Showing papers in "Journal of Atmospheric Chemistry in 2000"

Fast-J: Accurate Simulation of In- and Below-Cloud Photolysis in Tropospheric Chemical Models

Abstract: Photolysis rates in the troposphere are greatly affected by the presenceof cloud and aerosol layers. Yet, the spatial variability of theselayers along with the difficulty of multiple-scattering calculationsfor large particles makes their inclusion in 3-D chemical transportmodels computationally very expensive.This study presents a flexible and accurate photolysis scheme, Fast-J,which calculates photolysis rates in the presence of an arbitrary mix ofcloud and aerosol layers. The algorithm is sufficiently fast to allow thescheme to be incorporated into 3-D global chemical transport models andhave photolysis rates updated hourly. It enables tropospheric chemistrysimulations to include directly the physical properties of the scatteringand absorbing particles in the column, including the full, untruncatedscattering phase function and the total, uncorrected optical depth.The Fast-J scheme is compared with earlier methods that have been usedin 3-D models to parameterize the effects of clouds on photolysis rates.The impact of Fast-J on tropospheric ozone chemistry is demonstratedwith the UCI tropospheric CTM.

Impact of Non-Methane Hydrocarbons on Tropospheric Chemistry and the Oxidizing Power of the Global Troposphere: 3-Dimensional Modelling Results

Abstract: The impact of natural and anthropogenicnon-methane hydrocarbons (NMHC) on troposphericchemistry is investigated with the global,three-dimensional chemistry-transport model MOGUNTIA.This meteorologically simplified model allows theinclusion of a rather detailed scheme to describeNMHC oxidation chemistry. Comparing model resultscalculated with and without NMHC oxidation chemistryindicates that NMHC oxidation adds 40–60% to surfacecarbon monoxide (CO) levels over the continents andslightly less over the oceans. Free tropospheric COlevels increase by 30–60%. The overall yield of COfrom the NMHC mixture considered is calculated to beabout 0.4 CO per C atom. Organic nitrate formationduring NMHC oxidation, and their transport anddecomposition affect the global distribution of NO x and thereby O3 production. The impact of theshort-lived NMHC extends over the entire tropospheredue to the formation of longer-lived intermediateslike CO, and various carbonyl and carboxyl compounds.NMHC oxidation almost doubles the net photochemicalproduction of O3 in the troposphere and leads to20–80% higher O3 concentration inNO x -rich boundarylayers, with highest increases over and downwind ofthe industrial and biomass burning regions. Anincrease by 20–30% is calculated for the remotemarine atmosphere. At higher altitudes, smaller, butstill significant increases, in O3 concentrationsbetween 10 and 60% are calculated, maximizing in thetropics. NO from lightning also enhances the netchemical production of O3 by about 30%, leading to asimilar increase in the global mean OH radicalconcentration. NMHC oxidation decreases the OH radicalconcentrations in the continental boundary layer withlarge NMHC emissions by up to 20–60%. In the marineboundary layer (MBL) OH levels can increase in someregions by 10–20% depending on season and NO x levels.However, in most of the MBL OH will decrease by10–20% due to the increase in CO levels by NMHCoxidation chemistry. The large decreases especiallyover the continents strongly reduce the markedcontrasts in OHconcentrations between land and oceanwhich are calculated when only the backgroundchemistry is considered. In the middle troposphere, OHconcentrations are reduced by about 15%, although dueto the growth in CO. The overall effect of thesechanges on the tropospheric lifetime of CH4 is a 15%increase from 6.5 to 7.4 years. Biogenic hydrocarbonsdominate the impact of NMHC on global troposphericchemistry. Convection of hydrocarbon oxidationproducts: hydrogen peroxides and carbonyl compounds,especially acetone, is the main source of HO x in theupper troposphere. Convective transport and additionof NO from lightning are important for the O3 budgetin the free troposphere.

CAPRAM2.3: A chemical aqueous phase radical mechanism for tropospheric chemistry

Abstract: A Chemical Aqueous Phase Radical Mechanism (CAPRAM) for modelling tropospheric multiphase chemistry is described. CAPRAM contains (1) a detailed treatment of the oxidation of organic compounds with one and two carbon atoms, (2) an explicit description of S(IV)-oxidation by radicals and iron(III), as well as by peroxides and ozone, (3) the reactions of OH, NO 3 ,C l 2 , Br 2 ,a nd CO 3 radicals, as well as reactions of the transition metal ions (TMI) iron, manganese and copper. A modelling study using a simple box model was performed for three different tropospheric conditions (marine, rural and urban) using CAPRAM coupled to the RADM2-mechanism (Stockwell et al., 1990) for liquid and gas phase chemistry, respectively. In the main calculations the droplets are assumed as monodispersed with a radius of 1m and a liquid water content of 0.3 g m 3 .I n the coupled mechanism the phase transfer of 34 substances is treated by the resistance model of Schwartz (1989). Results are presented for the concentration levels of the radicals in both phases under variation of cloud duration and droplet radius. The effects of the multiphase processes are shown in the loss fluxes of the radicals OH, NO 3 and HO2 into the cloud droplets. From calculations under urban conditions considering gas phase chemistry only the OH maximum concentration level is found to be 5:5 10 6 cm 3 . In the presence of the aqueous phase (r D 1 m, LWC D 0: 3g m 3 ) the phase transfer constitutes the most important sink (58%) reducing the OH level to 1:0 10 6 cm 3 . The significance of the phase transfer during night time is more important for the NO3 radical (90%). Its concentration level in the gas phase (1:9 10 9 cm 3 ) is reduced to 1:4 10 6 cm 3 with liquid water present. In the case of the HO2 radical the phase transfer from the gas phase is nearly the only sink (99.8%). The concentration levels calculated in the absence and presence of the liquid phase again differ by three orders of magnitude, 6 10 8 cm 3 and 4:9 10 5 cm 3 , respectively. Effects of smaller duration of cloud occurrence and of droplet size variation are assessed. Furthermore, in the present study a detailed description of a radical oxidation chain for sulfur is presented. The most important reaction chain is the oxidation of (hydrogen) sulphite by OH and the subsequent conversion of SO 3 to SO 5 followed by the interaction with TMI (notably Fe 2C )a nd chloride to produce sulphate. After 36 h of simulation ((H2O2U0 D 1 ppb; (SO2U0 D 10 ppb) the direct oxidation pathway from sulfur(IV) by H2O2 and ozone contributes only to 8% (2:9 10 10 M s 1 ) of the total loss flux of S(IV) (3 :7 10 9 Ms 1 ).

Development and Intercomparison of Condensed Isoprene Oxidation Mechanisms for Global Atmospheric Modeling

Abstract: A new condensed isoprene oxidation mechanism forglobal atmospheric modeling (MIM) was derived from ahighly detailed master chemical mechanism (MCM). In abox model intercomparison covering a wide range ofboundary layer conditions the MIM was compared withthe MCM and with five other condensed mechanisms, someof which have already been used in global modelingstudies of nonmethane hydrocarbon chemistry. Theresults of MCM and MIM were generally in goodagreement, but the other tested mechanisms exhibitedsubstantial differences relative to the MCM as well asrelative to each other. Different formation yields,reactivities and degradation pathways of organicnitrates formed in the course of isoprene oxidationwere identified as a major reason for the deviations.The relevance of the box model results for chemistrytransport models is discussed, and the need for avalidated reference mechanism and for an improvedrepresentation of isoprene chemistry in global modelsis pointed out.

Seasonal variation in amount and composition of monoterpenes emitted by young Pinus pinea trees - implications for emission modeling

Abstract: Current inventories of terpenes released from vegetation consider only the short-term influences of light and temperature on emissions to simulate temporal variation during the year. We studied whole canopy emissions from young Pinus pinea during a 15-month enclosure in greenhouse chambers and examined data for other long-term influences. Mean daytime emission rates strongly increased during spring, reached an annual maximum of ≈ 200 pmol m−2 total needle area s−1 (1.1 μg g−1 leaf dry weight h−1) between mid June and mid August, strongly declined in fall and reached an annual minimum of ≈ 1 pmol m−2 s−1 (0.006 μg g−1 h−1) between January and February. Normalization to standard temperature and light conditions did not change the annual time course of emissions, but reduced summer to winter ratio from a factor of 200 to about 45. Seasonal variation was characterized also by changes in terpene composition: among the six main compounds, three (t-β-ocimene, linalool, 1.8-cineol) were exclusively emitted during sunlit hours in the main vegetation period, whereas the other (limonene, α-pinene, myrcene) were emitted day and night and throughout the seasons. The results suggest that different terpene sources in P. pinea foliage exist and that a great part of the annual emission course observed here results from seasonal influences on these sources. A global model to simulate plant emissions is proposed, which accounts for seasonal influences on emissions in addition to the short-term effects of temperature and light. The model is tested on field data and discussed for its general application.

Products and mechanism of the gas phase reaction of ozone with β-pinene

Abstract: Gas phase ozonolysis of β-pinene was performedin a 570 l static reactor at 730 Torr and 296 K insynthetic air and the products were analysed by acombination of gas phase FTIR spectroscopy, HPLC andIC analyses of gas phase and aerosol samples,respectively. The reaction mechanism was investigatedby adding HCHO, HCOOH and H2O as Criegeeintermediate scavenger and cyclohexane as OH radicalscavenger. Main identified products (yields inparentheses) in the presence of cyclohexane as OHradical scavenger were HCHO (0.65 ± 0.04),nopinone (0.16 ± 0.04), 3-hydroxy-nopinone (0.15± 0.05), CO2 (0.20 ± 0.04), CO (0.030± 0.002), HCOOH (0.020 ± 0.002), the secondaryozonide of β-pinene (0.16 ± 0.05), andcis-pinic acid (0.02 ± 0.01). The decompositionof the primary ozonide was found to yieldpredominantly the excited C9-Criegee intermediateand HCHO (0.84 ± 0.04) and to a minor extent theexcited CH2OO intermediate and nopinone (0.16± 0.04). Roughly 40% of the excitedC9-Criegee intermediate becomes stabilised andcould be shown to react with HCHO, HCOOH and H2O. The atmospherically important reaction of thestabilised C9-Criegee intermediate with H2Owas found to result in a nopinone increase of (0.35± 0.05) and in the formation of H2O2(0.24 ± 0.03). Based on the observed products,the unimolecular decomposition/isomerisationchannels of the C9-Criegee intermediate arediscussed in terms of the hydroperoxide and esterchannels. Subsequent reactions of the nopinonylradical, formed in the hydroperoxide channel, lead tomajor products like 3-hydroxy-nopinone but also tominor products like cis-pinic acid. A mechanismfor the formation of this dicarboxylic acid isproposed and its possible role in aerosol formationprocesses discussed.

Biomass Burning in Southern Africa: Individual Particle Characterization of Atmospheric Aerosols and Savanna Fire Samples

Abstract: Ambient atmospheric aerosols and savanna fireparticulate emission samples from southern Africa werecharacterised in terms of particle classes and theirnumber abundance by electron probe X-ray microanalysis(EPXMA). About ten particle classes were identifiedfor each sample. The major classes werealuminosilicates and sea salts for ambient coarse(2–10 μm equivalent aerodynamic diameter (EAD))samples, and K-S and S-only particles for ambient fine(<2 μm EAD) samples. The K-S particles are oneof the major products of biomass burning. The EPXMAresults were found to be consistent with the resultsfrom bulk analyses on a sample by sample basis. Forsavanna fire fine samples, quantitative EPXMA revealedthat many particles had a composition of simple saltssuch as KCl. Some particles had a deviatingcomposition in the sense that more ionic species wereinvolved in sustaining the balance between cations andanions, and they were composite or mixed salts.Because of extensive processing during the atmospherictransport, the composition of the K-S particles in theambient samples was different from K2SO4,and such particles were enriched with S. The finepyrogenic KCl particles and the fine sea-saltparticles were much depleted in chlorine.

Is the Arctic Surface Layer a Source and Sink of NOx in Winter/Spring?

Abstract: Measurements of NOx (NO +NO2) and the sum of reactive nitrogenconstituents, NOy, were made near the surface atAlert (82.5°N), Canada during March and April1998. In early March when solar insolation was absentor very low, NOx mixing ratios were frequentlynear zero. After polar sunrise when the sun was abovethe horizon for much or all of the day a diurnalvariation in NOx and NOy was observed withamplitudes as large as 30–40 pptv. The source ofactive nitrogen is attributed to release from the snowsurface by a process that is apparently sensitized bysunlight. If the source from the snowpack is a largescale feature of the Arctic then the diurnal trendsalso require a competing process for removal to thesurface. From the diurnal change in the NO/NO2ratio, mid-April mixing ratios for the sum of peroxyand halogen oxide radicals of ≤10 pptv werederived for periods when ozone mixing ratios were inthe normal range of 30–50 ppbv. Mid-day ozoneproduction and loss rates with the active nitrogensource were estimated to be ∼1–2 ppbv/day and in nearbalance. NOy mixing ratios which averaged only295±66 pptv do not support a large accumulation inthe high Arctic surface layer in the winter and springof 1998. The small abundance of NOy relative tothe elevated mixing ratios of other long-livedanthropogenic constituents requires that reactivenitrogen be removed to the surface during transport toor during residence within the high Arctic.

The Impact of Precipitation Scavenging on the Transport of Trace Gases: A 3-Dimensional Model Sensitivity Study

Abstract: With the global Chemistry-Transport model MATCHsensitivity simulations were performed to determinethe degree to which especially upward transport ofgases from the earth's surface is limited byconvective and large-scale precipitation scavenging.When only dissolution of species in the liquid phaseis taken into account, mixing ratio reductions in themiddle and upper troposphere by ≈10% arecalculated for gases with a Henry's Law constant H of103 mol/l/atm. The removal increases to ≈50% forH = 104 mol/l/atm, and to 90% for H =105 mol/l/atm. We also consider scavenging by theice phase, which is generally much less efficient thanby the aqueous phase. In fact, rejection of gases fromfreezing water droplets may be a source of trace gasat higher altitudes.H2O2 and the strong acids (H2SO4,HNO3, HCl, HBr, HI) have such large solubilitiesthat they become largely removed by precipitation.When significant concentrations of these gases andsulfate aerosol exist above the liquid water domain ofthe atmosphere, they have likely been produced thereor at higher altitudes, although some could have comefrom trace gas rejection from ice particles or fromevaporating hydrometeors. Several other gases areaffected by precipitation, but not strongly enough toprevent fractional transfer to the middle and uppertroposphere: e.g., HNO4, HNO2 at pH ≤5,CH2O, the organic acids at pH ≤6,CH3SOCH3, HOCl, HOBr, and HOI. NH3 islargely removed by liquid phase scavenging at pH ≤7 and SO2 atpH ≥7. At pH less thanabout 6, upward transport of SO2 should largelydepend on the efficiency of oxidation processes in thewater droplets by O3 and H2O2.Most gases have solubilities which are too low forsignificant precipitation scavenging and aqueous phaseoxidation to occur. This holds, e.g., for O3, CO,the hydrocarbons, NO, NO2, HCN, CH3CN,CH3SCH3, CH3O2H, CH3CHOandhigher aldehydes, CH3OH and higher alcohols,peroxyacetylnitrate (PAN), CH3COCH3 andother ketones (note that some of these are not listedin Table I because their solubilities are below 10mol/l/atm). Especially for the short-lived gases,transfer from the boundary layer to the middle andupper troposphere is actually promoted by the enhancedupward transport that occurs in clouds.

Contribution of Secondary VOC to the Composition of Aqueous Atmospheric Particles: A Modeling Approach

Abstract: Measurements show that 20–60% of the carbon mass present in fine atmospheric particulate matter consists of water soluble organic compounds (WSOC). However, only 5–20% of this WSOC has been identified, mainly as dicarboxylic acids. Because of their high solubility in water, multifunctional secondary compounds derived from the gas-phase oxidation of volatile organic compounds (VOC) are suspected to be key contributors to the WSOC. To test this assumption, an estimate of aqueous uptake of secondary VOC was included in a highly detailed gas-phase mechanism which treats explicitly the formation of the secondary VOC from a set of representative primary species. Simulations were conducted for 2 scenarios, representing typical rural and urban areas. It was observed that the uptake of secondary VOC can lead to WSOC mass concentrations in the range of a few μC m−3, in fairly good agreement with typical WSOC mass concentrations measured. Speciation of WSOC was found to be mainly as tri- or higher multifunctional hydroxy-carbonyl species and hydroxy-hydroperoxide-carbonyl species, in urban and rural environments, respectively. However, it was also found that taking into account only the absorption of secondary VOC does not bring the carboxylic acids mass concentration in agreement with measurements. An attempt was made to explain this discrepancy by introducing chemistry occurring within deliquescent aerosols.

Factors Influencing the Atmospheric Depositional Fluxes of Stable Pb, 210Pb, and 7Be into Chesapeake Bay

Abstract: Atmospheric depositional fluxes of 7Be and210Pb (bulk) and stable Pb (wet) were measuredsimultaneously for one year (from September 1995–August 1996) atStillpond, Maryland on the uppereastern shore of the Chesapeake Bay The annual total(bulk) depositional fluxes of 210Pb and 7Bewere 078 and 13 dpm cm-2, respectively, andagree well with other previously reported results atnearby locations such as Norfolk, VA and Lewes, DE The wet depositional flux of stable Pb (58 ng cm-2 yr-1)was also similar to thatmeasured at other Chesapeake sites during 1990–1991(55 ng cm-2 yr-1, for both Wye and Elms,Maryland) This suggests that a constant Pb flux hasbeen reached since the mandatory use of unleadedgasoline was instituted The concentrations of7Be, 210Pb, and to a lesser extent stable Pbwere diluted exponentially by precipitation, based onconcentrations versus precipitation plots Due tohigher enrichment of 210Pb in the lowertroposphere, the dilution effect was largest on210Pb (ie, controlled mainly by below-cloudscavenging processes), and thus its depositional fluxincrease is negligible as precipitation amountincreases A good correlation between the amount ofprecipitation and total depositional flux of 7Beand stable Pb, which are more enriched in the uppertroposphere, suggests that precipitation amount isimportant in controlling their fluxes (ie,controlled by both below-cloud scavenging and in-cloudcondensation processes) Based on 7Be versus210Pb plots, it appears that 7Be, relativeto 210Pb, is less efficiently scavenged bysnowfall Our results suggest that in addition toprecipitation amounts, marine air-mass transport orsnowfall may be important factors in controlling theseasonal variability of the fallout fluxes of tracemetals in coastal areas

Numerical Investigation of the Influence of Mineral Dust on the Tropospheric Chemistry of East Asia

Abstract: An investigation of the influence of mineral dust ontrace gas cycles in the troposphere is carried out inthis study. A 3D regional scale atmospheric chemistrymodel (STEM-III) which includes aerosol processes isused for the numerical simulations for May 1987.Heterogeneous interactions between gaseous species(SO2, N2O5, HNO3, HO2andH2O2) and the dust particles are considered.Emissions of dust behind convective cold fronts aremodeled. The transport and distribution of mineraldust predicted from the model is compared withsatellite measurements (aerosol index from TOMS). Themodel is shown to capture the synoptic variability inthe observed aerosol index. Calculations show twomajor dust events in May 1987, during which thedust levels close to the source reach more than500 μg/m3. The transport of dust is mostlyrestricted towards the north, with the net continentaloutflow of ∼6 Tg for the entire month. Results showthat the presence of mineral aerosol can greatlyimpact sulfate and nitrate distributions. Averagedover the month of May, the presence of dust isestimated to increase particulate sulfate and nitratelevels in east Asia by ∼40%. Furthermore, the sulfateand nitrate on the dust particles are predicted to beassociated with the coarse mode (3–5 μm particlediameter), consistent with observations over Japan.The influence of mineral dust on the photochemicaloxidant cycle is also investigated. For the entiremonth, a5–10% decrease in boundary layer ozone ispredicted by the model closer to regions of higherdust levels. The ratio of nitric acid to NOx overmarine regions is reduced by a factor between 1 and 2in the boundary layer to more than 2 in the freetroposphere as a result of aerosol processes.

Structural characterisation of organic matter in fine tropospheric aerosol by pyrolysis-gas chromatography-mass spectrometry

Abstract: The organic matter in tropospheric aerosol plays animportant, yet undetermined role in atmosphericprocesses. The bulk of organic carbon representing asignificant part of the fine aerosol mass is bound toa polymeric matter whose structure and properties areto date largely unknown. Here we use thefingerprinting technique of derivatization pyrolysis-gas chromatography-massspectrometry tosubstantiate that in terms of chemical structure partof this polymer in rural aerosol in summer can beregarded as natural humic matter. We suggest thatthese compounds are likely to be directly emitted bybiogenic sources.

Structure-reactivity based estimation of the rate constants for hydroxyl radical reactions with hydrocarbons

Abstract: The reaction with the OH radical constitutes the singlemost important removal process for most organiccompounds found in the atmosphere. Efforts to measurethe OH radical rate constants of all troposphericconstituents remain incomplete due to the largevariety of primary emitted compounds and theirtropospheric degradation products.Based on the measured rate constants of ≈250molecules with the OH radical, a structure-activityrelationship (SAR) for OH reactions has beendeveloped. The molecules used in the dataset includemost classes of tropospheric compounds (includingalkanes, alkenes, and oxygenated hydrocarbons), withthe exception of aromatic and halogen-containingcompounds. Using a new parameterization of themolecular structure, the overall agreement betweenmeasured values and those estimated using the SARdeveloped in this study is usually very good, with10% of the molecules showing deviations larger than50%. In particular, the estimated rate constants ofethers and ketones are in better agreement withexperimental data than with previous SARs (Kwok andAtkinson, Atmos. Environ. 29, 1685–1695,1995). Rate constants of organic nitrates werenot well described by the SAR used in thisstudy. The basic assumption that the additive rateconstant for a chemical group is only influenced byneighbouring functional groups did not allow a goodparameterization for the rate constants of organicnitrates. The use of a second parameter to alter thereactivity of C-H bonds in β-position to thefunctional group resulted in markedly better agreementbetween calculated and measured rate constants, butwas not extended due to the limited set of data. This indicates that strong electron withdrawing groups(e.g., nitrate groups) might influence the reactivityof C-H bonds that are not directly adjacent.

Intercomparison of formaldehyde measurements in clean and polluted atmospheres

Abstract: Three different techniques used tomeasure atmospheric formaldehyde were compared duringa field campaign carried out at a clean maritime siteon the West coast of Ireland. Two spectroscopictechniques Differential Optical AbsorptionSpectroscopy (DOAS) and Tunable Diode Laser AbsorptionSpectroscopy (TDLAS), together with a glass coil/Hantzschreaction/fluorescence technique, wereemployed for measurements of atmospheric formaldehydeof the order of a few hundred pptv. The betteragreement was observed between the fluorescence andDOAS instruments.Two DOAS instruments were compared to the glasscoil/Hantzsch reaction/fluorescence technique at asemi-polluted site on the North Norfolk coast, U.K.,where concentrations of formaldehyde were observed atlevels up to 4 ppbv. A very good agreement wasobserved between the two instruments.The glass coil/Hantzsch reaction/fluorescence and theTDLAS instruments were also deployed simultaneously inorder to measure indoor air inside a mobile laboratorylocated at the Imperial College Silwood Park site nearAscot, U.K. The doors of the mobile laboratory wereleft open in order to obtain the backgroundformaldehyde concentrations. Closing them afterwardsallowed us to observe the increase in concentrationsas a result of indoor emissions. The agreement betweenthe two instruments was outstanding (correlationcoefficient was 99%).The results from this study showed that of the fourinstruments included in this intercomparison the glasscoil/Hantzsch reaction/fluorescence technique provedthe most suitable for continuous measurements offormaldehyde in the background atmosphere.

A study of DMS oxidation in the tropics : Comparison of Christmas Island field observations of DMS, SO2, and DMSO with model simulations

Abstract: This study reports comparisonsbetween model simulations, based on current sulfurmechanisms, with the DMS, SO2 and DMSOobservational data reported by Bandy et al.(1996) in their 1994 Christmas Island field study. For both DMS and SO2, the model results werefound to be in excellent agreement with theobservations when the observations were filtered so asto establish a common meteorological environment. Thisfiltered DMS and SO2 data encompassedapproximately half of the total sampled days. Basedon these composite profiles, it was shown thatoxidation of DMS via OH was the dominant pathway withno more than 5 to 15% proceeding through Cl atoms andless than 3% through NO3. This analysis wasbased on an estimated DMS sea-to-air flux of 3.4 ×109 molecs cm-2 s-1. The dominant sourceof BL SO2 was oxidation of DMS, the overallconversion efficiency being evaluated at 0.65 ± 0.15. The major loss of SO2 was deposition to theocean's surface and scavenging by aerosol. Theresulting combined first order k value was estimated at 1.6 × 10-5 s-1. In contrast to the DMSand SO2 simulations, the model under-predictedthe observed DMSO levels by nearly a factor of 50. Although DMSO instrument measurement problems can notbe totally ruled out, the possibility of DMSO sourcesother than gas phase oxidation of DMS must beseriously considered and should be explored in futurestudies.

Temporal Variability of Iron Speciation in Coastal Rainwater

Abstract: Iron occurs in rain as particulateand dissolved Fe and includes both Fe(II) and Fe(III)species. Model calculations and correlation analysisindicate Fe(II)(aq) occurs almost exclusively as thefree ion whereas Fe(III)(aq) occurs as both ironoxalate and Fe(OH)2 +(aq) with largevariations over the pH range from 4.0 to 5.0. Complexation with humic-like compounds may also beimportant for Fe(III)(aq); however, the concentrationand structural characteristics of these compounds haveyet to be determined. 112 rain samples were collectedfor iron analysis in Wilmington, North Carolina,between 1 July 1997, and 30 June 1999. Total iron,particulate iron and Fe(III)(aq) were higher inconcentration in summer and spring rain relative towinter and autumn rain. Fe(II)(aq) concentrations, incontrast, did not vary seasonally. Particulate iron,which was approximately half the total rainwater iron,was highest between noon and 6 p.m. (EST), probably dueto more intense regional convection including land-seabreezes during that time. The ratio ofFe(II)(aq)/Fe(III)(aq) was also highest in rainreceived between noon and 6 p.m., which most likelyreflects photochemical reduction of Fe(III)(aq)complexes to form Fe(II)(aq). A conceptual modeldepicting the interplay between iron species, lightintensity and organic ligands in rainwater ispresented.

An examination of the atmospheric chemistry of mercury using 210Pb and 7Be.

Abstract: Measurements of Hg (total gas-phase, precipitation-phase andparticulate-phase), aerosol mass, particulate 210Pb and7Be and precipitation 210Pb were made at an atmosphericcollection station located in a near remote area of northcentral Wisconsin,U.S.A. (46°10′N, 89°50′W) during the summers of 1993, 1994and 1995. Total Hg and 210Pb were observed to correlate strongly(slope = 0.06 ± 0.03 ng mBq-1; r 2 =0.72) in rainwater. Mercury to 210Pb ratios in particulate matter(0.03 ± 0.02 ng mBq-1; r 2 = 0.06) wereconsistent with the ratio in rain. Enrichment of the Hg/mass ratio (approx.5–50×) relative to soil and primary pollutant aerosols indicatedthat gas-to-particle conversion had taken place during transport. Comparisonof these results with models for the incorporation of Hg into precipitationindicates that atmospheric particles deliver more Hg to precipitation than canbe explained by the presence of soot. A lack of correlation between totalgas-phase Hg (TGM) and a 7Be/210Pb function suggests novertical concentration gradient within the troposphere, and allows an estimateof TGM residence time of 1.5 ± 0.6 yr be made based on surface airsamples.

Scavenging Efficiency of ‘Aerosol Carbon’ and Sulfate in Supercooled Clouds at Mt. Sonnblick (3106 m a.s.l., Austria)

Abstract: Cloud water and interstitial aerosol samples collected at Mt. Sonnblick (SBO) were analyzed for sulfate and ‘aerosol carbon’ to calculate in-cloud scavenging efficiencies. Scavenging efficiencies for sulfate (eSO) ranged from 0.52 to 0.99 with an average of 0.80. ‘Aerosol carbon’ was scavenged less efficiently with an average value (eAC) of 0.45 and minimum and maximum values of 0.14 and 0.81, respectively. Both eSO and eAC showed a marked, but slightly different, dependence on the liquid water content (LWC) of the cloud. At low LWC, eSO increased with rising LWC until it reached a relatively constant value of 0.83 above an LWC of ≈ 0.3 g/m3. In the case of ‘aerosol carbon’, we obtained a more gradual increase of eAC up to an LWC of ≈ 0.5 g/m3. At higher LWCs, e_ remained relatively constant at 0.60. As the differences between eSO and eA varied across the LWC range observed at SBO, we assume that part of the ‘aerosol carbon’ was incorporated into the cloud droplets independently from sulfate. This hypothesis is supported by size classified aerosol measurements. The differences in the size distributions of sulfate and total carbon point to a partially external mixture. Thus, the different chemical nature and the differences in the size and mixing state of the aerosol particles are the most likely candidates for the differences in the scavenging behavior.

Cluster Ion Reactions: Insights into Processes of Atmospheric Significance

Abstract: Experimental results of ion cluster reactions withatmospherically relevant species are considered hereinto provide a unified overview of the current state ofunderstanding brought about through experimentalstudies of cluster ion reactions. This isaccomplished by first outlining the atmospheric issuesaddressed by cluster ion studies and then, summarizingthe results of numerous studies. The recent, as wellas previously published studies of cluster ionreactions, are considered in the context of gainingnew insights into the molecular details ofheterogeneous processes involving the interactions ofelectrolytes with cloud droplets, ice crystals andaerosol particles. In addition to elucidating themechanisms of these and other selected reactions andtransformations of atmospheric significance, thefindings of the uptake of acid molecules in waterclusters are shown to lead to the suggestion of a newmodel that accounts for the formation of reactioncenters involving charged sites at aqueous surfaces.

Gas-Phase Formation of Water-Soluble Organic Compounds in the Atmosphere: A Retrosynthetic Analysis

Abstract: Atmospheric particles contain a myriad of organiccompounds, including many multifunctional,water-soluble organic compounds. Many of thesecompounds are postulated to be secondary of origin. This work investigates the possible precursors ofseveral classes of multifunctional, water-solublesecondary organic compounds by analyzing the pathwaysleading to their formation, based on known gas-phasereactions. The analysis is termed `retrosynthetic'due to the backward direction of the analysis, fromproducts to precursors. Pathways for multi-functionalcompounds were generated combinatorially, consideringthe formation of one functional group at a time.Many multifunctional organic compounds with carboxylicacid, carbonyl, and hydroxy functional groups werefound to be first- or second-generation products ofcommon anthropogenic and biogenic volatile organiccompounds such as alkanes, alkenes, aromatics, andcyclic alkenes. The estimated yields of water-solubleorganic compounds from primary precursors ranged fromless than 1% to over 10%, based on stoichiometricconsiderations. The SOA formation index, whichcombines the concepts of yields and rates, was used tocompare the feasibility of the retrosyntheticpathways. Many of the candidate pathways involve theisomerization reaction of alkoxy radicals andoxygenated intermediate products such asmonocarboxylic acids and hydroxyaldehydes.

John H. Seinfeld and Spyros N. Pandis: Atmospheric Chemistry and Physics, from Air Pollution to Climate Change

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Potential atmospheric transport pathways for mercury measured in the Canadian High Arctic.

Abstract: Large-scale transport pathways for atmospheric gaseous mercury emitted from surrounding source regions to reach the Alert sampling site located at the Northwest Territories of Canada between August 1992 and August 1993 were investigated. The 12-month atmospheric mercury data set comprises blank-corrected total gaseous mercury (TGM) concentrations collected on gold-coated sand traps during one-week long sampling intervals. For TGM concentrations above the 12-month mean TGM level, the analysis reveals regions in Eastern Europe (close to East Germany and Poland), areas on Northeastern America in Canada (Northwest Territories), and central Siberia of the former U.S.S.R. Identification of the physical locations of the emission sources was limited by the temporal resolution of weekly averaged data; however, major atmospheric pathways of mercury transport to the Arctic were successfully resolved. The objective of this note is to demonstrate that the potential source contribution function could be applied to resolve the sources and recover transport pathways of atmospheric mercury over a large scale.

Stratospheric photolysis frequencies: Impact of an improved numerical solution of the radiative transfer equation

Abstract: Numerical schemes for the calculation of photolysis rates are usually employed in simulations of stratospheric chemistry. Here, we present an improvement of the treatment of the diffuse actinic flux in a widely used stratospheric photolysis scheme (Lary and Pyle, 1991). We discuss both the consequences of this improvement and the correction of an error present in earlier applications of this scheme on the calculation of stratospheric photolysis frequencies. The strongest impact of both changes to the scheme is for small solar zenith angles. The effect of the improved treatment of the diffuse flux is most pronounced in the lower stratosphere and in the troposphere. Overall, the change in the calculated photolysis frequencies in the region of interest in the stratosphere is below about 20%, although larger deviations are found for H2O, O2, NO, N2O, and HCl.

Chemical Composition of Wet Precipitation in Irbid, Jordan

Abstract: Rainwater samples were collected in Irbid city using 24 hour sampling periods from December 1996 to April 1998. All samples were analyzed for major cations (Na+, K+, Ca2+ and Mg2+), major anions (Cl−, NO3− and SO42−) and pH. High levels of Ca2+ and SO42− were observed. Together, Ca2+ and SO42− made up more than 52.4% of the total ion mass, while Ca2+ alone contributed over 39.0% of the total cation. The majority of the rain samples collected had pH values higher than 5.6. The average pH was 6.4±0.9. High values of pH are attributed to the neutralization by natural alkaline local dusts which contain large fractions of calcite. Correlation and mineralogical analyses indicated that Ca2+, K+ and fractions of Na+, SO42− and Mg2+ are of crustal origin. Results of the present study suggested that the atmospheric composition in the city is strongly influenced by natural sources rather than anthropogenic.

A Chamber Investigation of Nitric Acid-Soot Aerosol Chemistry at 298 K

Abstract: Long-pathlength infrared absorption spectroscopy wasused to investigate nitric acid-soot aerosol chemistryat 298 K and 0.5% relative humidity. Experimentswere performed by introducing nitric acid vapor(PHNO3 ∼ 3 Pa, Ptotal ∼ 40 kPa) intoateflon-coated chamber and initiating acquisition ofinfrared spectra at 3 minute time intervals. After 36minutes of data collection, soot powder was rapidlyexpanded into nitric acid contained in the chamber togenerate a soot-HNO3 aerosol. Infrared spectracollected before, and after, soot introduction to thechamber were used to characterize chamber wallreaction processes and soot aerosol chemistry,respectively. Three soot types were investigated(Degussa FW2, Cabot Monarch 1000, and crystallinegraphite), each yielding similar chemistry. Upon sootintroduction to the chamber both HNO3 uptake andNO2 production occurred, with the molar ratio ofHNO3 uptake to NO2 production varying from1.2 to 2.9 for the three soot types studied. Unreacted HNO3 was present at the conclusion ofeach of the aerosol experiments, indicating incompleteconversion of HNO3 into NO2. Thisobservation suggested that `active' sites at the sootsurface responsible for the reduction of HNO3 arenot regenerated (i.e., formed) in the reactionprocess. In essence, a titration occurred betweenthese active sites and HNO3. The NO2concentrations produced, the soot mass concentrationsused, and the BET measured specific surface area ofthe powders allowed computation of the surface densityof active sites of ∼4.0 × 10-18 m2/active site(describing all three powders studied). This is thefirst reported measurement of surface density ofactive sites for nitric acid chemistry on soot. Sinceatmospheric heterogeneous reactions that exhibitsurface deactivation may, in principle, affect tracegas concentration, we perform an assessment in thisregard.

Single particle analysis of aerosols. observed in the marine boundary layer during the Monterey Area Ship Tracks experiment (MAST), with respect to cloud droplet formation

Abstract: The chemical composition of individual particles >0.2 μm sampled duringthe MAST-experiment wereanalysed by SEM-EDX, in combination with multivariatetechniques. The objective of this experiment was toidentify the mechanisms responsible for themodification of marine stratocumulus clouds byemissions from ships and in a wider sense to provideinformation on the global processes involved inatmospheric modification of cloud albedo. Aerosolswere examined under different MBL pollution levels(clean, intermediately polluted and moderatelypolluted) in five different reservoirs: backgroundbelow-cloud and above-cloud aerosol; background clouddroplet residual particles; below-cloud ship plumeaerosol and ship track cloud droplet residualparticles.In this study a relation was provided between theaerosol emitted from the ship's stack to an effect incloud. Additionally, a large fraction of the ambientaerosol was found to be composed of organic materialor other compounds, consisting of low Z-elements,associated with chlorine. Their number fraction waslargest in clean marine boundary layers, and decreasedwith increasing pollution levels. The fraction of`transformed sea salt' (Na, Cl, S), on the other hand,increased with the pollution level in the MBL. Only20% of the particles fell within the detectable rangeof the analysis.

The Reactivity of Biogenic Monoterpenes towards OH· and SO4-· Radicals in De-Oxygenated Acidic Solution

Abstract: The reactivity of some selected biogenic monoterpenecompounds towards important aqueous phase free-radicaloxidants, namely OH· and SO4 -·, have beeninvestigated using the complementary experimentaltechniques of pulse radiolysis and laser flashphotolysis (λ = 248 nm). Rate constants forthe reactions of the OH· radical with cis-verbenol andmethacrolein have been determined to be (6.8 ± 0.5) ×109 dm3 mol-1 s-1 and (8.0± 0.7) × 109 dm3 mol-1s-1,respectively (T = 20 °C, pH 4.0, Ionic strength∼ 0 mol dm-3). Rate constants and activationenergies for the reactions of the SO4 -·radical have been measured for the following compounds(T = 20 °C, pH 4.0, Ionic strength = 0.03 moldm-3): α-pinene (k = (3.1 ± 0.1) ×109 dm3 mol-1 s-1;E act. =(8.9 ± 1.3) kJ mol-1), α-terpineol(k = (4.1 ± 0.1) × 109 dm3mol-1s-1; E act. = (13.4 ± 0.6) kJmol-1), cis-verbenol (k = (3.2 ± 0.2) ×109 dm3 mol-1 s-1;E act. =(10.0 ± 0.7) kJ mol-1), verbenone (k = (1.6± 0.1) × 109 dm3 mol-1s-1;E act. = (6.1 ± 0.7) kJ mol-1), myrtenal(k = (1.85 ± 0.1) × 109 dm3mol-1s-1; E act. = (7.5 ± 0.7) kJmol-1)and methacrolein (k = (1.18 ± 0.1) × 109dm3 mol-1 s-1). In most instances theabsorption spectra of the intermediate products formedby these reactions have been measured which, inconjunction with strategic conductiometric studies,have been used to suggest plausible mechanisms for theoxidation in acidic de-oxygenated solution.

The Influence of Oxalate on Fe-Catalyzed S(IV) Oxidation by Oxygen in Aqueous Solution

Abstract: This study demonstrates that oxalate has a strong inhibiting effect onFe-catalyzed S(IV) oxidation by oxygen in aqueous solution. While thepseudo-first order rate constant of S(IV) oxidation was determined to be1.6 × 103 M-1 s-1 in experimentswithout oxalate, the oxidation of S(IV) was totally inhibited at a molarconcentration ratio of iron:oxalate = 1:5 at an oxalate concentration of 4μM. Under these conditions, the Fe(II)/Fe(III) ratio remained nearlyconstant during the observed reaction time. The determined rate constants wereindependent of the initial oxidation state of iron. However, with increasingconcentrations of oxalate, a longer induction period is observed forexperiments with iron initially in the Fe(II) oxidation state.

Kinetic Studies of OH and O3 Reactions with Allyl and Isopropenyl Acetate

Abstract: Rate coefficients have been measured for the gas phasereactions of hydroxyl (OH) radicals and ozone with twounsaturated esters, allyl acetate(CH3C(O)OCH2CH=CH2) and isopropenylacetate (CH3C(O)OC(CH3)=CH2). The OHexperiments were carried out using the pulsed laserphotolysis – laser induced fluorescence technique overthe temperature range 243–372 K and the kinetic dataused to derive the following Arrhenius expressions (inunits of cm3 molecule-1 s-1): allylacetate, k 1 = (2.33 ± 0.27) ×10-12 exp[(732 ± 34)/T]; and isopropenyl acetate,k 2 = (4.52 ± 0.62) × 10-12exp[(809 ± 39)/T]. At 298 K, the rate coefficients obtained (inunits of 10-12 cm3 molecule-1 s-1)are: k 1 = (27.1 ± 3.0) and k 2= (69.6± 9.4). The relative rate technique has been usedto determine rate coefficients for the reaction ofozone with the acetates. Using methyl vinyl ketone asthe reference compound and a value of4.8 × 10-18 cm3 molecule-1s-1 asthe rate coefficient for its reaction with O3,the following rate coefficients were derived at 298 ± 4 K (in units of10-18 cm3molecule-1 s-1): allyl acetate, (2.4 ± 0.7) andisopropenyl acetate (0.7 ± 0.2). Theresults are discussed in terms of structure-activityrelationships and used to derive atmospheric lifetimesfor the acetates.