Hygroscopic Organic Aerosols during BRAVO?

Abstract

The hygroscopic properties of the organic fraction of aerosols are poorly understood. The ability of organic aerosols to absorb water as a function of relative humidity (RH) was examined using data collected during the 1999 Big Bend Regional Aerosol and Visibility Observational Study (BRAVO). (On average, organics accounted for 22% of fine particulate matter with an aerodynamic diameter less than 2.5 µm (PM_2.5) mass). Hourly RH exceeded 80% only 3.5% of the time and averaged 44%. BRAVO aerosol chemical composition and dry particle size distributions were used to estimate PM_2.5 light scattering (Bsp) at low and high ambient RH. Liquid water growth associated with inorganic species was sufficient to account for measured Bsp for RH between 70 and 95%.     

The effect of water on gas–particle partitioning of secondary organic aerosol. Part I: α-pinene/ozone system

The effect of relative humidity (RH) on aerosol formation by the semi-volatile oxidation products of the α-pinene/O₃ system has been comprehensively studied. Experiments were performed in the presence of ammonium sulfate (aqueous, dry), ammonium bisulfate seed (aqueous, dry), and aqueous calcium chloride seed aerosols to ascertain their effect on the partitioning of the oxidation products. The yield of organic aerosol varies little with RH, and is not affected by the presence of dry inorganic salt aerosols. Aqueous salt aerosols reduce the yield of organic aerosol compared to that under seed-free or dry seed conditions. The degree of reduction is electrolyte dependent, with aqueous ammonium sulfate leading to the largest reduction and aqueous calcium chloride the smallest. Hygroscopic growth of the organic aerosol from <2% to 85% RH was also monitored, and could be satisfactorily represented as the sum of the individual contributions of the organic and inorganic fractions. The implications of the growth factor measurements for concentration/activity relationships of the condensed phase organic material (assuming a liquid solution) was explored. The formation of the organic aerosol was investigated using a simple two component model, and also one including the 12 product compounds identified in a previous study. The experimental results for <2% and 50% RH (without salt seed aerosols) could be satisfactorily predicted. However, the aqueous salt seed aerosols are predicted to increase the overall yield due to the dissolution of the organic compounds into the water associated with the seed aerosol—the opposite effect to that observed. The implications of two distinct phases existing the aerosol phase were investigated.     

Performance evaluation of two protective treatments on salt-laden limestones and marble after natural and artificial weathering

Salt crystallization is a major damage factor in stone weathering, and the application of inappropriate protective products may amplify its effects. This research focuses on the evaluation of two protective products' performance (organic polydimethylsiloxane and inorganic ammonium oxalate (NH₄)₂(COO)₂·H₂O) in the case of a salt load from behind. Experimental laboratory simulations based on salt crystallization cycles and natural weathering in an urban area were carried out. The effects were monitored over time, applying different methods: weight loss evaluation, colorimetric and water absorption by capillarity measurements, stereomicroscope observations, FTIR and SEM-EDS analyses. The results showed minor impact exerted on the short term on stones, particularly those treated with the water repellent, by atmospheric agents compared to salt crystallization. Lithotypes with low salt load (Gioia marble) underwent minor changes than the heavily salt-laden limestones (Lecce and Ançã stones), which were dramatically damaged when treated with polysiloxane. The results suggest that the ammonium oxalate treatment should be preferred to polysiloxane in the presence of soluble salts, even after desalination procedures which might not completely remove them. In addition, the neo-formed calcium oxalate seemed to effectively protect the stone, improving its resistance against salt crystallization without occluding the pores and limiting the superficial erosion caused by atmospheric agents.     

Hygroscopic organic aerosols during BRAVO?

The hygroscopic properties of the organic fraction of aerosols are poorly understood. The ability of organic aerosols to absorb water as a function of relative humidity (RH) was examined using data collected during the 1999 Big Bend Regional Aerosol and Visibility Observational Study (BRAVO). (On average, organics accounted for 22% of fine particulate matter with an aerodynamic diameter less than 2.5 microm (PM2.5) mass). Hourly RH exceeded 80% only 3.5% of the time and averaged 44%. BRAVO aerosol chemical composition and dry particle size distributions were used to estimate PM2.5 light scattering (Bsp) at low and high ambient RH. Liquid water growth associated with inorganic species was sufficient to account for measured Bsp for RH between 70 and 95%.     

Organic acids in springtime wisconsin precipitation samples

A short-term study on organic acids in precipitation was conducted from 15 March to 1 June 1984 at two sites on the Wisconsin Acid Deposition Monitoring Network. Aliquots of collected samples were fixed with tetrachloromercurate (TCM) and analyzed for low molecular weight organic anions via ion-exclusion chromatography (ICE). Unfixed aliquots were subjected to standard network inorganic analyses. Of the 31 samples collected, 30 contained detectable concentrations of formate and acetate ions, with concentrations ranging from < 0.43 to 56μmol ℓ⁻¹ for the former and < 0.83 to 33 μmol ℓ⁻¹ for the latter. Propionate, oxalate and malonate ions were also detected in a number of samples. Statistical analysis of the combined organic and inorganic data set indicated that no significant differences existed between the concentrations of organic ions at the two sites; however, samples containing visible sediments had significantly greater concentrations than samples without sediments. Maximum contributions of the organic acids to precipitation-free acidity, calculated by assuming that the only sources of the measured formate and acetate were their respective acid forms, averaged 18.6% for samples with a pH ⩽ 5.0. Formate and acetate concentrations were highly correlated. Correlations between organic and inorganic ions were weak; NH⁺₄ and Ca²⁺ generally exhibited the highest, though still weak, correlations. The study emphasizes the need for further, longer-term investigations to determine the role of low molecular weight organic compounds in precipitation chemistry.     

Cloud condensation nucleus activity of organic compounds: a laboratory study

Critical or activation diameters of laboratory generated organic aerosols composed of succinic acid, adipic acid and glucose were determined. Measurements of sodium chloride and ammonium sulfate aerosols were performed for comparison. Our experimental approach involved producing single component aerosol particles of a known size, and measuring the fraction of aerosol number concentrations (CN) that act as CCN at several supersaturations. The particle diameter (D₅₀) at which the CCN/CN ratio of 0.50 is reached is defined as the critical, or activation, diameter. These experimentally derived diameters are compared with the theoretical critical diameter (D_C). The results indicate that highly water-soluble organic compounds exhibit critical diameters that approach that of ammonium sulfate.     

Assessment of the aerosol water content in urban atmospheric particles by the hygroscopic growth measurements in Sapporo,Japan

Aerosol water content (AWC) of urban atmospheric particles was investigated based on the hygroscopic growth measurements for 100 and 200 nm particles using a hygroscopicity tandem differential mobility analyzer in Sapporo, Japan in July 2006. In most of the humidogram measurements, presence of less and more hygroscopic mode was evident from the different dependence on relative humidity (RH). The volume of liquid water normalized by that of dry particle (V_w(RH)/V_dry) was estimated from the HTDMA data for 100 and 200 nm particles. The RH dependence of V_w(RH)/V_dry was well represented by a fitted curve with a hygroscopicity parameter κ_eff. The κ_eff values for 200 nm particles were in general higher than those for 100 nm particles, indicating a higher hygroscopicity of 200 nm particles. Based on the κ_eff values, the volume mixing ratios of water-soluble inorganic compounds (ammonium sulfate equivalent) were estimated to be on average 31% and 45% for 100 and 200 nm particles, respectively. The diurnal variation of κ_eff, with relatively higher values in the noontime and nighttime and lower values in the morning and evening hours, was observed for both particle sizes. The V_w(RH)/V_dry values under ambient RH conditions were estimated from κ_eff to range from 0.05 to 2.32 and 0.06 to 2.43 for 100 nm and 200 nm particles, respectively. The degree of correlation between κ_eff and V_w(RH)/V_dry at ambient RH suggests a significant contribution of the variation of κ_eff to atmospheric AWC in Sapporo.     

Size segregated water uptake of the urban submicrometer aerosol in Beijing

Physical and chemical properties of submicrometer aerosol particles were measured in summer 2004 (June/July) and winter 2005 (January/February) in Beijing, Peoples Republic of China, using a Twin-Differential Mobility Particle Sizer (T-DMPS), a Hygroscopicity-Tandem Differential Mobility Analyzer (H-TDMA), and a Micro Orifice Uniform Deposit Impactor (MOUDI). Particle number–size distributions were measured in the diameter range Dp = 3–800 nm and hygroscopic properties were determined at initial dry particle diameters of Dp_j (j = 30, 50, 80, 150, 250, and 350 nm) at a relative humidity (RH) of 90%. Hygroscopic properties were compared with chemical analyses of aerosol samples taken with the MOUDI. Based on the hygroscopicity data, the total hygroscopic particle volume was modeled, including dependence on dry particle size, season and level of pollution using a simple approach.Overall, the chemical analysis showed ammonium sulfate to be the major inorganic component of the urban submicrometer aerosol in Beijing along with relatively high fractions of elemental carbon (10–25%) and organic matter (15–60%) depending on particle size and season.The hygroscopic growth distributions (H-TDMA) subdivided the aerosol population into three different groups of particles with varying growth factors depending on dry particle size, namely nearly hydrophobic (growth factor = 0.96–1.07), less hygroscopic (1.06–1.29) and more hygroscopic (1.26–1.62).Hydrophobic particle fractions indicating freshly emitted soot/carbonaceous particles varied between 10 and 32% depending on dry particle size and season. During heavily polluted times, a decreasing number of hydrophobic particle fractions indicated that the urban submicrometer aerosol in Beijing was highly influenced by more aged aerosol transported from the industrial regions around Beijing containing sulfate as a major component.Based on model calculations, the urban submicrometer aerosol in Beijing showed strong compositional variations. The calculated total hygroscopic volume fractions varied between 16 and 65% depending on size, level of pollution and season.     

Phase transition behaviour of sodium oleate aerosol particles

Field measurements have shown that organic surfactants are significant components of atmospheric aerosols. While fatty acids, among other surfactants, are prevalent in the atmosphere, the influence of these species on the chemical and physical properties of atmospheric aerosols remains not fully characterized. In order to assess the phase in which particles may exist, a detailed study of the deliquescence of a model surfactant aerosol has been carried out. Sodium oleate was chosen as a surfactant proxy relevant in atmospheric aerosol. Sodium oleate micelle aerosol particles were generated nebulizing a sodium oleate aqueous solution. In this study, the water uptake and phase transition of sodium oleate aerosol particles have been studied in a room temperature aerosol flow tube system (AFT) using Fourier transform infrared (FTIR) spectroscopy. Aerosol morphology and elemental composition were also analysed using scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDX) techniques. The particles are homogeneously distributed as ellipsoidal-shape aggregates of micelles particles with an average size of ∼1.1 μm. The deliquescence by the sodium oleate aerosol particles was monitored by infrared extinction spectroscopy, where the dried aerosol particles were exposed to increasing relative humidity as they passed through the AFT. Observations of the infrared absorption features of condensed phase liquid water enable to determine the sodium oleate deliquescence phase transition at 88±2%.     

A GC-TCD method for measuring the liquid water mass of collected aerosols

This work presents a gas chromatographic method that uses a thermal conductivity detector (GC-TCD) to measure the liquid water mass (LWM) of collected aerosols. The method is a modification of the previously developed EA-TCD method (Journal of Aerosol Science 29, 827). A microcomputer was incorporated into the system to control the analytical procedures, improve the measurement precision, and make possible a continuous operation. To validate the method, the aerosol LWMs of NaCl, Na₂SO₄, NH₄NO₃, (NH₄)₂SO₄, NH₄Cl, and Na₂CO₃ were measured at room temperature under relative humidities (RHs) varying between 20% and 90%, in both humidifying and dehumidifying conditions. Estimates of aerosol LWMs for varying aerosol chemical compositions and RHs by various measurement methods and predictive models are comprehensively compared. The comparison shows that the GC-TCD measurements agree closely with those of the other methods. The GC-TCD measurements are closer to the ISORROPIA model predictions than those of the AIM2 model. Most notably, our method determines, for the first time, the hygroscopic behavior of Na₂CO₃ aerosol yielding the deliquescence relative humidity and crystallization relative humidity at 78% and 39% RH, respectively. The hygroscopic characteristics of various NaCl mole fractions in mixed NaCl–Na₂SO₄ aerosols, determined by GC-TCD, are used to show the discrepancy between the measurements and the model's prediction.     

Is the size distribution of urban aerosols determined by thermodynamic equilibrium?: An application to Southern California

A size-resolved equilibrium model, SELIQUID, is presented and used to simulate the size–composition distribution of semi-volatile inorganic aerosol in an urban environment. The model uses the efflorescence branch of aerosol behavior to predict the equilibrium partitioning of the aerosol components between the gas phase and a size-resolved aerosol population over the entire RH domain. Predictions of SELIQUID are compared against size-resolved composition measurements at different locations during the Southern California Air Quality Study. Based on the modeling results, the size distribution of sub-micrometer nitrate and ammonium can be determined by thermodynamic equilibrium when the RH>60%. In cases where the RH<60%, the assumption that all aerosol particles are metastable liquid solutions may introduce unacceptable errors. On the other hand, the equilibrium assumption, in some cases at least, introduces errors in the calculation of the coarse (particles with diameter >1 μm or so) nitrate and ammonium that increase with particle size. Finally, the inclusion of crustal species is important in modeling the size distribution of coarse inorganic aerosols when the concentration of these species is high. The effect of these crustal species can be complex and counterintuitive.     

Environmental scanning electron microscopy as a new technique to determine the hygroscopic behaviour of individual aerosol particles

The hygroscopic behaviour of NaCl, (NH₄)₂SO₄, Na₂SO₄ and NH₄NO₃ particles in the size range of 0.1–20 μm was studied by environmental scanning electron microscopy (ESEM). This technique allows the in-situ observation of individual aerosol particles while changing the temperature and/or relative humidity (RH) in the sample chamber. The hygroscopic behaviour of these particles (e.g., deliquescence, adsorption of water on the particle surface) becomes directly observable with a lateral resolution of the order of 8–15 nm. The deliquescence relative humidities (DRH) of the different salts, the temperature dependence of the DRH for NH₄NO₃, and the growth factors (at increasing relative humidities) for NaCl were determined. Generally, a good agreement between the values obtained by ESEM and those found in literature was achieved. However, the DRH of NaCl determined by ESEM is systematically higher (approximately 2%, absolute) than the values obtained by other techniques, which can be explained by the observed strong absorption of water onto the crystal surface prior to droplet formation. The efflorescence behaviour of individual particles can be studied only qualitatively due to influences of the sample substrate. Furthermore, it is demonstrated that the activation of soot can be studied at high resolution by ESEM.     

Water uptake characteristics of individual atmospheric particles having coatings

We used an environmental transmission electron microscope to observe deliquescence and hygroscopic growth of atmospheric particles with hygroscopic coatings over the range 0–100% relative humidity (RH). The particles were collected from polluted and clean environments. Types included a sulfate-coated NaCl/silicate aggregate particle, a sulfate-coated sea-salt particle, and a Mg-rich, chloride-coated sea-salt particle. They all exhibited initial water uptake between 50% and 60% RH, although the first major morphological changes occurred at 70% RH. A deliquescence sphere, adjacent to the core particle, formed between 70% and 76% RH when deliquescence occurred or when the liquid phase was able to break out of the solid exterior coating. The deliquescence sphere grew to engulf the particle with increasing RH. Some particles developed a splatter zone associated with a particle coating. Efflorescence occurred over the range 49–44% RH. Our results indicate that some coated particles undergo a multi-step deliquescence process and that composition of the different phases within the coating affects deliquescence and hygroscopic growth below 76% RH. Above 76% RH, the dominant hygroscopic growth was due to water uptake by NaCl. Efflorescence of these particles also was strongly linked to NaCl, although the presence of other phases inhibited formation of a single NaCl crystal. Our results show that the observed coatings can both enhance particle solubility and lower the effective deliquescence RH of the particle. Thus, these coatings cause important phase and size changes for aerosol particles that could feed back into many other chemical and physical processes that contribute to radiative forcing within the atmosphere.     

The effect of water on gas–particle partitioning of secondary organic aerosol: II. m-xylene and 1,3,5-trimethylbenzene photooxidation systems

An investigation of the effect of relative humidity on aerosol formation from m-xylene and 1,3,5-trimethylbenzene photooxidation is reported. Experiments were performed in the presence and absence of ammonium sulfate seed particles (both aqueous and dry) to ascertain the effect of partitioning of oxidation products into a strong electrolytic solution or onto dry crystalline seed particles. In marked contrast to the α-pinene/ozone system, the final measured secondary organic aerosol yield was unaffected by the presence of gas-phase or liquid-phase water at relative humidities (RH) up to 50%. The hygroscopic nature of the aerosol generated upon photooxidation of m-xylene and 1,3,5-trimethylbenzene was examined; the hygroscopicity of the aerosol at 85% RH for both parent organics increased with the extent of the reaction, indicating that the first-generation oxidation products undergo further oxidation. Limited identification of the gas- and aerosol-phase products of m-xylene and 1,3,5-trimethylbenzene photooxidation is reported. It is evident that a more complete molecular identification of aromatic photooxidation aerosol awaits analytical techniques not yet brought to bear on this problem.     

On the sensitivity of particle size to relative humidity for Los Angeles aerosols

A TDMA system (Tandem Differential Mobility Analyzer; Rader D.J. and McMurry P.H. J. Aerosol Sci. 17, 771–787, 1986) was used to measure the sensitivity of particle size to relative humidity for monodisperse Los Angeles aerosols. Measurements were made at Claremont, CA on 13 days between 19 June and 3 September 1987, in conjunction with the Southern California Air Quality Study (SCAQS). The particle sizes that were studied ranged from 0.05 μm to 0.5 μm diameter at ambient relative humidity (typically 45–65%).The data provide clear evidence that these atmospheric aerosols were externally mixed. Monodisperse ambient aerosols were often found to split into nonhygroscopic (no water uptake) and hygroscopic portions when humidified. An average of 30% of the particles in the 0.2–0.5 μm range were nonhygroscopic. However, the proportion of the particles that was nonhygroscopic varied considerably from day to day and was, on occasion, as high as 70–80% of the particles. There was no clear evidence for nonhygroscopic 0.05 μm particles, but the data are not definitive on this point.The data also show that for the hydrophilic aerosol fraction, the larger particles (0.4–0.5 μm) grew more when humidified than did smaller particles (0.05–0.2 μm). As relative humidities were increased from 50% to 90%, particle diameters grew by average factors of 1.46 ±0.02 (for 0.5 μm particles), 1.49 ± 0.08 (0.4 μm), 1.19 ± 0.08 (0.2 μm) and 1.12 ± 0.05 (0.05 μm). Similarly, when particles were dried from 50% RH to 6–10% RH, particle diameters changed by factors ranging from 0.94 ± 0.03 (0.5 μm) to 0.98 ± 0.01 (0.05 μm).     

Influence of NO2 on S(IV) oxidation in aqueous suspensions of aerosol particles from two different origins

The influence of soluble compounds leached from real atmospheric aerosol particles (size range D_ae: 0.17–1.6 μm) and dissolved NO₂ on S(IV) oxidation in aqueous solution is presented. Experiments were conducted with aerosol particles of two different origins (i.e., urban and industrial) and at concentrations of trace gases in the gas mixtures (SO₂/air and SO₂/NO₂/air) typical for a polluted atmosphere. During the introduction of SO₂/air into the aqueous aerosol suspensions under dark conditions at pH 4, the formation of SO₄²⁻ was very slow with a long induction period. However, in the presence of NO₂ the oxidation rate of dissolved SO₂ in suspensions of aerosols from both origins increased substantially (about 10 times). The results suggest that soluble compounds eluted from atmospheric aerosols have not only a catalytic (e.g. Fe, Mn), but also a pronounced inhibiting effect (e.g., oxalate, formate, acetate, glycolate) on S(IV) autoxidation. When NO₂ was also introduced into the aerosol suspensions, the inhibition was not so highly expressed. An explanation for this is that the radical chain mechanism is mainly initiated by the interaction of dissolved NO₂ and HSO₃⁻. Therefore, at conditions typical for a polluted atmosphere dissolved NO₂ can have a significant influence on the secondary formation of SO₄²⁻.     

Assessing the Relative Contribution of Biogenic and Fossil Processes to Visibility-Scattering Aerosols Found in Remote Areas: Near-Term Organic Research Program

Abstract

Organic carbon has been found to be a significant component of aerosols that impair visibility in remote areas across the country. Organic aerosols are particularly important in western areas of the United States and contribute roughly equally with sulfate aerosols and dust in the total extinction budget. Potential visibility enhancement resulting from various future energy management options that reduce volatile organic carbon and particulate material emissions from fossil-energy-related processes hinges on the relative contribution of the fossil-fuel-derived organic component to the extinction budget. Thus, additional studies are needed to quantify the partitioning of organic carbon between biogenic and fossil sources. Relative humidity (RH) also plays an important role in visibility impairment. It is well known that water soluble aerosol species, such as sulfate and nitrate, can increase light-scattering efficiencies of fine particles by more than an order of magnitude as RH is increased from 20-30% to 90-95%. Organic carbon aerosol has been found to be a mixture of more soluble and less soluble components, but few studies have been performed to evaluate the RH response function of aerosols composed of these components, either separately or in combination, especially at high relative humidities. The purpose of this paper is to describe some experiments that could address the major uncertainties of biogenic and fossil carbon contributions to the fine particle extinction budget and visibility impairment.     

Coarse–fine and day–night differences of water-soluble ions in atmospheric aerosols collected in Christchurch and Auckland,New Zealand

The water-soluble ions in fine (PM_<2.5) and coarse (PM_2.5−10) atmospheric aerosols collected in Christchurch during winter 2001, spring 2000 and summer 2001, and in Auckland during winter 2001 have been studied in terms of coarse–fine and day–night differences. Although the chemical characteristics of the coarse particles were similar in both cities, those of the fine particles collected in the Christchurch winter were significantly different, as manifested by higher concentrations of nss-K⁺, nss-Cl⁻, nss-Ca²⁺, nss-SO₄²⁻, NO₃⁻ and NH₄⁺. It was found that nighttime PM₁₀ and nss-K⁺ concentrations were much higher than their daytime concentrations in the Christchurch winter but a clear day–night difference was not apparent in the Auckland winter. Moreover, in the winter, sea-salt ions did not show a day–night difference; however, nss-SO₄²⁻ had opposite day–night variation in the two cities. An ion balance calculation has shown that in most samples, coarse particles can be neutral or alkaline, however, fine particles can be neutral or acidic. The possibility of ammonium salts existing in the fine particles collected in the Christchurch winter is discussed and it is concluded that a variety of ammonium salts were present. Equivalent ratios suggest that the fine particles may be significantly aged in the Christchurch winter.The evidence from our soluble ion study strongly suggests that wood and coal burning and secondary aerosols make a significant contribution to fine particulate mass in the Christchurch atmosphere. Thus, home-heating, a sheltered geographic location and relatively calm atmospheric condition are thought to be the major causes for the serious atmospheric particulate pollution in the Christchurch winter.     

A case study of optical and chemical ground apportionment for urban aerosols in Thessaloniki

Urban aerosol characterization gathering ground-based in situ and sunphotometer measurements have been performed for the city of Thessaloniki for two specific days: the 12th and 13th of June 1997. A representative aerosol model for Thessaloniki aerosols was tentatively constructed for each day. Four components have been selected from our chemical measurements: black carbon (BC), particulate organic matter (POM), inorganic fine water soluble particles (WS) and a residue coarse component which mainly contains coarse dust and sea-salt particles (CC). Size distribution and complex refractive index for (WS) and (CC) components were determined from published data. (CC) has been shown to have a small optical effect compared to the submicron components. Size distribution for carbonaceous particles was obtained from sensitivity tests on particulate number and visible Angström exponent. The impact of relative humidity on extinction and scattering coefficients has been calculated on 13 June with Mie theory and Hänel relationships. Parameters needed for this calculation were well known for WS particles only. For POM particles we have used the experimental curve of hygroscopic factors obtained by Hobbs et al. (1997) for urban aerosols sampled on the East coast of United States to determine the hydrophilic dependency of POM particles. Relative humidity has been shown to be an important parameter even for values lower than 50%. Optical apportionment calculation has been realized pointing out that more than 45% of the total extinction coefficient is due to (POM) particles and about 20 and 30% to (WS) and (BC), respectively.     

Carbonaceous and inorganic composition in long-range transported aerosols over northern Japan: Implication for aging of water-soluble organic fraction

To better understand the influence of sources and atmospheric processing on aerosol chemical composition, we collected atmospheric particles in Sapporo, northern Japan during spring and early summer 2005 under the air mass transport conditions from Siberia, China and surrounding seas. The aerosols were analyzed for inorganic ions, organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), and the major water-soluble organic compound classes (i.e., dicarboxylic acids and sugars). SO₄^2? is the most abundant inorganic constituent (average 44% of the identified inorganic ion mass) followed by NH₄⁺ (21%) and NO₃^? (13%). Concentrations of OC, EC, and WSOC ranged from 2.0–16, 0.24–2.9, and 0.80–7.9 μg m^?3 with a mean of 7.4, 1.0, and 3.1 μg m^?3, respectively. High OC/EC ratios (range: 3.6–19, mean: 8.7) were obtained, however WSOC/OC ratios (0.23–0.69, 0.44) do not show any significant diurnal changes. These results suggest that the Sapporo aerosols were already aged, but were not seriously affected by local photochemical processes. Identified water-soluble organic compounds (diacids + sugars) account for <10% of WSOC. Based on some marker species and air mass back trajectory analyses, and using stable carbon isotopic compositions of shorter-chain diacids (i.e., C₂–C₄) as photochemical aging factor of organic aerosols, the present study suggests that a fraction of WSOC in OC is most likely influenced by aerosol aging, although the OC loading in aerosols may be more influenced by their sources and source regions.