerosol, and by implementing both types of processes in a diagnostic model, in a 3-D global Chemical Transport Model, and in a General Circulation Model. Initial results are presented regarding the formation, transformation, and cycling of aerosols within the global troposphere.

The aim of this series of two publications is to characterise important radical reactions of organic molecules in atmospheric water phases, such as cloud, fog, and rain droplets, or on wetted aerosols. In this first study the reactions occurring in a mixed peroxyl radical system generated from the OH radical attack on a model compound, 2-butoxyethanol, in oxygenated aqueous solution are investigated in the absence of trace compounds typically found in such natural waters. Using γ-radiation emitted from a 60 Co source, we have produced steady-state concentrations of O2 /HO2- and OH radicals comparable to those predicted for daytime conditions in cloud or fog droplets. The reactions of the peroxyl radicals derived from 2-butoxyethanol (C4H9OCH2CH2OH) in oxygenated aqueous solution have been investigated by measuring detailed product distributions resulting from the oxidation process. The observed and quantified products are mainly esters, ketones, aldehydes and hydroperoxides and accounted for 83% of the removal of 2-butoxyethanol (expressed as a carbon balance). A mechanism for the formation of the reaction products is proposed. It includes the OH attack at four carbon centers of the molecule producing carbon centered radicals, the formation of peroxyl radicals by addition of oxygen to these radicals, the decay of the peroxyl radicals in a reaction system of six different peroxyl radicals and superoxide via bimolecular and unimolecular reactions and the reactions of intermediary produced oxyl radicals.
Keywords: Cloud chemistry; Superoxide radical; Hydrogen peroxide; Radical–radical reactions; Water droplet;

The aim of this series of two publications is to characterise radical reactions of organic molecules, occurring in atmospheric water phases. In this second study, the reactions occurring in a mixed peroxyl radical system generated from the OH radical attack on a model compound, 2-butoxyethanol, in oxygenated aqueous solution are investigated in the presence of O2 /HO2 radicals, H2O2, and of copper (I and II)- and iron (II and III) species. A general mechanism of the reactions occurring in such systems is discussed and compared to the formation of organic hydroperoxides and hydrogen peroxide from this oxidation reactions. The main features of the mechanism are as follows: (i) production of peroxyl radicals following OH-attack on the model compound; (ii) cycling of the transition metals between the oxidation states given above, arising from a reduction reaction with superoxide, O2 , and oxidation reactions by organic peroxyl radicals, organic hydroperoxides, superoxide, and oxygen; (iii) reactions of organic peroxyl radicals, in contrast to superoxide radicals, occur only with the reduced forms of the transition metals and lead to the formation of organic hydroperoxides; (iv) the reaction of organic peroxyl radicals with Cu (I) is fast (k>108  M−1  s−1); (v) the reaction of organic hydroperoxides with Cu (I) can be much faster (2 orders of magnitude) than the analogous reaction of hydrogen peroxide and leads to the production of oxyl radicals; and (vi) oxalate, a strong ligand for Cu (II), reduces the activity of copper, owing to its stabilisation of the copper (II) complex. The mechanism is discussed in relation to atmospheric conditions.
Keywords: Copper; Iron; Cloud chemistry; Organic hydroperoxides; Hydrogen peroxide;

Concentrations of seven elements (As, Cd, Cr, Mn, Pb, V, Zn) in mosses (Hylocomium splendens, Pleurozium schreberi, Eurhynchium angustirete) and needles of Norway spruce (Picea abies) and juniper (Juniperus communis) were determined at 48 sites in Lithuania. Conifer needles consistently showed many times lower concentrations than mosses collected at the same site. Correlations between heavy-metal concentrations in needles and mosses indicated that accumulation processes may be similar, but mosses appear to be clearly preferable as biomonitors of atmospheric deposition because of their higher elemental concentrations and more quantitative reflection of deposition rates. Precipitation in the open field and under the canopy was investigated at two stations with respect to the same metals. The canopy was shown to retain a considerable part of lead, whereas elements such as Zn and Mn were enriched in precipitation under the canopy. Study of metal concentrations in moss growing, respectively, below and outside the canopy showed that none of so studied elements was significantly retained by the canopy. Most of the metals (Cu, Fe, Zn, Cr, Ni, V) were leached from the canopy to a smaller or greater extent.
Keywords: Needles; Moss; Biomonitors; Trace metals; Canopy; Precipitation;

Study of humic-like substances in fog and interstitial aerosol by size-exclusion chromatography and capillary electrophoresis by Z Krivácsy; Gy Kiss; B Varga; I Galambos; Zs Sárvári; A Gelencsér; Á Molnár; S Fuzzi; M.C Facchini; S Zappoli; A Andracchio; T Alsberg; H.C Hansson; L Persson (4273-4281).
Recently, there have been implications that the bulk of the organic carbon in the atmospheric aerosol and fog is contained in an “air polymer” whose chemical nature is poorly understood. Since several properties (e.g. acidity, UV–VIS absorbance, fluorescence) of this polymer were found to be very similar to those of humic substances the term humic-like substances (HULIS) was proposed. In this work size-exclusion chromatography and capillary electrophoresis are used to obtain new information about some properties of the HULIS found in fog water and aqueous extract of interstitial aerosol. Retention and migration behaviour as well as mass spectra are investigated and compared to those of reference humic substances. The capillary electrophoresis is applied for the determination of the range of protonation constants of the HULIS, as well. Scavenging ratio for the HULIS is calculated and found to be about the same as those of ammonium and sulphate. The results imply that such compounds may play a role in cloud condensation.
Keywords: Cloud condensation; Fog; Interstitial aerosol; Humic-like substances; Scavenging ratio; Size-exclusion chromatography; Capillary electrophoresis;

During two balloon flights in the Arctic winter in February 1994 two distinctly different vertical profiles of ClO were measured. On 1 February enhanced values of ClO were observed between about 420 and 620 K inside the polar vortex in an air mass that had experienced moderately cold temperatures about 10 h prior to our measurements. In contrast, on 9 February an unstructured vertical profile was observed outside of the vortex with lower mixing ratios of ClO. Based on calculations with a photochemical box trajectory model we show that the observed enhancement in ClO can be explained by heterogeneous processing of chlorine reservoir species on supercooled liquid aerosols. When NAT formation is forced in the model or when lee wave processing is assumed, the mixing ratios of ClO inside the vortex are overpredicted. Under conditions where air masses are only cooled to low temperatures for a short period of time this may be a very important chlorine activation mechanism.
Keywords: Chlorine; Heterogeneous Chemistry; Stratosphere; Stratospheric aerosol;

A high-flow humidograph for testing the water uptake by ambient aerosol by Harry M ten Brink; Andrey Khlystov; Gerard P.A Kos; Thomas Tuch; Christa Roth; Wolfgang Kreyling (4291-4300).
A “humidograph” with an air flow rate of 0.4 m3  s−1 was built to investigate the uptake of water and its effect on sizing, collection and light scattering of ambient aerosol. In the humidograph the relative humidity (RH) can be scanned over a large RH trajectory. Its performance was assessed with laboratory particles of ammonium nitrate, ammonium sulfate and sodium chloride that are the major hygroscopic components of ambient aerosol. The increase in size at the deliquescence points, which ideally is a stepwise function of RH, occurs over a range of 3% RH units. This is shown to be an optimum value in a system of such large dimensions. Because the vapor pressure of ammonium nitrate increases significantly with temperature, its evaporative loss was investigated as a function of heating/drying temperature. The loss of pure test aerosol, with a mass distribution similar to that in the ambient atmosphere, was found to be acceptable for drying temperatures of up to 40°C. The sizing of deliquesced aerosol by LAS-X monitors was tested and found to be a complex function of RH. In Berner low-pressure impactors growth of hygroscopic aerosol was not observed, not even at an RH approaching saturation.
Keywords: Aerosol; Sizing; Collection; Deliquescence; Ammonium nitrate;

A modified semi-implicit method to obtain the evolution of an aerosol by coagulation by J.M. Fernández-Dı́az; C.González-Pola Muñiz; M.A.Rodrı́guez Braña; B.Arganza Garcı́a; P.J.Garcı́a Nieto (4301-4314).
In the present work we improve the semi-implicit method previously developed by Jacobson et al. (1994. Atmospheric Environment 28, 1327–1338). In this the particle size distribution has to be discretized in several volume bins, and we must use partition coefficients that relate the particle formed in thecollision to the bin discretization. The original method has the problem of some numerical diffusion and we present two different methods for calculating the coefficients that provide better results. We compare the original and our methods with several analytical solutions for concrete cases of coagulation kernels. Finally, we apply our methods to a typical urban mono-component aerosol and to a multi-component case. More work has to be developed in the comparison of our methods with the original one when growth is present, because this phenomenon swamps out coagulation at large sizes.
Keywords: Particle size distribution; Coagulation; Numerical method; Geometrical and potential bins; Multi-component aerosol;

Nocturnal secondary ozone concentration maxima analysed by sodar observations and surface measurements by Oliver Reitebuch; Anja Strassburger; Stefan Emeis; Wilhelm Kuttler (4315-4329).
The occurrence of nocturnal secondary ozone maxima was investigated by ground-level meteorological and air quality measurements in an urban park in Essen, Germany, during 29 individual summer measurement campaigns between May 1995 and September 1997. In addition, during an intensive measurement campaign in May 1997, SODAR measurements of wind and turbulence were made. The spatial and temporal distribution of nocturnal ozone maxima within the measurement network of the Environmental Protection Office of North Rhine–Westphalia was also analysed. Two case studies of nocturnal secondary ozone maxima are discussed in more detail. They represent two different types of this phenomenon, associated with a nocturnal low-level jet (LLJ) and the passage of a front, respectively. In both cases, the nocturnal increase in ozone concentration was accompanied by a significant increase in the standard deviation of the vertical wind speed σ w across the lower stable boundary layer, indicating enhanced vertical mixing. During the LLJ case, ozone maxima were observed at 33% of the stations of the Environmental Protection Office network at approximately the same time. In the case of the front, the time of the ozone concentration rise could be allocated to the time of the passage of the front, moving from northwest to southeast across the study area. For the first time, the measurements presented here document turbulent mixing induced by shear forces in the whole layer between the core of the LLJ and the ground surface during secondary nocturnal ozone maxima.
Keywords: Tropospheric ozone; Nocturnal ozone maxima; SODAR; LLJ; turbulence;

Iodine concentration and availability in atmospheric aerosol by A.R Baker; D Thompson; M.L.A.M Campos; S.J Parry; T.D Jickells (4331-4336).
Iodine has been determined in aerosol samples collected at a coastal site in southeast England using instrumental neutron activation analysis (INAA) and by an electrochemical technique, after aqueous extraction. Size distribution and enrichment factor data for the samples are consistent with a non-sea-salt source of iodine, presumably gas-to-particle conversion of volatile iodocarbons. On average, only ∼70% of INAA (i.e. total) iodine could be released from the aerosols as inorganic iodine by aqueous extraction at 95°C. Extraction at a more environmentally realistic temperature (20°C) decreased this yield to ∼25%. Through the use of high-energy UV light, which is known to destroy organic matter, the yield of aqueous extractable iodine at the lower temperature was increased for some samples. Thus, it appears likely that iodine is present in aerosol in varying proportions as soluble inorganic iodine, soluble organic iodine and insoluble, or unextractable, iodine. The different characteristics of these fractions are likely to have significant impacts on the cycling and reactivity of iodine in the atmosphere.
Keywords: Inorganic iodine; Organic iodine; Neutron activation; Voltammetry; Aqueous extraction;

Because the major atmospheric reaction products for aromatic hydrocarbons are unknown, they are represented in air quality models using parameterized mechanisms derived by modeling environmental chamber data. Uncertainties in rate constants, experimental conditions, and chamber artifacts affect the resulting parameter estimates. The SAPRC-97 mechanism represents aromatic ring fragmentation products by model species MGLY (α-dicarbonyls) and AFG2 (other photoreactive products) with yields derived from aromatics–NO x experiments with blacklight or xenon arc light sources. In this study, parameter estimates for nine aromatic compounds were estimated using stochastic programming. The uncertainties estimated for these parameters range from about 29% (1σ relative to the mean) for the yield of AFG2 in the 135-trimethylbenzene mechanism to 71% for the yield of MGLY for p-xylene. Major causes are uncertainties in rate constants for the aromatics+OH and NO2+OH reactions, and the light intensity, parameters representing unknown chamber radical sources, and initial aromatic concentrations in the experiments. The chamber radical source parameters are estimated from CO–NO x and n-butane–NO x experiments, and are sensitive to uncertainties in the rate constants for n-butane or CO+OH, NO2+OH, HONO photolysis and the experimental light intensity. A companion paper examines how uncertainties in the chamber-derived aromatics parameters affect incremental reactivity estimates for volatile organic compounds.
Keywords: Ozone; Photochemistry; Uncertainty analysis;

Incremental reactivity estimates for aromatic compounds have been considered highly uncertain due to gaps in understanding the reaction mechanism for their atmospheric oxidation. In this study, uncertainties in the parameters of the SAPRC-97 chemical mechanism, including uncertainties in aromatic mechanism parameters estimated by fitting chamber data, are propagated through incremental reactivity calculations using Monte Carlo analysis with Latin hypercube sampling. For 34 volatile organic compounds, absolute incremental reactivity estimates calculated with the SAPRC-97 mechanism are generally higher, but with lower associated uncertainties, than those calculated previously with the SAPRC-90 mechanism. The uncertainty levels found for the maximum incremental reactivities (MIRs) of the aromatic compounds range from 27 to 32% (1σ relative to the mean) and are similar to those for VOCs with relatively well-established mechanisms. The uncertainly levels for the maximum ozone incremental reactivities (MOIRs) and equal benefit incremental reactivities (EBIRs) of the aromatics range from 38 to 75% and 30 to 520%, respectively. Uncertainties in relative MIRs, MOIRs and EBIRs for the aromatic compounds range from 13 to 24%, 21 to 62% and 21 to 360%, respectively. The chamber-derived aromatics oxidation parameters strongly influence the uncertainties in the incremental reactivities of most of the VOCs studied.
Keywords: Ozone; Photochemistry; Uncertainty analysis;

During the last 20 years a variety of models have been developed to study the transport, transformation and deposition of pollutants on the regional scale. Different levels of complexity have been considered to depict these phenomena. The representation of the chemical transformations has been one of the most challenging tasks. A detailed non-linear Eulerian chemistry module (CheM) has here been added to the hybrid single-particle Lagrangian integrated trajectory (HY-SPLIT) model. The model is applied to analyze changes in hourly averaged ozone concentration over the period of 8–12 July 1996. Comparisons between simulation results and measurements from 11 monitoring stations throughout Pennsylvania show that hybrid Lagrangian–Eulerian modeling techniques are promising and may be useful for predicting air quality variables in general.
Keywords: Photochemical modeling; Air quality; Particle-in-grid; Smog; HY-SPLIT;

Atmospheric nonmethane hydrocarbons (NMHCs: C2–C6) were measured over the western North Pacific and eastern Indian Ocean between 25°N and 40°S during oceanographic cruise from December 1996 to February 1997 using a fused-silica-lined stainless-steel canister and GC technique. Averaged mixing ratios of individual NMHCs were 0.61 ppbv (ethane), 0.42 ppbv (ethylene), 0.17 ppbv (acetylene), 0.24 ppbv (propane), 0.60 ppbv (propylene), 0.07 ppbv (i-butane), 0.13 ppbv (n-butane), 0.04 ppbv (i-pentane), 0.06 ppbv (n-pentane), and 0.06 ppbv (n-hexane). Although there are few reported data available for comparison in these regions of marine boundary layer, mixing ratios of the NMHCs are within the range of previous results reported in the similar latitudes from the Atlantic and Pacific Ocean. NMHCs with lifetimes more than a week (C2–C4 alkanes and acetylene) showed a significant latitudinal decrease from north to south in the Northern Hemisphere, suggesting an important source strength in East Asian region. In contrast, NMHCs with lifetimes less than a week (alkenes and C5–C6 alkanes) did not show any significant latitudinal trends. Based on a comparison with the results of the backward trajectories for 2 days, light alkenes were suggested to be derived from ocean surfaces.
Keywords: Nonmethane hydrocarbons; Marine atmosphere; Western North Pacific; Eastern Indian Ocean; Long-range transport;

Determination of elemental and organic carbon on damaged stone monuments by N Ghedini; G Gobbi; C Sabbioni; G Zappia (4383-4391).
An analytical methodology was developed for the discrimination and evaluation of the different types of carbon matter, particularly carbonate, elemental and organic carbon, present on monuments and historical buildings, due to interaction between materials and atmospheric pollution. With this aim samples of black patinas were analysed by a procedure consisting of three different steps. Total, noncarbonate and elemental carbon were measured by combustion-chromatographic CO2 determination: C t was obtained by burning the bulk samples (step 1), while C nc and C e were quantified after elimination of C c with acid treatment (step 2) and elimination of C o by means of alternate attacks, followed by centrifugation, with concentrate acid and base solutions at high temperature and pressure (step 3); the carbonate carbon and the organic carbon were then calculated. Furthermore, for a complete sample characterization, oxalate, acetate, formate and the main anion contents were detected by ion chromatography. The methodology was also tested on standard samples containing the same carbon species as the black crusts. The results obtained indicate that this approach satisfactorily distinguishes between elemental and organic carbon and allows reliable elemental carbon determination at the ppm level in black damage crust samples from historic monuments and buildings.
Keywords: Stone damage; Black crusts; Elemental carbon; Organic carbon;

Using atmospheric CCl4 as an internal reference in gas standard preparation by Jia-Lin Wang; Wen-Chain Lin; Tai-Yih Chen (4393-4398).
This study investigates the feasibility of using CCl4 in the background air as a concentration reference to calibrate synthetic mixtures. A metal vacuum line was constructed for preparing diluted standard mixtures at parts per billion by volume (ppbv) or trillion by volume levels (pptv) from pure substance. Component mixing ratios in the synthetic air were roughly estimated based upon the volume and pressure changes between the dilution glass bulbs on the vacuum line. More accurate estimates of the dilution factors relied on calibrating the absolute concentration of CCl4 in the synthetic mixture, acting as a surrogate, against its atmospheric levels.
Keywords: Surrogate; Dilution; Volatile organic compounds; Variability;

Future directions: Satellite observations of tropospheric chemistry by Hanwant B Singh; Daniel J Jacob (4399-4401).