Aqueous Oxidation of Sulfur(IV) Catalyzed by Manganese(II): A Generalized Simple Kinetic Model

The reaction kinetics of S(IV) autoxidation catalyzed by Mn(II) in the pH range 3–5 typical for atmospheric liquid water, was investigated. For reactions with pH maintained constant during the reaction course, the predictions obtained by a simple integral approach cover kinetic results only for concentrations of HSO ₃ ⁻ up to 0.2 mM at pH 4.5. Thus, a generalized simple kinetic model, which can be used for predicting the reaction kinetics in wider concentration, pH and temperature ranges, was derived. This model is based on the assumption that the reaction rate is proportional to the concentration of a transient manganese-sulfito complex formed in the initial step of a radical chain mechanism. In the proposed power law rate equation

Multi-Phase Chemistry of C<Subscript>2</Subscript> and C<Subscript>3</Subscript> Organic Compounds in the Marine Atmosphere

A box model is used to explore the detailed chemistry of C₂ and C₃ organic compounds in the marine troposphere by tracing the individual reaction paths resulting from the oxidation of ethane, ethene, acetylene, propane, propene and acetic acid. The mechanisms include chemical reactions in the gas phase and in the aqueous phase of clouds and aerosol particles at cloud level under conditions resembling those in the northern hemisphere. Organic hydroperoxides are found to be important intermediate products, with subsequent reactions leading partly to the formation of mixed hydroxy or carbonyl hydroperoxides that are readily absorbed into cloud water, where they contribute significantly to the formation of multifunctional organic compounds and organic acids. Organic hydroperoxides add little to the oxidation of sulfur dioxide dissolved in the aqueous phase, which is dominated by H₂O₂. Next to acetaldehyde and acetone, glycol aldehyde, glyoxal, methyl glyoxal and hydroxy propanone are prominent oxidation products in the gas and the aqueous phase. Acetaldehyde is not efficiently converted to acetic acid in clouds; the major local sources of acetic acid are gas-phase reactions. Other acids produced include hydroperoxy acetic, glycolic, glyoxylic, oxalic, pyruvic, and lactic acid. The mechanism of Schuchmann et al. (1985), which derives glycolic and glyoxylic acid from the oxidation of acetate, is found unimportant in the marine atmosphere. The principal precursors of glyoxylic acid are glyoxal and glycolic acid. The former derives mainly from acetylene and ethene, the latter from glycolaldehyde, also an oxidation product of ethene. The oxidation of glyoxylic acid leads to oxalic acid, which accumulates and is predicted to reach steady state concentrations in the range 30–90 ng m⁻³. This is greater, yet of the same magnitude, than the concentrations observed over the remote Pacific Ocean.     

Processing of oxidised nitrogen compounds by passage through winter-time orographic cloud

Four case studies are described, from a three-site field experiment in October/November 1991 using the Great Dun Fell flow-through reactor hill cap cloud in rural Northern England. Measurements of total odd-nitrogen nitrogen oxides (NO _y ) made on either side of the hill, before and after the air flowed through the cloud, showed that 10 to 50% of the NO _y , called NO _z , was neither NO nor NO₂. This NO _z failed to exhibit a diurnal variation and was often higher after passage through cloud than before. No evidence of conversion of NO _z to NO₃ ^- in cloud was found. A simple box model of gas-phase chemistry in air before it reached the cloud, including scavenging of NO₃ and N₂O₅ by aerosol of surface area proportional to the NO₂ mixing ratio, shows that NO₃ and N₂O₅ may build up in the boundary layer by night only if stable stratification insulates the air from emissions of NO. This may explain the lack of evidence for N₂O₅ forming NO₃ ^- in cloud under well-mixed conditions in 1991, in contrast with observations under stably stratified conditions during previous experiments when evidence of N₂O₅ was found. Inside the cloud, some variations in the calculated total atmospheric loading of HNO₂ and the cloud liquid water content were related to each other. Also, indications of conversion of NO _x to NO _z were found. To explain these observations, scavenging of NO _x and HNO₂ by cloud droplets and/or aqueous-phase oxidation of NO₂ ^- by nitrate radicals are considered. When cloud acidity was being produced by aqueous-phase oxidation of NO _x or SO₂, NO₃ ^- which had entered the cloud as aerosol particles was liberated as HNO₃ vapour. When no aqueous-phase production of acidity was occurring, the reverse, conversion of scavenged HNO₃ to particulate NO₃ ^-, was observed.     

Impact of cloud dynamics on tropospheric chemistry: Advances in modeling the interactions between microphysical and chemical processes

A chemical module describing the tropospheric photochemistry of ozone precursors in both gaseous and aqueous phases for a remote continental atmosphere has been developed within the framework of a two-dimensional cloud model. Dynamical, microphysical and chemical processes are fully interacting in order to study the influence of clouds on ozone chemistry and to quantify the relative importance of the different processes on the budget and evolution of 12 chemical species. Whereas the concentrations of highly soluble species are strongly affected by evaporation and sedimentation, less soluble species are affected primarily by accretion. The model reproduces previously observed chemical phenomena such as the enrichment of formic acid at the top of the cloud.     

Simulations of seasonal variations of sulfur compounds in the remote marine atmosphere

A photochemical box model is used to simulate seasonal variations in concentrations of sulfur compounds at latitude 40° S. It is assumed that the hydroxyl radical (OH) addition reaction to sulfur in the dimethyl sulfide (DMS) molecule is the predominant pathway for methanesulfonic acid (MSA) production, and that the rate constant increases as the air temperature decreases. Concentration of the nitrate radical (NO₃) is a function of the DMS flux, because the reaction of DMS with NO₃ is the most important loss mechanism of NO₃. While the diurnally averaged concentration of OH in winter is a factor of about 8 smaller than in summer, due to the weak photolysis process, the diurnally averaged concentration of NO₃ in winter is a factor of about 4–5 larger than in summer, due to the decrease of DMS flux. Therefore, at middle and high latitudes in winter, atmospheric DMS is mainly oxidized by the reaction with NO₃. The calculated ratio of the MSA to SO₂ production rates is smaller in winter than in summer, and the MSA to non-sea-salt sulfate (nssSO₄ ^2-) molar ratio varies seasonally. This result agrees with data on the seasonal variation of the MSA/nssSO₄ ^2- molar ratio obtained at middle and high latitudes. The calculations indicate that during winter the reaction of DMS with NO₃ is likely to be a more important sink of NO_x (NO+NO₂) than the reaction of NO₂ with OH, and to serve as a significant pathway of the HNO₃ production. If dimethyl sulfoxide (DMSO) is produced through the OH addition reaction and is heterogeneously oxidized in aqueous solutions, half of the nssSO₄ ^2- produced in summer may be through the oxidation process of DMSO. It is necessary to further investigate the oxidation products by the reaction of DMS with OH, and the possibility of the reaction of DMS with NO₃ during winter.     

Urban and rural observations of carboxylic acids in rainwater in Southwest of China: the impact of urbanization

Seven carboxylic acids in rainwater were simultaneously determined using ion chromatography for 13 months in two different sites, Guiyang and Shangzhong, southwest of China. Results showed formic, acetic and oxalic acids were the three predominant carboxylic acids. Their volume-weighted average concentrations were 14.24, 9.35, 2.79 μmol/L in Guiyang and 4.95, 1.35, 2.31 μmol/L in Shangzhong, respectively. A distinctive diurnal pattern in carboxylic acid concentrations (daytime>nighttime, t test, p < 0.05) was observed during the growing season in Guiyang. Shangzhong witnessed higher concentration of these acids during the growing season than that during the non-growing season. Direct emissions from growing vegetation or soils probably account for the main provenance of the acids in the rural area. However, the opposite trend were found in Guiyang and the anthropogenic sources during the non-growing season were the main reason. By comparison of our result with the previous data about 20 years ago, we calculated that at least 42% of acetic acids and 69% of formic acid originated from the anthropogenic sources in Guiyang. Furthermore, the ratio of formate/acetate in gas phase larger than 1 suggest the oxidation of unsaturated hydrocarbons from the human activity and/or natural sources were the main origin of carboxylic acids in Guiyang. While The F/A ratio in gas phase was less than 1 in Shangzhong which indicate the direct emissions from biogenic sources. Oxalic acid was in similar amounts in both sites, indicating the common source of the acid.     

基于CMAQ-MCM模型的长三角地区夏季臭氧和大气氧化性影响的研究

传统的空气质量模型多使用简化的光化学反应机制来模拟大气污染物的形成.这些机制主要基于烟雾箱实验拟合的反应速率和产物来模拟二次产物（如臭氧（O₃））前体物的氧化反应,具有一定的不确定性,导致模拟结果产生偏差.针对该问题,本研究将详细的大气化学机理（MCMv3.3.1）与美国国家环境保护局研制的第三代空气质量预报和评估系统CMAQ相结合（CMAQ-MCM）,模拟研究长三角地区2015年8月27—9月5日臭氧高发时段的空气质量.CMAQ-MCM模型可以较好地模拟长三角地区6个代表城市O₃和其前体物随时间的变化趋势.对模拟的O₃日最大8 h平均浓度的统计分析表明,徐州表现最好（标准平均误差=-0.15,标准平均偏差=0.23）.在长三角地区,居民源对挥发性有机物（VOCs）的贡献最大,占39.08%,其次是交通运输（33.25%）和工业（25.56%）.能源对总VOCs的贡献最小,约为2.11%.对活性氧化氮（NO_y）的分析表明,其主要组分是NO_x（80%）,其次是硝酸（HNO₃）（<10%）.O₃的空间分布与NO_y和NO_x非常相似.HCHO等其他氧化产物的分布与NO_x相似,这很可能是由于在高NO_x条件下VOCs氧化产生的产物.甲基乙烯基酮（MVK）和甲基丙烯醛（MACR）的空间分布与自然源VOCs （BVOCs）非常相似,表明长三角地区MVK和MACR主要由BVOCs氧化生成.长三角地区受到人为源和自然源排放相互作用的影响.     

Measurements of stratospheric HO2 and NO2 by matrix isolation and ESR spectroscopy

Vertical profiles of stratospheric HO₂ and NO₂ concentrations were determined using matrix isolation and ESR. Up to 10 different samples per flight were collected in situ by a balloon borne cryosampler. Free radicals and trace constituents which are condensable at 68 K are trapped in a polycristalline H₂O or D₂O matrix. After collection, the samples are stored at a temperature below 83 K until they are analysed in the laboratory by X-band ESR spectroscopy at 4 K. The HO₂ and NO₂ were identified and calibrated by comparison with standard samples collected in the laboratory under typical stratospheric sampling conditions. From several flights over Southern France (44°N) we obtained two profiles of the stratospheric NO₂ mixing ratio. One, from 21 October 1982, agrees well with previous measurements. The other, from 8 October 1981, is lower by one order of magnitude. The few HO₂ data obtained around 35 km altitude agree with previous measurements. An isolated measurement at 17 km altitude is one order of magnitude higher than the model predicted HO₂ concentration.     

The influence of light intensity,SO2, NOx and C3H6 on sulfate aerosol formation

The photochemical oxidation of SO₂ in the presence of NO and C₃H₆ was studied in a 18.2 liter pyrex reactor. When light intensity, irradiation time and SO₂ concentration were constant, SO₄ ^2- concentration, derived from the total volume of aerosol produced, peaked when [C₃H₆]/[NO] was approximately 6.0. Another increase im SO₄ ^2- formation was reached at very high ratios (>50). The experimental observations are consistent with the two SO₂ oxidation mechanisms. At low [C₃H₆]/[NO] ratios, the processes proceed via the HO–SO₂ reaction, while at high ratios the O₃–C₃H₆ adduct is assumed to oxidize SO₂ to produce SO₄ ^2- aerosols.     

Behavior of atmospheric formic and acetic acid in the presence of hydrometeors

The partitioning of formic and acetic acid between the atmospheric liquid and gaseous phase is modelled for a range of liquid water contents. At low liquid water content, formic acid is dissolved preferentially over acetic acid. Applying these results to the analysis of processes taking place in clouds, one can explain the frequently found enrichment of formic over acetic acid in rainwater, which results from selective transport by washout. We assess the ability of dew to act as a temporary sink and source for organic acids, and propose that the diel variation of mixing ratios often found during surface measurements, may in part be due to the dissolution in dew and subsequent evaporation on the following day.     

Modified HNO3 seasonality in volcanic layers of a polar ice core: Snow-pack effect or photochemical perturbation?

Using the chemical composition of snow and ice of a central Greenland ice core, we have investigated changes in atmospheric HNO₃ chemistry following the large volcanic eruptions of Laki (1783), Tambora (1815) and Katmai (1912). The concentration of several cations and anions, including SO ₄ ^2– and NO ₃ ^– , were measured using ion chromatography. We found that following those eruptions, the ratio of the concentration of NO ₃ ^– deposited during winter to that deposited during summer was significantly higher than during nonvolcanic periods. Although we cannot rule out that this pattern originates from snow pack effects, we propose that increased concentrations of volcanic H₂SO₄ particles in the stratosphere may have favored condensation and removal of HNO₃ from the stratosphere during Arctic winter. In addition, this pattern might have been enhanced by slower formation of HNO₃ during summer, caused by direct consumption of OH through oxidation of volcanic SO₂.     

Characteristics of ionic components in precipitation in Kitakyushu City,Japan

Precipitation samples were collected by filtrating bulk sampler in Kitakyushu City, Japan, from January 1988 to December 1990. Volume weighted annual mean of pH was 4.93, but the pH distribution indicated that most probable value lay in the range pH 6.0–6.4. Volume weighted annual mean concentrations of major ionic components were as follows; SO ₄ ^2– : 84.2, NO ₃ ^– : 28.1, Cl^–: 86.3, NH ₄ ⁺ : 45.5, Ca²⁺: 63.3, Mg²⁺: 27.0, K⁺: 3.4, Na⁺: 69.0 µ eq l^–1. The highest concentrations of these ionic components were observed in winter and the lowest occurred in the rainy season. The ratio of ex-SO ₄ ^2– /NO ₃ ^– exhibited the lowest ratio in summer, and the highest ratio in winter. Good correlations were obtained between Cl^– and Na⁺, ex-SO ₄ ²⁺ and ex-Ca²⁺, NO ₃ ^– and ex-Ca²⁺, and NH ₄ ⁺ and ex-SO ₄ ^2– , respectively. However, no correlation between Cl^– and Na⁺ with Ca²⁺ was observed. The relationship of H⁺ with (ex-SO ₄ ^2– + NO ₃ ^– ) - (ex-Ca²⁺ + NH ₄ ⁺ ) indicated positive correlation.     

S(IV) Autoxidation in Atmospheric Liquid Water: The Role of Fe(II) and the Effect of Oxalate

The reactivity of dissolved iron compounds towards different pollutants and photooxidants in atmospheric liquid water depends upon the oxidation state and speciation of iron. Our measurements of the oxidation state of dissolved iron eluted from aerosol particles (Dae: 0.4–1.6 m) collected in the urban atmosphere of Ljubljana showed that a large fraction of the iron content is present as Fe(II). The concentration ratio [Fe(II)]/[Fe(III)] varied between 0.9 and 3.1. The kinetics of S(IV) autoxidation catalyzed by Fe(II) under the conditions representative for acidified atmospheric liquid water and the influence of oxalate on this reaction under dark conditions was investigated. The reaction rate is the same if Fe(II) or Fe(III) is used as a catalyst under the condition that Fe(II) can be oxidized in Fe(III), which is the catalytically active species. Oxalate has a strong inhibiting effect on the S(IV) autoxidation in the presence of Fe(II). The reaction is autocatalytic with an induction period, that increases with higher concentrations of oxalate. The inhibiting effect of oxalate differs according to whether iron is initially in the Fe(II) or Fe(III) state. However, in both cases the inhibition by oxalate is a result of the formation of complexes with the catalyst.     

Products and Mechanism of the NO<Subscript>3</Subscript> Reaction with Thiophene

Products of the gas-phase reaction of the NO₃ radical with thiophene have been investigated using different experimental systems. On the one hand, experiments have been conducted in our laboratory using two different methods, a Teflon static reactor coupled to a gas chromatograph combined with mass-spectrometry (GC-MS) and a discharge flow tube with direct MS spectroscopic detection. A qualitative analysis in these cases indicates that possible products for the reaction of thiophene+NO₃ at room temperature include: sulphur dioxide, acetic and formic acids, a short-chain aldehyde, 2-nitrothiophene and 3-nitrothiophene. On the other hand, quantitative experiments have been performed in the European Photoreactor (EUPHORE) in Valencia, Spain. In this case, the major products were: HNO₃ (≈80%), nitrothiophenes (≈30%), SO₂ (≈20%), propanal (3%) and a fraction of particles (≈10%). The results obtained indicate that at least 70% of the reaction of NO₃ with thiophene proceeds by an H-abstraction process at room temperature. The mechanism of the reaction studied is proposed on the basis of experimental results.     

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

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.     

Products and mechanisms of the gas phase reactions of NO3 with CH3SCH3, CD3SCD3, CH3SH and CH3SSCH3

Products and mechanisms of the reaction between the nitrate radical (NO₃) and three of the most abundant reduced organic sulphur compounds in the atmosphere (CH₃SCH₃, CH₃SH and CH₃SSCH₃), have been studied in a 480 L reaction chamber using in situ FT-IR and ion chromatography as analytical techniques. In the three reactions, methanesulphonic acid was found to be the most abundant sulphur containing product. In addition the stable products SO₂, H₂SO₄, CH₂O, and CH₃ONO₂ were identified and quantified and thionitric acid-S-methyl ester (CH₃SNO₂) was observed in the i.r. spectrum from all of the three reactions. Deuterated dimethylsulphide (CD₃SCD₃) showed an isotope effect on the reaction Deuterated dimethylsulphide (CD₃SCD₃) showed an isotope effect on the reaction rate constant (k_H/k_D) of 3.8±0.6, indicating that hydrogen abstraction is the first step in the NO₃+CH₃SCH₃ reaction, probably after the formation of an inital adduct.Based on the products and intermediates identified, reaction mechanisms are proposed for the three reactions.     

A Temperature-Dependent Study of the Ozonolysis of Propene

The ozonolysis of propene has been investigated in a temperature controlled reaction chamber at 295, 260, and 230 K. Experiments were performed using a total zero air pressure of 760 Torr (STP) and propene/ozone reactant mixing ratios ranging from 2.3 to 23 ppmv. An analysis of FTIR spectra collected at the conclusion of each reaction revealed that methane was formed with a yield of 0.14 ± 0.03 (precision) for all the temperatures investigated.In addition, the yield of HCHO decreased from 0.67 ± 0.04 to 0.43± 0.03 upon cooling from 295 to 230 K, whereas the yield of HCOOH increased from 0.11 ± 0.02 to 0.53 ± 0.04. Experiments were also performedusing an excess of cyclohexane (to scavenge OH) and it was found that the formaldehyde yield was 0.79 ± 0.05 and 0.61 ± 0.04 at 295 and260 K, respectively. Finally, to more fully understand the reaction energies involved in product formation, we have performed molecular orbital calculations of heats of formation of reactants, stable intermediates, and products. Three conclusions can be made of this work. First, the reaction CH₂OO + Aldehyde Secondary Ozonide HCOOH + Aldehyde is not an important mechanism in formic acid production. Second, the decomposition of the primary ozonide products (e.g., C₂ radical species) appears to occur, in part, by a thermal mechanism (e.g., thermalized to chamber temperature). Third, ab initio resultscombined with experiment reveal no correlation between reaction exothermicity and products formed (e.g., kinetically dictated product formation occurs). The abinitio database is provided nevertheless as a starting point for transition state calculations to be performed in the future. Finally, since formaldehyde yield decreases by at most 35% with decreasing temperature and formic acid is relatively unreactive in the atmosphere, our results suggest that temperature-dependent HCHO yield will constitute only a minor perturbation to HO_x formation in the middle troposphere.     

NO<Subscript>3</Subscript> Vertical Profile Measurements from Remote Sensing Balloon-Borne Spectrometers and Comparison with Model Calculations

Eleven vertical profiles of stratospheric NO₃ have been obtained since 1992 using the AMON and SALOMON balloon-borne UV-visible spectrometers. The measurements are compared to the SLIMCAT 3D model and calculations based on the steady-state hypothesis for NO₃. The calculations cannot reproduce some parts of the profiles which exhibit strong concentration fluctuations over few kilometres, as a consequence of the dependence of NO₃ on local temperature variations. A statistical use of the data allows us to estimate the influence of the temperature dependence of the absorption cross-section on the data analysis, and the validity of the recommended reaction rates available in the literature. Discrepancies exist between the model based on recommended kinetics and observations at warmer temperatures. Nevertheless, the analysis is biased by local temperature inhomogeneities, and only a low-resolution vertical shape of the NO₃ profiles can be retrieved.     

Influence of traffic emissions on the composition of atmospheric particles of different sizes—Part 2: SEM–EDS characterization

In urban areas traffic is the major contributor to atmospheric particulate matter and exposure to these particles currently represents a serious risk to human health. The attention has been recently focused more on the particles of smaller sizes (PM_2.5) which penetrate deeper in respiratory system causing severe health effects. Therefore, more information on PM_2.5 should be provided, namely concerning morphological and chemical characterization. Aiming further evaluation of the impact of traffic emissions on public health, this work evaluated the influence of traffic on the chemical and morphological characteristics of PM₁₀ and PM_2.5, collected at one site influenced by traffic emissions and at one reference site. Chemical and morphological characteristics of 1,000 individual particles were determined by scanning electron microscopy combined with energy dispersive spectrometer (SEM–EDS). Cluster analysis (CA) was used to identify different types of particles that occurred in PM, aiming the identification of the respective emission sources. Traffic PM_2.5 were dominated by particles composed of Fe oxides and alloys (67%) which were related to traffic emissions (this percentage was 3.7 times higher than at the background site); in PM_2.5–10 the abundance of Fe oxides and alloys were 20% and 0% for the traffic and background sites, respectively. Background PM_2.5 were mainly constituted by aluminum silicates (63%) related to natural sources (this percentage was 2.5 times higher than at the traffic site); the abundances of aluminum silicates in PM_2.5–10 were 74% and 73% for traffic and background sites, respectively. It was concluded that traffic emissions were mainly present in PM_2.5 (the percentage of particles associated to these emissions was 3.4 times higher than in PM_2.5–10), while coarse particles were dominated by material of natural origin (the percentage of particles associated was 1.2 and 3.0 times higher than in PM_2.5 for traffic and background sites, respectively). Previous results obtained by proton induced X-ray emission (PIXE) were consistent with SEM–EDS analysis that showed to be very useful to complement elemental analysis of different PM_2.5 and PM_2.5–10.