A case study of urban particle acidity and its influence on secondary organic aerosol

Size-resolved indicators of aerosol acidity, including H+ ion concentrations (H+Aer) and the ratio of stoichiometric neutralization are evaluated in submicrometer aerosols using highly time-resolved aerosol mass spectrometer (AMS) data from Pittsburgh. The pH and ionic strength within the aqueous particle phase are also estimated using the Aerosol Inorganics Model (AIM). Different mechanisms that contribute to the presence of acidic particles in Pittsburgh are discussed. The largest H+Aer loadings and lowest levels of stoichiometric neutralization were detected when PM1 loadings were high and dominated by SO4(2-). The average size distribution of H+Aer loading shows an accumulation mode at Dva approximately 600 nm and an enhanced smaller mode that centers at Dva approximately 200 nm and tails into smaller sizes. The acidity in the accumulation mode particles suggests that there is generally not enough gas-phase NH3 available on a regional scale to completely neutralize sulfate in Pittsburgh. The lack of stoichiometric neutralization in the 200 nm mode particles is likely caused by the relatively slow mixing of gas-phase NH3 into SO2-rich plumes containing younger particles. We examined the influence of particle acidity on secondary organic aerosol (SOA) formation by comparing the mass concentrations and size distributions of oxygenated organic aerosol (00A--surrogate for SOA in Pittsburgh) during periods when particles are, on average, acidic to those when particles are bulk neutralized. The average mass concentration of ODA during the acidic periods (3.1 +/- 1.7 microg m(-3)) is higher than that during the neutralized periods (2.5 +/- 1.3 microg m(-3)). Possible reasons for this enhancement include increased condensation of SOA species, acid-catalyzed SOA formation, and/or differences in air mass transport and history. However, even if the entire enhancement (approximately 0.6 microg m(-3)) can be attributed to acid catalysis, the upperbound increase of SOA mass in acidic particles is approximately 25%, an enhancement that is much more moderate than the multifold increases in SOA mass observed during some lab studies and inferred in SO2-rich industrial plumes. In addition, the mass spectra of OOA from these two periods are almost identical with no discernible increase in relative signal intensity at larger m/z's (>200 amu), suggesting that the chemical nature of SOA is similar during acidic and neutralized periods and that there is no significant enhancement of SOA oligomer formation during acidic particle periods in Pittsburgh.     

Heterogeneous reactions of glyoxal on particulate matter: identification of acetals and sulfate esters

Reactive uptake of glyoxal onto particulate matter has been studied in laboratory experiments in a 2 m3 Teflon reaction chamber. Inorganic seed particles of different composition were utilized, including (NH4)2SO4, (NH4)2SO4/ H2SO4, NaNO3, and simulated sea salt, while the relative humidity and acid concentration were varied. The organic composition of the growing particles was measured in situ with an aerosol mass spectrometer, providing particle mass spectra as a means of product identification. Aerosol physical characteristics were also measured with a differential mobility analyzer and condensation nucleus counter. Regardless of seed composition, particle growth was rapid and continuous over the course of several hours. Identification of several mass fragments greater than the glyoxal monomer suggested that heterogeneous reactionsto form glyoxal adducts of lowvolatility had occurred. Temporal analysis of the mass fragments was consistent with a proposed acid-catalyzed mechanism whereby glyoxal is first hydrated, followed by self-reaction to form cyclic acetal structures. Increased relative humidity slowed the formation of higher order oligomers, also consistent with the proposed mechanism. The relative contribution of various oligomers to the overall organic composition was strongly dependent on the relative humidity and hence the particulate water concentration. A mild acid catalysis was also observed upon increasing the acidity of the seed particles. Specific mass fragments were found that could only arise from sulfate esters and were not present on the non-sulfur-containing seed particles. This first evidence of the formation of organic sulfates in particles is presented together with a proposed mechanism and molecular structure. These results suggest that the formation of these products of glyoxal uptake can contribute significantly to secondary organic aerosol.     

Effect of acidity on secondary organic aerosol formation from isoprene

The effect of particle-phase acidity on secondary organic aerosol (SOA) formation from isoprene is investigated in a laboratory chamber study, in which the acidity of the inorganic seed aerosol was controlled systematically. The observed enhancement in SOA mass concentration is closely correlated to increasing aerosol acidity (R2 = 0.979). Direct chemical evidence for acid-catalyzed particle-phase reactions was obtained from the SOA chemical analyses. Aerosol mass concentrations for the 2-methyltetrols, as well as the newly identified sulfate esters, both of which serve as tracers for isoprene SOA in ambient aerosols, increased significantly with enhanced aerosol acidity. Aerosol acidities, as measured in nmol of H+ m(-3), employed in the present study are in the same range as those observed in tropospheric aerosol collected from the eastern U.S.     

Particle growth by acid-catalyzed heterogeneous reactions of organic carbonyls on preexisting aerosols

Aerosol growth by the heterogeneous reactions of different aliphatic and alpha,beta-unsaturated carbonyls in the presence/absence of acidified seed aerosols was studied in a 2 m long flow reactor (2.5 cm i.d.) and a 0.5-m3 Teflon film bag under darkness. For the flow reactor experiments, 2,4-hexadienal, 5-methyl-3-hexen-2-one, 2-cyclohexenone, 3-methyl-2-cyclopentenone, 3-methyl-2-cyclohexenone, and octanal were studied. The carbonyls were selected based on their reactivity for acid-catalyzed reactions, their proton affinity, and their similarity to the ring-opening products from the atmospheric oxidation of aromatics. To facilitate acid-catalyzed heterogeneous hemiacetal/acetal formation, glycerol was injected along with inorganic seed aerosols into the flow reactor system. Carbonyl heterogeneous reactions were accelerated in the presence of acid catalysts (H2SO4), leading to higher aerosol yields than in their absence. Aldehydes were more reactive than ketones for acid-catalyzed reactions. The conjugated functionality also resulted in higher organic aerosol yieldsthan saturated aliphatic carbonyls because conjugation with the olefinic bond increases the basicity of the carbonyl leading to increased stability of the protonated carbonyl. Aerosol population was measured from a series of sampling ports along the length of the flow reactor using a scanning mobility particle sizer. Fourier transform infrared spectrometry of either an impacted liquid aerosol layer or direct reaction of carbonyls as a thin liquid layer on a zinc selenide FTIR disk was employed to demonstrate the direct transformation of chemical functional groups via the acid-catalyzed reactions. These results strongly indicate that atmospheric multifunctional organic carbonyls, which are created by atmospheric photooxidation reactions, can contribute significantly to secondary organic aerosol formation through acid-catalyzed heterogeneous reactions. Exploratory studies in 25- and 190-m3 outdoor chambers were also implemented to demonstrate the formation of high molecular weight organic structures. The reaction of ozone with alpha-pinene to generate secondary organic aerosols (SOAs) was performed in the presence of background aerosol consisting of a mixture of wood soot and diesel soot. Results strongly suggest that indigenous sulfuric acid associated with the combustion of fossil fuels (e.g., diesel soot) can initiate acid-catalyzed heterogeneous reactions of SOAs on the particle phase.     

Depression of ammonia uptake to sulfuric acid aerosols by competing uptake of ambient organic gases

The neutralization of acidic aerosols by ammonia has been studied through experiments which combine ambient air with laboratory generated sulfuric acid aerosol. Results indicated that acidic aerosol mixed with organic free air and ammonia was neutralized on a time scale<1 min, consistent with expectations. However, in the presence of ambient organic gases and ammonia, the rate of aerosol neutralization is significantly reduced. This reduction in ammonia uptake was concurrent with an increase in the amount of particle phase organics. A steady state in the NH4+/SO4(2-) in the presence of organic gases was established on time scales of 10 min to several hours, corresponding to NH3 uptake coefficients in the range of 4×10(-3)-2×10(-4). The degree to which neutralization was slowed was dependent upon the initial ammonia concentration and the organic mass added to the aerosols. These results suggest that inorganic equilibrium thermodynamic models may overestimate the rate of ammonia uptake and that ambient particles may remain acidic for longer than previously expected.     

Uptake of glyoxal by organic and Inorganic aerosol

The uptake of glyoxal by a variety of organic and inorganic aerosol types was examined in a Teflon chamber. Rapid glyoxal uptake was observed for all liquid-phase aerosols at all relative humidity levels tested (< 5 to 50% RH). Even for aerosol with known water content, Henry's Law cannot predict glyoxal uptake: H* > (3 +/- 1.5) x 10(8) mol kg(-1) atm(-1) for l-tartaric acid, H* > (1 +/- 0.5) x 10(8) for dl-malic acid and H* = (2 +/- 1) x 10(7) for malonic acid aerosol. Other liquid-phase aerosol particles containing amine functional groups (arginine, aspartic acid, and glutamic acid) took up even more glyoxal (H* > 3 x 10(8)). The trend of higher glyoxal uptake onto aerosol containing more nucleophilic organic compounds suggests that glyoxal is reacting with organic compounds in the aerosol phase. Solid-phase aerosol showed RH-dependent glyoxal uptake, likely due to the existence of surface water layers. However, particle growth rates were the highestfor sodium sulfate aerosol. For organic aerosol, growth rates correlated with the acidity of the carboxylic acid groups of the aerosol material, suggesting that glyoxal uptake is enhanced by mildly acidic conditions.     

Isoprene epoxydiols as precursors to secondary organic aerosol formation: acid-catalyzed reactive uptake studies with authentic compounds

Isoprene epoxydiols (IEPOX), formed from the photooxidation of isoprene under low-NO(x) conditions, have recently been proposed as precursors of secondary organic aerosol (SOA) on the basis of mass spectrometric evidence. In the present study, IEPOX isomers were synthesized in high purity (>99%) to investigate their potential to form SOA via reactive uptake in a series of controlled dark chamber studies followed by reaction product analyses. IEPOX-derived SOA was substantially observed only in the presence of acidic aerosols, with conservative lower-bound yields of 4.7-6.4% for β-IEPOX and 3.4-5.5% for δ-IEPOX, providing direct evidence for IEPOX isomers as precursors to isoprene SOA. These chamber studies demonstrate that IEPOX uptake explains the formation of known isoprene SOA tracers found in ambient aerosols, including 2-methyltetrols, C(5)-alkene triols, dimers, and IEPOX-derived organosulfates. Additionally, we show reactive uptake on the acidified sulfate aerosols supports a previously unreported acid-catalyzed intramolecular rearrangement of IEPOX to cis- and trans-3-methyltetrahydrofuran-3,4-diols (3-MeTHF-3,4-diols) in the particle phase. Analysis of these novel tracer compounds by aerosol mass spectrometry (AMS) suggests that they contribute to a unique factor resolved from positive matrix factorization (PMF) of AMS organic aerosol spectra collected from low-NO(x), isoprene-dominated regions influenced by the presence of acidic aerosols.     

Semiempirical model for organic aerosol growth by acid-catalyzed heterogeneous reactions of organic carbonyls

Aerosol growth by heterogeneous reactions of diverse carbonyls in the presence and absence of acidified seed aerosols was studied in a 4 m long flow reactor (2.5 cm i.d.) and a 2-m3 indoor Teflon film chamber under darkness. The acid catalytic effects on heterogeneous aerosol production were observed for diverse carbonyls in various ranges of humidities and compositions of seed inorganic aerosols. Particle population data measured by a scanning mobility particle sizer were used to calculate organic aerosol growth. To accountforthe aerosol growth contributed by heterogeneous reactions, the increase in organic aerosol mass was normalized bythe organic mass predicted by partitioning or the square of predicted organic mass. The carbonyl heterogeneous reactions were accelerated in the presence of acid catalysts (H2SO4), leading to higher aerosol yields than in their absence. The experimental data from aerosol yields in the flow reactorwere semiempirically fitted to the model parameters to predict the organic aerosol growth. The model parameters consist of environmental characteristics and molecular structure information of organic carbonyls. Basicity constants of carbonyls were used to describe the proton affinity of carbonyls for the acid catalysts. Particle environmental factors, such as humidity, temperature, and inorganic seed composition, were expressed by excess acidity and the parameters obtained from an inorganic thermodynamic model. A stepwise regression analysis of the aerosol growth model for the experimental data revealed that either the chemical structure information of carbonyls or characteristic environmental parameters are statistically significant in the prediction of organic aerosol growth. It was concluded thatthis model approach is applicable to predict secondary organic aerosol formation by heterogeneous reaction.     

Evidence for organosulfates in secondary organic aerosol

Recent work has shown that particle-phase reactions contribute to the formation of secondary organic aerosol (SOA), with enhancements of SOA yields in the presence of acidic seed aerosol. In this study, the chemical composition of SOA from the photooxidations of alpha-pinene and isoprene, in the presence or absence of sulfate seed aerosol, is investigated through a series of controlled chamber experiments in two separate laboratories. By using electrospray ionization-mass spectrometry, sulfate esters in SOA produced in laboratory photooxidation experiments are identified for the first time. Sulfate esters are found to account for a larger fraction of the SOA mass when the acidity of seed aerosol is increased, a result consistent with aerosol acidity increasing SOA formation. Many of the isoprene and alpha-pinene sulfate esters identified in these chamber experiments are also found in ambient aerosol collected at several locations in the southeastern U.S. It is likely that this pathway is important for other biogenic terpenes, and may be important in the formation of humic-like substances (HULIS) in ambient aerosol.     

SOA formation from partitioning and heterogeneous reactions: model study in the presence of inorganic species

A predictive model for secondary organic aerosol (SOA) formation by both partitioning and heterogeneous reactions was developed for SOA created from ozonolysis of alpha-pinene in the presence of preexisting inorganic seed aerosols. SOA was created in a 2 m3 polytetrafluoroethylene film indoor chamber under darkness. Extensive sets of SOA experiments were conducted varying humidity, inorganic seed compositions comprising of ammonium sulfate and sulfuric acid, and amounts of inorganic seed mass. SOA mass was decoupled into partitioning (OM(P)) and heterogeneous aerosol production (OM(H)). The reaction rate constant for OM(H) production was subdivided into three categories (fast, medium, and slow) to consider different reactivity of organic products for the particle phase heterogeneous reactions. The influence of particle acidity on reaction rates was treated in a previous semiempirical model. Model OM(H) was developed with medium and strong acidic seed aerosols, and then extrapolated to OM(H) in weak acidic conditions, which are more relevant to atmospheric aerosols. To demonstrate the effects of preexisting glyoxal derivatives (e.g., glyoxal hydrate and dimer) on OM(H), SOA was created with a seed mixture comprising of aqueous glyoxal and inorganic species. Our results show that heterogeneous SOA formation was also influenced by preexisting reactive glyoxal derivatives.     

Semicontinuous measurements of organic carbon and acidity during the Pittsburgh Air Quality Study: implications for acid-catalyzed organic aerosol formation

Laboratory evidence suggests that inorganic acid seed particles may increase secondary organic aerosol yields secondary organic aerosol (SOA) through heterogeneous chemistry. Additional laboratory studies, however, report that organic acidity generated in the same photochemical process by which SOA is formed may be sufficient to catalyze these heterogeneous reactions. Understanding the interaction between inorganic acidity and SOA mass is important when evaluating emission controls to meet PM2.5 regulations. We examine semicontinuous measurements of organic carbon (OC), elemental carbon (EC), and inorganic species from the Pittsburgh Air Quality Study to determine if we can detect coupling in the variations of inorganic acidity and OC. We were not able to detect significant enhancements of SOA production due to inorganic acidity in Western Pennsylvania most of the time, but its signal might have been lost in the noise. If we assume a causal relationship between inorganic acidity and OC, reductions in OC for Western Pennsylvania that might result from drastic reductions in inorganic acidity were estimated to be 2 +/- 4% by a regression technique, and an upper bound for this geographic area was estimated to be 5 +/- 8% based on calculations from laboratory measurements.     

Secondary organic carbon and aerosol yields from the irradiations of isoprene and alpha-pinene in the presence of NOx and SO2

A laboratory study was carried out to investigate the secondary organic carbon (SOC) yields of alpha-pinene and isoprene in the presence of SO2, which produces acidic aerosol in the system. Experiments were based on irradiating each hydrocarbon (HC) with NOx in a 14.5 m3 smog chamber operated in the dynamic mode. The experimental design consisted of several multi-part experiments for each HC. In the first part of each experiment, an HC/NOx irradiation was conducted in the absence of SO2 and was followed by irradiations with the addition of SO2 in subsequent parts. Filter-based analyses for organic carbon were made using a thermal-optical approach either with an off-line instrument or in situ with an automated instrument. For isoprene in the absence of SO2, the SOC yield was approximately 0.001, a value consistent with earlier work from this laboratory. With the addition of up to 200 ppb SO2, the yield increased by a factor of 7. For alpha-pinene in the absence of SO2, the SOC yield of the irradiated mixture was found to average 0.096 from two experiments. With SO2 in the system, the SOC yield increased on average to 0.132. These results suggest that SO2, and by inference acidic aerosol, may play a role in increasing the yield of SOC from the photooxidation products of biogenic hydrocarbons or by the direct uptake of biogenic hydrocarbons onto acidic aerosol.     

固体酸SO42-/ZrO2催化合成大豆油基多元醇

采用溶胶-凝胶-浸渍法制备SO42-/ZrO2固体酸催化剂,将其应用于环氧大豆油和甲醇的开环加成反应,系统考察了沉淀pH、H2SO4浸渍液浓度、焙烧温度对SO42-/ZrO2固体酸催化剂催化活性的影响。采用XRD、NH3-TPD、氮吸附等方法对SO42-/ZrO2催化剂的结构和表面酸性进行了表征。结果表明:催化剂的表面强酸位是促进开环加成反应得到大豆油基多元醇的催化活性中心。当沉淀pH为9,H2SO4浸渍液浓度为2 mol/L,焙烧温度为550℃时,催化剂表面具有更多的强酸中心,催化活性最高,110℃反应2 h后,环氧大豆油中环氧基团转化率大于98%,大豆油基多元醇的羟值(KOH)达到203.7 mg/g,且产物易与催化剂分离,简化了后处理过程。     

Secondary organic aerosol formation from m-xylene in the absence of NOx

Formation of secondary organic aerosol (SOA) from m-xylene photoxidation in the absence of NOx was investigated in a series of smog chamber experiments. Experiments were performed in dry air and in the absence of seed aerosol with H2O2 photolysis providing a stable hydroxyl radical (OH radical) source. SOA formation from this study is exceptionally higher than experiments with existence of NOx. The experiments with elevated HO2 levels indicate that organic hydroperoxide compounds should contribute to SOA formation. Nitrogen oxide (NO) is shown to reduce aerosol formation; the constant aerosol formation rate obtained before addition of NO and after consumption of NO strongly suggests that aerosol formation is mainlythrough reactions with OH and HO2 radicals. In addition, a density of 1.40 +/- 0.1 g cm(-3) for the SOA from the photooxidation of m-xylene in the absence of NOx has been measured, which is significantly higherthan the currently used unit density.     

Comparative analysis of urban atmospheric aerosol by particle-induced X-ray emission (PIXE), proton elastic scattering analysis (PESA), and aerosol mass spectrometry (AMS)

A multifaceted approach to atmospheric aerosol analysis is often desirable in field studies where an understanding of technical comparability among different measurement techniques is essential. Herein, we report quantitative intercomparisons of particle-induced X-ray emission (PIXE) and proton elastic scattering analysis (PESA), performed of fline under a vacuum, with analysis by aerosol mass spectrometry (AMS) carried out in real-time during the MCMA-2003 Field Campaign in the Mexico City Metropolitan Area. Good agreement was observed for mass concentrations of PIXE-measured sulfur (assuming it was dominated by SO4(2-)) and AMS-measured sulfate during most of the campaign. PESA-measured hydrogen mass was separated into sulfate H and organic H mass fractions, assuming the only major contributions were (NH4)2SO4 and organic compounds. Comparison of the organic H mass with AMS organic aerosol measurements indicates that about 75% of the mass of these species evaporated under a vacuum. However approximately 25% of the organics does remain under a vacuum, which is only possible with low-vapor-pressure compounds, and which supports the presence of high-molecular-weight or highly oxidized organics consistent with atmospheric aging. Approximately 10% of the chloride detected by AMS was measured by PIXE, possibly in the form of metal-chloride complexes, while the majority of Cl was likely present as more volatile species including NH4Cl. This is the first comparison of PIXE/PESA and AMS and, to our knowledge, also the first report of PESA hydrogen measurements for urban organic aerosols.     

Formation of oligomers in secondary organic aerosol

The formation of oligomeric molecules, an important step in secondary organic aerosol production, is reported. Aerosols were produced by the reaction of alpha-pinene and ozone in the presence of acid seed aerosol and characterized by exact mass measurements and tandem mass spectrometry. Oligomeric products between 200 and 900 u were detected with both electrospray ionization and matrix-assisted laser desorption ionization. The exact masses and dissociation products of these ions were consistent with various combinations of the known primary products of this reaction ("monomers") with and/or without the expected acid-catalyzed decomposition products of the monomers. Oligomers as large as tetramers were detected. Both aldol condensations and gem-diol reactions are suggested as possible pathways for oligomer formation. Exact mass measurements also revealed reaction products that cannot be explained by simple oligomerization of monomers and monomer decomposition products, suggesting the existence of complex reaction channels. Chemical reactions leading to oligomer formation provide a reasonable answer to a difficult problem associated with secondary organic aerosol production in the atmosphere. It is unlikely that monomers alone play an important role in the formation and growth of nuclei in the atmosphere as their Kelvin vapor pressures are too high for them to significantly partition into the particle phase. Polymerization provides a mechanism by which partitioning to the particle phase becomes favored.     

Sampling artifacts of acidity and ionic species in PM2.5

Although sampling artifacts of acidity, ammonium, nitrate, and chloride in airborne particulate pollutants can be reduced by the use of denuders to absorb interfering gases, artifacts due to interparticle interactions still remain. In this study, the contribution of individual artifact reactions to particle evaporation and the effects of aerosol composition on the extents of sampling artifacts in PM2.5 were investigated. Samples were collected using a Harvard honeycomb denuder/filter-pack system at an urban site and a rural site in Hong Kong. The results show that the formation of artifacts can be categorized into two regimes: ammonium rich (AR) samples with a molar ratio [NH4+]/ [SO4(2-)] greater than 1.5 and ammonium poor (AP) samples with a molar ratio [NH4+]/[SO4(2-)] less than or equal to 1.5. The urban samples were all AR samples, and they were characterized by high nitrate and low in situ free acid concentrations. In contrast, the rural samples were all AP samples and they were characterized by low nitrate and high in situ free acid concentrations. We have developed a methodology to estimate the contribution of each artifact reaction to the sampling loss of nitrate, chloride, ammonium, and acidity. In the AR samples, the evaporation of HNO3 and HCl and concomitant evaporation of NH3 were the principal reactions in determining the extent of the sampling loss of nitrate and chloride. In the AP samples, the evaporation of HNO3 and HCl alone was the principal reaction instead, especially at high sampling loss. The in situ free acid concentration, a function of aerosol composition and ambient conditions, is a more useful parameter than strong acidity in understanding the sampling loss of acidity, nitrate, and chloride from the collected particles.     

Optical properties of the products of α-dicarbonyl and amine reactions in simulated cloud droplets

Secondary organic aerosol makes up a significant fraction of the total aerosol mass, and a growing body of evidence indicates that reactions in the atmospheric aqueous phase are important contributors to aerosol formation and can help explain observations that cannot be accounted for using traditional gas-phase chemistry. In particular, aqueous phase reactions between small organic molecules have been proposed as a source of light absorbing compounds that have been observed in numerous locations. Past work has established that reactions between α-dicarbonyls and amines in evaporating water droplets produces particle-phase products that are brown in color. In the present study, the complex refractive indices of model secondary organic aerosol formed by aqueous phase reactions between the α-dicarbonyls glyoxal and methylglyoxal and the primary amines glycine and methylamine have been determined. The reaction products exhibit significant absorption in the visible, and refractive indices are similar to those for light absorbing species isolated from urban aerosol. However, the optical properties are different from the values used in models for secondary organic aerosol, which typically assume little to no absorption of visible light. As a result, the climatic cooling effect of such aerosols in models may be overestimated.     

Laboratory investigation of heterogeneous interaction of sulfuric acid with soot

The internal mixing state of soot with sulfuric acid is believed to significantly impact the optical, cloud-forming, and chemical properties of soot-containing aerosols, but little is known about the interaction between soot and sulfuric acid. We report the first measurements of the uptake of H2SO4 on three types of soot generated from methane, hexane, and kerosene combustion. H2SO4 loss on soot is found to be irreversible. The measured uptake coefficients are 0.018 +/- 0.007 for kerosene soot, 0.012 +/- 0.006 for methane soot, and 0.0076 +/- 0.0016 for hexane soot at a total pressure of 1-2 Torr and 298 K assuming a geometric surface area, likely corresponding to the upper limits. Additional experiments using the differential mobility analysis and Fourier transform infrared spectroscopy techniques are carried out to further characterize the interaction of H2SO4 with soot. The results suggest that uptake of H2SO4 takes place efficiently on soot particles, representing an important route to convert hydrophobic soot to hydrophilic aerosols.