SOA Formation Research Articles

Estimations of primary and secondary organic carbon formation in PM 2.5 aerosols of Santiago City, Chile

High concentration of fine airborne particulates is considered one of the major environmental pollutants in Santiago, the Chilean Capital city, which in 1997 was declared a PM 10 saturated zone. To date there is no control of the amounts of fine and coarse aerosols concentrations and the source and chemical characterizations of the PM 2.5 particulates in the carbonaceous fractions are not well known even though this fraction could be represented almost the 50% in mass of the PM 2.5. In this work, we present for the first time determinations of primary organic aerosol (POA) and secondary organic aerosol composition (SOA) fractions of the total mass of PM 2.5 particulates collected in the urban atmosphere of Santiago City. Our purpose is to know the anthropogenic contributions to the formation of SOA. To accomplish this we used the elemental carbon (EC) and organic carbon (OC) determinations developed by automatic monitoring stations installed in the city during the period 2002–2005, with a particular analysis of the summer time occurred in February 2004. Based on the EC tracer method, we have estimated the POA and SOA fraction and our data permit us to estimate the SOA reaching up to 20% of total organic aerosol matter, in good agreement to other measurements observed in large cities of Europe and U.S.A.

Secondary organic aerosol production from aqueous photooxidation of glycolaldehyde: Laboratory experiments

Organic particulate matter (PM) formed in the atmosphere (secondary organic aerosol; SOA) is a substantial yet poorly understood contributor to atmospheric PM. Aqueous photooxidation in clouds, fogs and aerosols is a newly recognized SOA formation pathway. This study investigates the potential for aqueous glycolaldehyde oxidation to produce low volatility products that contribute SOA mass. To our knowledge, this is the first confirmation that aqueous oxidation of glycolaldehyde via the hydroxyl radical forms glyoxal and glycolic acid, as previously assumed. Subsequent reactions form formic acid, glyoxylic acid, and oxalic acid as expected. Unexpected products include malonic acid, succinic acid, and higher molecular weight compounds, including oligomers. Due to (1) the large source strength of glycolaldehyde from precursors such as isoprene and ethene, (2) its water solubility, and (3) the aqueous formation of low volatility products (organic acids and oligomers), we predict that aqueous photooxidation of glycolaldehyde and other aldehydes in cloud, fog, and aerosol water is an important source of SOA and that incorporation of this SOA formation pathway in chemical transport models will help explain the current under-prediction of organic PM concentrations.

Correlations between Water-Soluble Organic Aerosol and Water Vapor: A Synergistic Effect from Biogenic Emissions?

Ground-based measurements of meteorological parameters and water-soluble organic carbon in the gas(WSOCg) and particle (WSOCp) phases were carried out in Atlanta, Georgia, from May to September 2007. Fourteen separate events were observed throughout the summer in which WSOCp and water vapor concentrations were highly correlated (average WSOCp-water vapor r = 0.92); however, for the entire summer, no well-defined relationship existed between the two. The correlation events, which lasted on average 19 h, were characterized by a wide range of WSOCp and water vapor concentrations. Several hypotheses for the correlation are explored, including heterogeneous liquid phase SOA formation and the co-emission of biogenic VOCs and water vapor. The data provide supporting evidence for contributions from both and suggest the possibility of a synergistic effect between the co-emission of water vapor and VOCs from biogenic sources on SOA formation. Median WSOCp concentrations were also correlated with elemental carbon (EC), although this correlation extended over the entire summer. Despite the emission of water vapor from anthropogenic mobile sources and the WSOCp-EC correlation, mobile sources were not considered a potential cause for the WSOCp-water vapor correlations because of their low contribution to the water vapor budget. Meteorology could perhaps have influenced the WSOCp-EC correlation, but other factors are implicated as well. Overall, the results suggest that the temperature-dependent co-emission of water vapor through evapotranspiration and SOA precursor-VOCs by vegetation may be an important process contributing to SOA in some environments.

Correlation of secondary organic aerosol with odd oxygen in Mexico City

Photochemically processed urban emissions were characterized at a mountain top location, free from local sources, within the Mexico City Metropolitan Area. Analysis of the Mexico City emission plume demonstrates a strong correlation between secondary organic aerosol and odd oxygen (O3 + NO2). The measured oxygenated‐organic aerosol correlates with odd oxygen measurements with an apparent slope of (104–180) μg m−3 ppmv−1 (STP) and r2 > 0.9. The dependence of the observed proportionality on the gas‐phase hydrocarbon profile is discussed. The observationally‐based correlation between oxygenated organic aerosol mass and odd oxygen may provide insight into poorly understood secondary organic aerosol production mechanisms by leveraging knowledge of gas‐phase ozone production chemistry. These results suggest that global and regional models may be able to use the observed proportionality to estimate SOA as a co‐product of modeled O3 production until more complete models of SOA formation become available.

Gas/particle partitioning of carbonyls in the photooxidation of isoprene and 1,3,5-trimethylbenzene

Abstract. A new denuder-filter sampling technique has been used to investigate the gas/particle partitioning behaviour of the carbonyl products from the photooxidation of isoprene and 1,3,5-trimethylbenzene. A series of experiments was performed in two atmospheric simulation chambers at atmospheric pressure and ambient temperature in the presence of NOx and at a relative humidity of approximately 50%. The denuder and filter were both coated with the derivatizing agent O-(2,3,4,5,6-pentafluorobenzyl)-hydroxylamine (PFBHA) to enable the efficient collection of gas- and particle-phase carbonyls respectively. The tubes and filters were extracted and carbonyls identified as their oxime derivatives by GC-MS. The carbonyl products identified in the experiments accounted for around 5% and 10% of the mass of secondary organic aerosol formed from the photooxidation of isoprene and 1,3,5-trimethylbenzene respectively. Experimental gas/particle partitioning coefficients were determined for a wide range of carbonyl products formed from the photooxidation of isoprene and 1,3,5-trimethylbenzene and compared with the theoretical values based on standard absorptive partitioning theory. Photooxidation products with a single carbonyl moiety were not observed in the particle phase, but dicarbonyls, and in particular, glyoxal and methylglyoxal, exhibited gas/particle partitioning coefficients several orders of magnitude higher than expected theoretically. These findings support the importance of heterogeneous and particle-phase chemical reactions for SOA formation and growth during the atmospheric degradation of anthropogenic and biogenic hydrocarbons.

Ozonolysis of β-Pinene: Temperature Dependence of Secondary Organic Aerosol Mass Fraction

The SOA formation from beta-pinene ozonolysis at modest precursor concentrations (2-40 ppb) was investigated in the temperature range of 0-40 degrees C. The presence of inert seeds and high ozone concentrations is necessary to minimize losses of semivolatile vapors to the walls of the smog chamber. beta-pinene secondary organic aerosol production increases significantly with decreasing temperature. An increase by a factor of 2-3, depending on the reacted beta-pinene concentration, was observed as the temperature decreased from 40 to 0 degrees C. This increase appearsto be due mainly to the shifting of partitioning of the semivolatile SOA componentstoward the particulate phase and not to a change of the beta-pinene product distribution with temperature. The measurements are used to develop a new temperature-dependent parametrization for the four-component basis-set. The parametrization predicts much higher SOA production for beta-pinene ozonolysis for typical atmospheric conditions than the values that have been suggested by previous studies.

Global modeling of secondary organic aerosol formation from aromatic hydrocarbons: high- vs. low-yield pathways

Abstract. Formation of SOA from the aromatic species toluene, xylene, and, for the first time, benzene, is added to a global chemical transport model. A simple mechanism is presented that accounts for competition between low and high-yield pathways of SOA formation, wherein secondary gas-phase products react further with either nitric oxide (NO) or hydroperoxy radical (HO2) to yield semi- or non-volatile products, respectively. Aromatic species yield more SOA when they react with OH in regions where the [NO]/[HO2] ratios are lower. The SOA yield thus depends upon the distribution of aromatic emissions, with biomass burning emissions being in areas with lower [NO]/[HO2] ratios, and the reactivity of the aromatic with respect to OH, as a lower initial reactivity allows transport away from industrial source regions, where [NO]/[HO2] ratios are higher, to more remote regions, where this ratio is lower and, hence, the ultimate yield of SOA is higher. As a result, benzene is estimated to be the most important aromatic species with regards to global formation of SOA, with a total production nearly equal that of toluene and xylene combined. Global production of SOA from aromatic sources via the mechanisms identified here is estimated at 3.5 Tg/yr, resulting in a global burden of 0.08 Tg, twice as large as previous estimates. The contribution of these largely anthropogenic sources to global SOA is still small relative to biogenic sources, which are estimated to comprise 90% of the global SOA burden, about half of which comes from isoprene. Uncertainty in these estimates owing to factors ranging from the atmospheric relevance of chamber conditions to model deficiencies result in an estimated range of SOA production from aromatics of 2–12 Tg/yr. Though this uncertainty range affords a significant anthropogenic contribution to global SOA, it is evident from comparisons to recent observations that additional pathways for production of anthropogenic SOA still exist beyond those accounted for here. Nevertheless, owing to differences in spatial distributions of sources and seasons of peak production, regions exist in which aromatic SOA produced via the mechanisms identified here are predicted to contribute substantially to, and even dominate, the local SOA concentrations, such as outflow regions from North America and South East Asia during the wintertime, though total modeled SOA concentrations there are small (~0.1 μg/m3).

Interactions between boreal wildfire and urban emissions

A suite of particulate, gaseous and meteorological measurements during the Pittsburgh Supersite experiment were used to characterize the impact of the 2002 Quebec wildfires on pollutant concentrations and physical and chemical processes dominant in the region. Temporal trends in the number distribution of wildfire particles (isolated using Rapid Single‐ultrafine‐particle Mass Spectrometry data) combined with CO, NOx and O3 mixing ratios identified two separate periods (Periods I and II) when the measurement site was directly impacted by plumes of relatively unprocessed wildfire emissions; i.e., increases in primary ultrafine wildfire particles, CO and NOx concomitant with a decrease in O3 from intraplume NOx titration. Carbonaceous particle number distributions predominantly associated with vehicular emissions, PM2.5 sulfate mass concentration and SO2 mixing ratio resolved individual components of local and regional sources. Single particle signatures indicated a period of intense atmospheric processing following Period II that caused rapid growth of the ultrafine mode due to simultaneous sulfate and secondary organic mass accumulation, resulting in significant changes to particle physical and chemical properties. Particle growth was concurrent with large increases in O3 and maxima in incoming solar radiation and ambient temperature and is posited to have occurred in situ as the air mass, containing a mixture of urban and wildfire emissions, was advected past the site. In total, the current work demonstrates significant added severity for pollution episodes in an area already burdened by large anthropogenic emission rates due to the impact of the 2002 Quebec wildfires. High levels of atmospheric processing increased sulfate accumulation and SOA formation and brought PM2.5 mass concentrations close to, and O3 mixing ratios in excess of, the National Ambient Air Quality Standards. Projections of increasing wildfire activity under a warming climate may increase the frequency and severity of such events.

Photooxidation of α-pinene at high relative humidity in the presence of increasing concentrations of NO x

The photooxidation of ∼1 ppm α-pinene in the presence of increasing concentrations of NO 2 was studied in a Teflon ® chamber at 72–88% relative humidity and 296–304 K. The loss of α-pinene and formation of gas-phase products were followed using proton-transfer reaction mass spectrometry (PTR-MS). Gas-phase reaction products (and their yields) include formaldehyde (5±1%), formic acid (2.5±1.4%), methanol (0.6±0.3%), acetaldehyde (3.9±1.7%), acetic acid (8.6±1.9%), acetone (12±3%), pinonaldehyde (22±6%), and pinene oxide (0.9±0.1%). There was evidence of organic nitrates; and small peaks were tentatively assigned to norpinonaldehyde, 4-oxopinonaldehyde, propanedial, 2,3-dioxobutanal and 3,5,6-trioxoheptanal or 3-hydroxymethyl-2,2-dimethylcyclobutylethanone. The formation and growth of new particles were followed using a scanning mobility particle sizer (SMPS), and their chemical composition and density probed using single particle mass spectrometry (SPLAT II). SPLAT II showed that the suspended SOA consisted of a complex mixture of organic nitrates and oxygenates having a density of 1.21±0.02 g cm −3, 20% larger than often assumed in calculating SOA yields. Three-wavelength light scattering measurements were consistent with particles having a refractive index characteristic of organic compounds, but the data could not be well matched at all three wavelengths with a single refractive index. The effect of addition of cyclohexane or NO on particle formation showed that ozonolysis was the major mechanism of SOA formation in this system. However, unlike simple ozonolysis, organic nitrates are formed in both the gas and particle phases. Identifying and measuring specific organic nitrates in both the gas and particle phases in air may help to elucidate why SOA formation has been reported in field studies to be associated with polluted urban areas, yet the carbon in these particles is largely contemporary, i.e., non-fossil fuel carbon.

Modeling exceptional high concentrations of carbonaceous aerosols observed at Pic du Midi in spring–summer 2003: Comparison with Sonnblick and Puy de Dôme

Carbonaceous aerosols have been sampled weekly in 2002 and 2003 at Pic du Midi (PdMO), an isolated high-altitude Pyrenean station. High concentrations of both black carbon (BC) and total organic carbon (OC) have been observed during the exceptional prolonged warm dry spell over western Europe in spring–summer 2003, culminating during the first 2 weeks in August. The aerosol ORISAM-TM4 global model [Guillaume, B., Liousse, C., Rosset, R., Cachier, H., Van Velthoven, P., Bessagnet, B., Poisson, N., 2007. ORISAM-TM4: a new global sectional multi-component aerosol model including SOA formation—focus on carbonaceous BC and OC aerosols. Tellus B 59, 283–302.], together with new updated European emission inventories, appears apt to closely simulate at this PdMO site the BC and OC aerosol components during the whole 2002–2003 period, including the August 2003 heat wave. Further, ORISAM-TM4 provides unique detailed information on both primary and secondary OC fractions together with differentiated aerosol secondary anthropogenic (SOAA) vs. biogenic OC (SOAB) contributions, not accessible to measurements. Such comparisons have been extended to two other high-altitude European sites of the CARBOSOL programme, Sonnblick (Austrian Alps) and Puy de Dôme (central France). At these two sites, the agreement between simulations and measured values is clearly not so close as at PdMO, with systematic more elevated measured values of both BC and OC, presumably due to local and regional sources.

Ozonolysis of α-pinene: parameterization of secondary organic aerosol mass fraction

Abstract. Existing parameterizations tend to underpredict the α-pinene aerosol mass fraction (AMF) or yield by a factor of 2–5 at low organic aerosol concentrations (<5 µg m−3). A wide range of smog chamber results obtained at various conditions (low/high NOx, presence/absence of UV radiation, dry/humid conditions, and temperatures ranging from 15–40°C) collected by various research teams during the last decade are used to derive new parameterizations of the SOA formation from α-pinene ozonolysis. Parameterizations are developed by fitting experimental data to a basis set of saturation concentrations (from 10−2 to 104 µg m−3) using an absorptive equilibrium partitioning model. Separate parameterizations for α-pinene SOA mass fractions are developed for: 1) Low NOx, dark, and dry conditions, 2) Low NOx, UV, and dry conditions, 3) Low NOx, dark, and high RH conditions, 4) High NOx, dark, and dry conditions, 5) High NOx, UV, and dry conditions. According to the proposed parameterizations the α-pinene SOA mass fractions in an atmosphere with 5 µg m−3 of organic aerosol range from 0.032 to 0.1 for reacted α-pinene concentrations in the 1 ppt to 5 ppb range.

ORISAM-TM4: a new global sectional multi-component aerosol model including SOA formation - Focus on carbonaceous BC and OC aerosols

ORISAM-TM4: a new global sectional multi-component aerosol model including SOA formation - Focus on carbonaceous BC and OC aerosols

Direct polymerization of isoprene and α‐pinene on acidic aerosols

The direct polymerization of isoprene and α‐pinene on acidic sulfate aerosols has been studied in a reaction chamber utilizing aerosol mass spectrometry. Results indicated that both species can be directly taken up into acidic aerosols to a significant extent, forming polymers that contain at least 4 isoprene or 2 α‐pinene repeating units. Aerosol mass spectra indicate that double bonds in the polymers hydrate under acid catalysis, leading to partial oxygenation of the polymers. This reactive uptake depends highly upon relative humidity and particle acidity. This process is rapid and reaches equilibrium in less than 50 minutes, with effective partition coefficients (Kp,eff) between 1.2–14.1 × 10−6 m3μg−1, from which it is estimated <0.5–5 ng m−3 of polymers may be present from both species in acidic aerosols. The formation of biogenic polymers is an important mechanism for incorporating hydrophobic, unsaturated species into polar aerosols and enhanced SOA formation.

Gas‐phase products and secondary aerosol yields from the photooxidation of 16 different terpenes

The photooxidation of isoprene, eight monoterpenes, three oxygenated monoterpenes, and four sesquiterpenes were conducted individually at the Caltech Indoor Chamber Facility under atmospherically relevant HC:NOx ratios to monitor the time evolution and yields of SOA and gas‐phase oxidation products using PTR‐MS. Several oxidation products were calibrated in the PTR‐MS, including formaldehyde, acetaldehyde, formic acid, acetone, acetic acid, nopinone, methacrolein + methyl vinyl ketone; other oxidation products were inferred from known fragmentation patterns, such as pinonaldehyde; and other products were identified according to their mass to charge (m/z) ratio. Numerous unidentified products were formed, and the evolution of first‐ and second‐generation products was clearly observed. SOA yields from the different terpenes ranged from 1 to 68%, and the total gas‐ plus particle‐phase products accounted for ∼50–100% of the reacted carbon. The carbon mass balance was poorest for the sesquiterpenes, suggesting that the observed products were underestimated or that additional products were formed but not detected by PTR‐MS. Several second‐generation products from isoprene photooxidation, including m/z 113, and ions corresponding to glycolaldehyde, hydroxyacetone, methylglyoxal, and hydroxycarbonyls, were detected. The detailed time series and relative yields of identified and unidentified products aid in elucidating reaction pathways and structures for the unidentified products. Many of the unidentified products from these experiments were also observed within and above the canopy of a Ponderosa pine plantation, confirming that many products of terpene oxidation can be detected in ambient air using PTR‐MS, and are indicative of concurrent SOA formation.

Acidity effects on the formation of α-pinene ozone SOA in the presence of inorganic seed

A 2 m 3 chamber was used to explore the effect of relative humidity (RH) and inorganic composition on aerosol yield due to aerosol phase heterogeneous processes in the α-pinene ozone oxidation system. The inorganic compositions considered range from pure aqueous sulfuric acid to a completely neutralized acid, ammonium sulfate. The heterogeneous organic mass (OM H) and the organic mass from partitioning (OM P) were decoupled from the total organic mass to consider the effect of acidity parameters solely on the aerosol mass fraction they impact the most (OM H). An experiment with aqueous ammonium sulfate was preformed in a 25 m 3 chamber to determine the distribution of α-pinene ozone products for an explicit multiproduct model used in the decoupling process. Both RH and inorganic seed composition were found to influence heterogeneous aerosol yield, due to the acid catalytic nature of the reactions that lead to this mass. The highest heterogeneous aerosol yields were found under conditions of low RH and strongly acidic inorganic seed compositions. A multiple linear regression model is presented and analyzed to show that both RH and inorganic seed composition significantly influence the production of aerosol mass through heterogeneous processes. The results presented here aid our understanding of the development of heterogeneous mass in the presence of inorganic aerosols and will be useful for modeling heterogeneous aerosol mass.

Coupled Partitioning, Dilution, and Chemical Aging of Semivolatile Organics

A unified framework of semi-volatile partitioning permits models to efficiently treat both semi-volatile primary emissions and secondary organic aerosol production (SOA), and then to treat the chemical evolution (aging) of the aggregate distribution of semi-volatile material. This framework also reveals critical deficiencies in current emissions and SOA formation measurements. The key feature of this treatment is a uniform basis set of saturation vapor pressures spanning the range of ambient organic saturation concentrations, from effectively nonvolatile material at 0.01 microg m(-3) to vapor-phase effluents at 100 mg m(-3). Chemical evolution can be treated by a transformation matrix coupling the various basis vectors. Using this framework, we show that semi-volatile partitioning can be described in a self-consistent way, with realistic behavior with respect to temperature and varying organic aerosol loading. The time evolution strongly suggests that neglected oxidation of numerous "intermediate volatility" vapors (IVOCs, with saturation concentrations above approximately 1 mg m(-3)) may contribute significantly to ambient SOA formation.

Estimation of secondary organic aerosol formation from semi-continuous OC–EC measurements in a Madrid suburban area

An analysis of hourly measurements of gaseous pollutants and fine carbonaceous aerosol (PM 2.5) divided into two fractions, elemental and organic carbon (EC and OC), in a suburban Madrid metropolitan area site is presented. Data were obtained using a semi-continuous thermal analyser for two periods of time during the summer and late autumn–early winter of 2003. In the summer period, correlation between EC and OC was poor ( r = 0.4 6 ), and high OC/EC ratios were usually found. In contrast, during the late autumn–early winter period the two carbonaceous aerosol fractions showed a high degree of correlation ( r = 0.9 4 ). Coincident peaks of OC and secondary gases (NO 2+O 3) were observed during summer episodes and also on some winter episodic days, suggesting a significant non-primary contribution to OC in fine aerosol, linked to photochemical oxidation of gaseous pollutants. OC apportionment between primary and secondary origins has been attempted for the two measuring periods using the EC tracer model. In the summer period, the results of this apportionment were very dependent on the y-intercept value (considered as the non-combustion OC background concentration) of selected linear regressions. Calculation of the secondary contribution to the OC fraction during polluted days gave diurnal maximum hourly estimations in a range from 35% to 55% and daily estimations averaged during the 10–19 h period ranging from 25% to 45%. Thus, this would confirm a daily urban photochemical origin for the secondary organic aerosol, although biogenic contribution to both the OC background and the SOA formation may be significant in the area during summer. During late autumn–early winter episodes primary contribution to OC was more prevailing, although the model also estimated maximum hourly values of secondary OC in the order of 35% of the measured OC.

Formation of 2-methyl tetrols and 2-methylglyceric acid in secondary organic aerosol from laboratory irradiated isoprene/NOX/SO2/air mixtures and their detection in ambient PM2.5 samples collected in the eastern United States

Abstract Recent observations in ambient PM 2.5 of 2-methylthreitol, 2-methylerythritol and 2-methylglyceric acid, proposed isoprene oxidation products, suggest the contribution of isoprene to SOA formation, long thought to be relatively unimportant, should be reexamined. To address this issue, an isoprene/NO X /air mixture was irradiated in a flow reactor smog chamber in both the absence and presence of SO 2 to measure the SOA yield of isoprene and to establish whether the two 2-methyl tetrols and 2-methylglyceric acid are present in isoprene SOA and could serve as SOA indicator compounds. In the absence of SO 2 , the SOA yield of 0.002 was low, as expected, although uncertain because the SOA concentration was near chamber background levels. However, in the presence of SO 2 , the SOA yield increased significantly to 0.028. Analysis of the trimethylsilyl derivatives of the SOA samples by gas chromatography/mass spectrometry showed chamber concentrations of the two 2-methyl tetrols totaling 0.1 μg m −3 and a 2-methylglyceric acid concentration of 0.4 μg m −3 in the absence of SO 2 , with the levels increasing significantly to 6.3 μg m −3 and 1.2 μg m −3 , respectively, when SO 2 was added. The laboratory data suggest that these compounds are possible indicator compounds for isoprene SOA and that the presence of SO 2 enhances significantly SOA formation from isoprene photooxidation, with acid-catalyzed reactions possibly playing a major role. The importance of these findings was supported by the detection of the two 2-methyl tetrols and 2-methylglyceric acid in summertime ambient PM 2.5 samples collected at three locations in the eastern United States. However, additional mechanistic studies are required to predict the contributions of the SO 2 -assisted isoprene SOA formation to ambient PM 2.5 concentrations.

Time‐ and size‐resolved chemical composition of submicron particles in Pittsburgh: Implications for aerosol sources and processes

An Aerodyne aerosol mass spectrometer (AMS) was deployed at the Pittsburgh Environmental Protection Agency Supersite from 7 to 22 September 2002 as part of the Pittsburgh Air Quality Study (PAQS). The main objectives of this deployment were to characterize the concentrations, size distributions, and temporal variations of nonrefractory (NR) chemical species in submicron particles (approximately PM1) and to further develop and evaluate the AMS. Reasonably good agreement was observed on particle concentrations, composition, and size distributions between the AMS data and measurements from collocated instruments (given the difference between the PM1 and PM2.5 size cuts), including TEOM, semicontinuous sulfate, 2‐hour‐ and 24‐hour‐averaged organic carbon, SMPS, 4‐hour‐averaged ammonium, and micro‐orifice uniform deposit impactor. Total NR‐PM1 mass concentration in Pittsburgh accumulates over periods of several days punctuated with rapid cleaning due to rain or air mass changes. Sulfate and organics are the major NR‐PM1 components while the concentrations of nitrate and chloride are generally low. Significant amounts of ammonium, which most of the time are consistent with sulfate present as ammonium sulfate, are also present in particles. However, there are periods when the aerosols are relatively acidic and more than 50% of sulfate is estimated to be in the form of ammonium bisulfate. No major enhancement of the organic concentration is observed during these acidic periods, which suggests that acid‐catalyzed SOA formation was not an important process during this study. Size distributions of particulate sulfate, ammonium, organics, and nitrate vary on timescales of hours to days, showing unimodal, bimodal and even trimodal characteristics. The accumulation mode (peaking around 350–600 nm in vacuum aerodynamic diameter for the mass distributions) and the ultrafine mode (<100 nm) are observed most frequently. The accumulation mode is dominated by sulfate that appears to be internally mixed with oxidized organics, while combustion‐emitted organics are often the main component of the ultrafine particles (except during nucleation events). The ultrafine‐mode organic aerosols are mainly associated with combustion sources (likely traffic).

PM2.5 episodes as observed in the speciation trends network

Abstract Urban PM2.5 speciation data from the 2000–2002 Speciation Trends Network were analyzed to gain an understanding of the various species contributing to regional PM2.5 episodes. The seasonal differences of these episodes and their relationship to meteorology were also studied. The results of this analysis revealed that summertime PM2.5 episodes are dominated by high concentration of acidic sulfate particles and some elevated organic aerosols. There is clear evidence that simultaneously elevated organics coincide with high concentration of acidic sulfate particles in many areas in the summer PM2.5 episode and they peaked on a day when solar radiation was obviously not very strong. This suggests that in summer, in addition to the conventional explanation of SOA formation through photochemical oxidation and partitioning, some other mechanism such as the presence of acidic particles could also play a role in enhancing the SOA production under heterogeneous reactions. In the cold seasons, however, nitrate and organic particles are the major contributors to the observed PM2.5 episodes with organic particles peaking in the fall and nitrates peaking in the winter. Given the weak solar radiations, cold temperatures, and an obvious decoupling of high organic aerosol concentrations with acidic particles in the winter, it suggests that the observed wintertime organic aerosols are essentially primary in nature. Meteorologically, regional PM2.5 episodes are predominantly triggered by stagnant high pressure systems with most of the high values occurred on the backside of the high pressure system or ahead of a quasi-stationary front where temperatures and relative humidity are high. The fact that the regional PM2.5 episodes can occur in almost any season and their major components are essentially secondary aerosols suggests that meteorology and aerosol thermodynamics are at least as important as the photochemistry in determining the high airborne PM2.5 concentrations. The dominance of nitrate episodes in the cold seasons reflects the fact that the wintertime ammonia emissions are generally high enough to neutralize most of the acids produced in the major industrial source regions. This is supported by the ammonium availability calculations.