Environmental factors driving the formation of water-soluble organic aerosols: A comparative study under contrasting atmospheric conditions

Water-soluble organic carbon (WSOC), as major fractions of atmospheric aerosols, have gained attention due to their light-absorption properties. To illustrate the sources and key environmental factors driving WSOC formation under different atmospheric conditions, a comparative study was conducted by summarizing the results obtained from five field campaigns at inland (urban, suburban or regional) sites and a coastal site during different seasons. Organic carbon concentrations varied from 8.5 μg/m³ at the summer regional site to 17.5 μg/m³ at the winter urban site, with 46 %- 89 % of the mass as WSOC. Based on correlation analysis, primary combustion emissions were more important in winter than in summer, and secondary formation was an important source of WSOC during winter, summer and autumn. Atmospheric oxidants (NO₂, O₃), aerosol liquid water (ALW) and ambient RH were important factors influencing the WSOC formation, while their roles varied in different atmospheres. We observed a seasonal transition of atmospheric oxidants dominating the WSOC formation from O₃ and NO₂-driven conditions in summer to NO₂-driven conditions in winter. Elevated ALW or ambient RH generally favor the WSOC formation, while the WSOC dependence of ALW varied among different ALW ranges. As the increasing of ALW or ambient RH, a transition of WSOC formation from "RH/ALW-limited regime" under low-ALW conditions, to "RH/ALW and precursor-driven regime" under medium-ALW/RH, and to "precursor-limited (RH/ALW-excess) regime" were observed for the inland atmospheric conditions. Under the high-RH and ALW conditions in coastal areas, ALW or ambient RH was generally not a limiting factor for WSOC formation.

Humic-like substances (HULIS) in springtime aerosols at a high-altitude background station in the western North Pacific: Source attribution, abundance, and light-absorption

Atmospheric humic-like substances (HULIS) are important components of biomass-burning (BB) emissions and highly associated with light-absorbing organic aerosols (often referred to as brown carbon). This study highlights the importance of BB-emitted HULIS aerosols in peninsular Southeast Asian outflow to the subtropical western North Pacific. We determined various key light-absorbing characteristics of HULIS i.e. mass absorption cross-section (MAC_HULIS), absorbing component of the refractive index (k_HULIS), and absorption Ångström exponent (AAE_HULIS) based on ground-based aerosol light absorption measurements along with HULIS concentrations in springtime aerosols at Lulin Atmospheric Background Station (LABS; 2862 m above mean sea level), which is a representative high-altitude remote site in the western North Pacific. Daily variations of HULIS (0.58-12.92 μg m^-3) at LABS were mostly linked with the influence from incoming air-masses, while correlations with BB tracers and secondary aerosols indicated the attribution of primary and secondary sources. Stronger light absorption capability of HULIS was clearly evident from MAC_HULIS and k_HULIS values at 370 nm, which were about ~1.5 times higher during BB-dominated days (1.16 ± 0.75 m² g^-1 and 0.05 ± 0.03, respectively) than that during non-BB days (0.77 ± 0.89 m² g^-1 and 0.03 ± 0.04, respectively). Estimates from a simple radiative transfer model showed that HULIS absorption can add as much as 15.13 W g^-1 to atmospheric warming, and ~46% more during BB-dominated than non-BB period, highlighting that HULIS light absorption may significantly affect the Earth-atmosphere system and tropospheric photochemistry over the western North Pacific.

Concentrations, optical properties and sources of humic-like substances (HULIS) in fine particulate matter in Xi'an, Northwest China

Humic-like substances (HULIS) are ubiquitous in the atmospheric environment, which affects both human health and climate. We present here the mass concentration and optical characteristics of HULIS isolated from aerosol samples collected in Xi'an, China. Both mass concentration and absorption coefficient (Abs₃₆₅) of HULIS show clear seasonal differences, with the highest average in winter (3.91 μgC m^-3 and 4.78 M m^-1, respectively) and the lowest in summer (0.65 μgC m^-3 and 0.55 M m^-1, respectively). The sources of HULIS_C and light absorption of HULIS were analyzed by positive matrix factorization (PMF) and four major sources were resolved, including secondary formation, biomass burning, coal burning, and vehicle emission. Our results show that secondary formation (i.e., gas-to-particle conversion from e.g., photochemical oxidation) was the major contributor to both HULIS_C (50%) and light absorption (55%) of HULIS in summer, biomass burning and coal burning were major sources of HULIS_C (~70%) and light absorption (~80%) of HULIS in winter. It is worth noting that biomass burning and coal burning had higher contribution to HULIS light absorption (47% in spring, 37% in summer, 73% in fall, and 77% in winter) than their corresponding contribution to HULIS_C concentration (41% in spring, 37% in summer, 54% in fall, and 69% in winter). However, vehicle emission had lower contribution to HULIS light absorption (26% in spring, 8% in summer, 18% in fall, and 11% in winter) than to HULIS_C concentration (24% in spring, 13% in summer, 28% in fall, and 18% in winter). These results suggest that HULIS from biomass burning and coal burning have higher light absorption ability than from vehicle emission.

Aqueous-phase oxidation of syringic acid emitted from biomass burning: Formation of light-absorbing compounds

Syringic acid is a methoxyphenol model compound derived from biomass burning, and its photooxidation processes have important effects on atmospheric chemistry. However, its aqueous-phase photochemistry remains unclear. In this study, we systematically report the photooxidation of syringic acid induced by OH radicals in the aqueous phase. Employing the relative rate technique, the bimolecular rate constant for syringic acid reaction with OH radicals was acquired to be (1.1 ± 0.3) × 10¹⁰ M^-1 s^-1. Notably, colored products were formed as the reaction progressed. Furthermore, the UV-vis and fluorescence spectra confirmed the formation of light-absorbing organic species, and the results agreed well with previous results on atmospheric and natural humic-like substances (HULIS). The photooxidation products were detected by high performance liquid chromatography mass spectrometry (HPLC/MS), and a possible reaction mechanism was proposed. The aqueous-phase reaction of syringic acid would undergo functionalization process forming a hydroxylation product that enhances the degree of oxidation of aqueous secondary organic aerosol (aqSOA), and goes through dimerization process by C-C or C-O coupling of phenoxy radicals which may conduce to the formation of HULIS. These findings suggest that the photooxidation of syringic acid is an important pathway for highly oxygenated phenolic aqSOA formation, providing a secondary source for HULIS in a liquid phase or in deliquescent particles surrounded by a layer of water.

Photoenhanced heterogeneous reaction of O3 with humic acid: Focus on O3 uptake and changes in the composition and optical property

Heterogeneous photochemical reaction of O₃ with humic acid (HA) under simulated sunlight was performed using a flow tube reactor coupled to an O₃ analyzer at ambient pressure. It was confirmed that light significantly enhanced the uptake of O₃ on HA. The initial uptake coefficient (γ_i) and the steady-state uptake coefficient (γ_ss) of O₃ under irradiation increased by 1.6 and 3.8 times compared to those in the dark, respectively. The γ_i and γ_ss on HA varied in the range of 0.76-2.77 × 10^-5 and 1.50-9.55 × 10^-6, respectively, which were dependent on various environmental factors including HA mass, total irradiance, initial O₃ concentration, O₂ content, temperature, relative humidity (RH) and HA solution pH. Both γ_i and γ_ss showed linear dependence on the total irradiance (0-2.07 × 10¹⁶ photons/(cm²⋅s)) of the light source, and increased with the HA mass (0-3.2 μg/cm²), temperature (278-298 K) and HA solution pH (4.0-9.6). However, they showed negative correlations with the initial O₃ concentration and O₂ content. The γ_i remained constant in the RH range of 7%-60%, while γ_ss exhibited the maximum value at RH = 20%. During the ozonization of HA under irradiation, some functional groups were consumed, including CH₂, CH₃, aromatic CC, OH, CO, COOH and COO^-. HA aged by O₃ exhibited a decrease in the mass absorption efficiency (MAE) and a small increase in the absorption Ångström exponent between 300 and 600 nm wavelength (AAE_300,600), which was ascribed to changes in the composition of HA during the photochemical ozonization process.

Speciation of carboxylic components in humic-like substances (HULIS) and source apportionment of HULIS in ambient fine aerosols (PM2.5) collected in Hong Kong

Humic-like substances (HULIS) are an important mixture of organic compounds, which account for a great part of water-soluble organic compounds in ambient aerosols. In this study, individual carboxylic and hydroxylic species in HULIS extracts of PM_2.5 samples collected in Hong Kong during summer were measured by gas chromatography mass spectrometry with prior chemical derivatization. Significantly higher levels of HULIS were observed on days mainly impacted by regional pollution (regional days, 4.11 ± 1.76 μg m^-3) than on days under local emission influences (local days, 0.56 ± 0.30 μg m^-3). Positive matrix factorization was applied to identify the major sources and apportion their contributions to HULIS. Simultaneous monitoring and analysis data from four different sampling sites showed that sources of HULIS in Hong Kong were mainly regional with small spatial variations. Secondary aerosol formation (both organic and inorganic) had a predominant contribution (52.7%) to HULIS during the whole sampling period. It accounted for 1.88 ± 0.91 μg m^-3 of HULIS on regional days, which was about 5 times higher than its contribution (0.39 ± 0.34 μg m^-3) on local days. Of the three identified primary sources, biomass burning had the largest contribution on both regional (34.9%) and local days (24.6%). Marine vessels were also a significant contributor, especially on local days (20.3%). Vehicle exhaust, on the other hand, showed a negligible contribution to HULIS (2.1%) in Hong Kong in this study.

Sources of humic-like substances (HULIS) in PM2.5 in Beijing: Receptor modeling approach

Recent work has identified the presence of humic-like substances (HULIS) in ambient fine particulate matter (PM_2.5) in Beijing, China and that residential coal combustion as well as biomass burning are significant contributors to its presence. These results were based on the characterization of emissions from representative stoves and modeling of the aerosol with the Community Multiscale Air Quality (CMAQ) chemical transport model. The CMAQ source apportionment estimated that residential coal and biofuel burning and secondary aerosol formation were important annual sources of ambient HULIS, contributing 47.1%, 15.1%, and 38.9%, respectively. In this study, chemical composition data including concentrations of water-soluble organic carbon and HULIS across four seasons during 2012-2013 were analyzed with positive matrix factorization (PMF) to provide a complementary source apportionment. The PMF results indicate that the identified sources were Traffic, Biomass Burning, Nitrate/Sulfate, Incineration, Sulfate, Coal Combustion/Ammonium Chloride, Residential Coal/Biofuel Combustion, and Road Dust/Soil with mass contributions (fractions) to PM_2.5 of 12.35 (10.4%), 8.70 (8.9%), 24.51 (22.4%), 5.64 (7.2%), 25.14 (24.5%), 7.10 (6.2%), 14.18 (15.4%), and 5.33 μg/m³ (5.0%), respectively. The contributions to the observed HULIS concentrations were 0.63 (10.9%), 0.38 (6.4%), 0.07 (1.7%), 0.00 (0%), 1.12 (28.8%), 0.00 (0%), 1.50 (52.2%), and 0.01 μg/m³ (0.3%), respectively. These PMF modeling results were in reasonable agreement with the CMAQ values supporting the attribution of significant amounts of primary HULIS to residential coal and biofuel combustion. Currently, efforts are underway in China to replace solid fuel combustion for heating and cooking with natural gas and electricity by 2020. Thus, future studies should be able to see substantial reductions in both PM_2.5 and HULIS in the near term future.

Cloud droplet activation of organic-salt mixtures predicted from two model treatments of the droplet surface

The droplet surface plays important roles in the interaction between organic aerosols with clouds and climate. Surface active organic compounds can partition to the droplet surface, depleting the solute from the droplet bulk or depressing the droplet surface tension. This may in turn affect the shape of the droplet growth curve, threshold of aerosol activation into cloud droplets, activated droplet size distributions, and cloud radiative effects. In this work, a new monolayer model along with a traditional Gibbs adsorption isotherm model was used in conjunction with equilibrium Köhler theory to predict cloud condensation nuclei (CCN) activation of both simple and complex surface active model aerosol systems. For the surface active aerosol considered, the monolayer droplet model produces similar results to the Gibbs model as well as comparable results to CCN measurements from the literature, even for systems where specific molecular identities and aqueous properties are unknown. The monolayer model is self-contained and fully prognostic, and provides a versatile, conceptually simple, yet physically based model for understanding the role of organic surfactants in cloud droplet formation.

Size-Resolved Surface-Active Substances of Atmospheric Aerosol: Reconsideration of the Impact on Cloud Droplet Formation

Our current understanding of the importance of surface-active substances (SAS) on atmospheric aerosol cloud-forming efficiency is limited, as explicit data on the content of size-resolved ambient aerosol SAS, which are responsible for lowering the surface tension (σ) of activating droplets, are not available. We report on the first data comprising seasonal variability of size-segregated SAS concentrations in ambient aerosol particulate matter (PM). To assess the impact of SAS distribution within PM on cloud droplet activation and growth, a concept of surfactant activity was adopted and a parametrization developed; i.e., surfactant activity factor (SAF) was defined, which allowed translation of experimental data for use in cloud parcel modeling. The results show that SAS-induced σ depression during cloud activation may affect droplet number ( N_d) as much as a 2-fold increase in particle number, whereas by considering also the size distribution of particulate SAS, N_d may increase for another 10%. This study underscores the importance of size-resolved SAS perspective on cloud activation, as data typically obtained from aqueous extracts of PM_2.5 and PM₁₀ may result in misleading conclusions about droplet growth due to large mass fractions of supermicron particles with SAS deficit and little or no influence on CCN and N_d.

Speciation of organic fraction does matter for source apportionment. Part 1: A one-year campaign in Grenoble (France)

PM₁₀ source apportionment was performed by positive matrix factorization (PMF) using specific primary and secondary organic molecular markers on samples collected over a one year period (2013) at an urban station in Grenoble (France). The results provided a 9-factor optimum solution, including sources rarely apportioned in the literature, such as two types of primary biogenic organic aerosols (fungal spores and plant debris), as well as specific biogenic and anthropogenic secondary organic aerosols (SOA). These sources were identified thanks to the use of key organic markers, namely, polyols, odd number higher alkanes, and several SOA markers related to the oxidation of isoprene, α-pinene, toluene and polycyclic aromatic hydrocarbons (PAHs). Primary and secondary biogenic contributions together accounted for at least 68% of the total organic carbon (OC) in the summer, while anthropogenic primary and secondary sources represented at least 71% of OC during wintertime. A very significant contribution of anthropogenic SOA was estimated in the winter during an intense PM pollution event (PM₁₀>50μgm^-3 for several days; 18% of PM₁₀ and 42% of OC). Specific meteorological conditions with a stagnation of pollutants over 10days and possibly Fenton-like chemistry and self-amplification cycle of SOA formation could explain such high anthropogenic SOA concentrations during this period. Finally, PMF outputs were also used to investigate the origins of humic-like substances (HuLiS), which represented 16% of OC on an annual average basis. The results indicated that HuLiS were mainly associated with biomass burning (22%), secondary inorganic (22%), mineral dust (15%) and biogenic SOA (14%) factors. This study is probably the first to state that HuLiS are significantly associated with mineral dust.

Persistence of urban organic aerosols composition: Decoding their structural complexity and seasonal variability

Organic Aerosols (OAs) are typically defined as highly complex matrices whose composition changes in time and space. Focusing on time vector, this work uses two-dimensional nuclear magnetic resonance (2D NMR) techniques to examine the structural features of water-soluble (WSOM) and alkaline-soluble organic matter (ASOM) sequentially extracted from fine atmospheric aerosols collected in an urban setting during cold and warm seasons. This study reveals molecular signatures not previously decoded in NMR-related studies of OAs as meaningful source markers. Although the ASOM is less hydrophilic and structurally diverse than its WSOM counterpart, both fractions feature a core with heteroatom-rich branched aliphatics from both primary (natural and anthropogenic) and secondary origin, aromatic secondary organics originated from anthropogenic aromatic precursors, as well as primary saccharides and amino sugar derivatives from biogenic emissions. These common structures represent those 2D NMR spectral signatures that are present in both seasons and can thus be seen as an "annual background" profile of the structural composition of OAs at the urban location. Lignin-derived structures, nitroaromatics, disaccharides, and anhydrosaccharides signatures were also identified in the WSOM samples only from periods identified as smoke impacted, which reflects the influence of biomass-burning sources. The NMR dataset on the H-C molecules backbone was also used to propose a semi-quantitative structural model of urban WSOM, which will aid efforts for more realistic studies relating the chemical properties of OAs with their atmospheric behavior.

Molecular Characterization of Nitrogen-Containing Organic Compounds in Humic-like Substances Emitted from Straw Residue Burning

The molecular composition of humic-like substances (HULIS) in different aerosol samples was analyzed using an ultrahigh-resolution mass spectrometer to investigate the influence of biomass burning on ambient aerosol composition. HULIS in background aerosols were characterized with numerous molecular formulas similar to biogenic secondary organic aerosols. The abundance of nitrogen-containing organic compounds (NOC), including nitrogen-containing bases (N-bases) and nitroaromatics, increased dramatically in ambient aerosols affected by crop residue burning in the farm field. The molecular distribution of N-bases in these samples exhibited similar patterns to those observed in smoke particles freshly emitted from lab-controlled burning of straw residues but were significantly different with those observed from wood burning. Signal intensity of the major N-bases correlated well with the atmospheric concentrations of potassium and levoglucosan. These N-bases can serve as molecular markers distinguishing HULIS from crop residue burning with from wood burning. More nitroaromatics were detected in ambient aerosols affected by straw burning than in fresh smoke aerosols, indicating that many of them are formed in secondary oxidation processes as smoke plumes evolve in the atmosphere. This study highlights the significant contribution of crop residue burning to atmospheric NOC. Further study is warranted to evaluate the roles of NOC on climate and human health.

Two-Dimensional Offline Chromatographic Fractionation for the Characterization of Humic-Like Substances in Atmospheric Aerosol Particles

Organic carbon in atmospheric particles comprises a large fraction of chromatographically unresolved compounds, often referred to as humic-like substances (HULIS), which influence particle properties and impact climate, human health, and ecosystems. To better understand its composition, a two-dimensional (2D) offline method combining size-exclusion (SEC) and reversed-phase liquid chromatography (RP-HPLC) using a new spiked gradient profile is presented. It separates HULIS into 55 fractions of different size and polarity, with estimated ranges of molecular weight and octanol/water partitioning coefficient (log P) from 160-900 g/mol and 0.2-3.3, respectively. The distribution of HULIS within the 2D size versus polarity space is illustrated with heat maps of ultraviolet absorption at 254 nm. It is found to strongly differ in a small example set of samples from a background site near Leipzig, Germany. In winter, the most intense signals were obtained for the largest molecules (>520 g/mol) with low polarity (log P ∼ 1.9), whereas in summer, smaller (225-330 g/mol) and more polar (log P ∼ 0.55) molecules dominate. The method reveals such differences in HULIS composition in a more detailed manner than previously possible and can therefore help to better elucidate the sources of HULIS in different seasons or at different sites. Analyzing Suwannee river fulvic acid as a common HULIS surrogate shows a similar polarity range, but the sizes are clearly larger than those of atmospheric HULIS.

Chemical characterization of humic-like substances (HULIS) in PM2.5 in Lanzhou, China

Evaporative light scattering detection (ELSD) was applied to quantify HULIS (humic-like substances) for the first time in 2012 winter and 2013 summer at an urban site in Lanzhou. Water soluble organic carbon (WSOC), water soluble inorganic ions, and carbonaceous species (OC/EC) were also analyzed. The results show that OM (Organic Matter=OC×1.6, constituting 45.8% to PM_2.5) was the most abundant species, followed by SNA (SO₄^2-+NO₃^-+NH₄⁺, constituting 23.4% to PM_2.5). The chemical species were in the order of: OC>EC>SO₄^2->NO₃^->NH₄⁺>Cl^->Ca²⁺>K⁺. The annual average concentration of HULIS was 4.70μg/m^-3 and HULISc (carbon content of HULIS) contributed 6.19% to PM_2.5 and 45.6% to WSOC, indicating that HULIS was the most important components of WSOC. The concentration of HULIS was 2.14±0.80μg/m³ in summer and 7.24±2.77μg/m³ in winter, respectively. The concentrations of HULIS were relatively low and stable in summer, while high and varied dramatically in winter. The abundance of HULISc in WSOC shows a more concentrated distribution in Lanzhou, with a range between 0.28-0.57. The ratios of HULIS/K⁺ were 6.25±1.41 and 6.14±1.96 in summer and winter, respectively, suggesting there were other significant sources in addition to biomass burning emissions. HULIS and WSOC exhibited similar seasonal variation and had a strong positive correlation. In addition to the good relationship (0.89) between HULIS and Cl^- in winter, the great enhancement of HULIS with significantly high Cl^- and relatively low K⁺ in winter indicated that residential coal burning was probably an important HULIS source in winter. Correlation and back trajectory analysis suggested that biomass burning and secondary formation were also important HULIS sources and the contribution of HULIS from dust could be neglected. Adverse meteorological conditions were also important factors for the accumulation of HULIS in winter.

Springtime precipitation effects on the abundance of fluorescent biological aerosol particles and HULIS in Beijing

Bioaerosols and humic-like substances (HULIS) are important components of atmospheric aerosols, which can affect regional climate by acting as cloud condensation nuclei and some of which can damage human health. Up to date, release of bioaerosols and HULIS initiated by precipitation is still poorly understood. Here we present different release processes for bioaerosols, non-bioaerosols and HULIS during a precipitation event in Beijing, China. Large fungal-spore-like aerosols were emitted at the onset and later weak stage of precipitation, the number concentration of which increased by more than two folds, while the number concentration of bacteria-like particles doubled when the precipitation strengthened. Besides, a good correlation between protein-like substances that were measured simultaneously by on-line and off-line fluorescence techniques consolidated their applications to measure bioaerosols. Furthermore, our EEM results suggest that the relative contribution of water-soluble HULIS to microbial materials was enhanced gradually by the rain event.

Two chemically distinct light-absorbing pools of urban organic aerosols: A comprehensive multidimensional analysis of trends

The chemical and light-absorption dynamics of organic aerosols (OAs), a master variable in the atmosphere, have yet to be resolved. This study uses a comprehensive multidimensional analysis approach for exploiting simultaneously the compositional changes over a molecular size continuum and associated light-absorption (ultraviolet absorbance and fluorescence) properties of two chemically distinct pools of urban OAs chromophores. Up to 45% of aerosol organic carbon (OC) is soluble in water and consists of a complex mixture of fluorescent and UV-absorbing constituents, with diverse relative abundances, hydrophobic, and molecular weight (Mw) characteristics between warm and cold periods. In contrast, the refractory alkaline-soluble OC pool (up to 18%) is represented along a similar Mw and light-absorption continuum throughout the different seasons. Results suggest that these alkaline-soluble chromophores may actually originate from primary OAs sources in the urban site. This work shows that the comprehensive multidimensional analysis method is a powerful and complementary tool for the characterization of OAs fractions. The great diversity in the chemical composition and optical properties of OAs chromophores, including both water-soluble and alkaline-soluble OC, may be an important contribution to explain the contrasting photo-reactivity and atmospheric behavior of OAs.

Size distribution and sources of humic-like substances in particulate matter at an urban site during winter

This study investigates the size distribution and possible sources of humic-like substances (HULIS) in ambient aerosol particles collected at an urban site in Gwangju, Korea during the winter of 2015. A total of 10 sets of size-segregated aerosol samples were collected using a 10-stage Micro-Orifice Uniform Deposit Impactor (MOUDI), and the samples were analyzed to determine the mass as well as the presence of ionic species (Na(+), NH4(+), K(+), Ca(2+), Mg(2+), Cl(-), NO3(-), and SO4(2-)), water-soluble organic carbon (WSOC) and HULIS. The separation and quantification of the size-resolved HULIS components from the MOUDI samples was accomplished using a Hydrophilic-Lipophilic Balanced (HLB) solid phase extraction method and a total organic carbon analyzer, respectively. The entire sampling period was divided into two periods: non-Asian dust (NAD) and Asian dust (AD) periods. The contributions of water-soluble organic mass (WSOM = 1.9 × WSOC) and HULIS (=1.9 × HULIS-C) to fine particles (PM1.8) were approximately two times higher in the NAD samples (23.2 and 8.0%) than in the AD samples (12.8 and 4.2%). However, the HULIS-C/WSOC ratio in PM1.8 showed little difference between the NAD (0.35 ± 0.07) and AD (0.35 ± 0.05) samples. The HULIS exhibited a uni-modal size distribution (@0.55 μm) during NAD and a bimodal distribution (@0.32 and 1.8 μm) during AD, which was quite similar to the mass size distributions of particulate matter, WSOC, NO3(-), SO4(2-), and NH4(+) in both the NAD and AD samples. The size distribution characteristics and the results of the correlation analyses indicate that the sources of HULIS varied according to the particle size. In the fine mode (≤1.8 μm), the HULIS composition during the NAD period was strongly associated with secondary organic aerosol (SOA) formation processes similar to those of secondary ionic species (cloud processing and/or heterogeneous reactions) and primary emissions during the biomass burning period, and during the AD period, it was only associated with SOA formation. In the coarse mode (3.1-10 μm), it was difficult to identify the HULIS sources during the NAD period, and during the AD period, the HULIS was most likely associated with soil-related particles [Ca(NO3]2 and CaSO4) and/or sea-salt particles (NaNO3 and Na2SO4).

Measurement of humic-like substances in aerosols: a review

Aerosol-phase humic-like substances (HULIS) have received increasingly attention due to their universal ambient presence, active participation in atmospheric chemistry and important environmental and health effects. In last decade, intensive field works have promoted development of quantification and analysis method, unearthed spatio-temporal variation, and proved evidence for source identification of HULIS. These important developments were summarized in this review to provide a global perspective of HULIS. The diverse operational HULIS definitions were gradually focused onto several versions. Although found globally in Europe, Asia, Australasia and North America, HULIS are far more typical in continental and near-ground aerosols. HULIS concentrations varied from <1 μg/m(3) to >13 μg/m(3), with their carbon fraction making up 9%-72% of water soluble organic carbon. Dominant HULIS source was suggested as secondary processes and biomass burning, with the detailed formation pathways suggested and verified in laboratory works.

Oxidation of atmospheric humic like substances by ozone: a kinetic and structural analysis approach

This work explores the heterogeneous reaction between HUmic-LIke Substances (so-called HULIS) and ozone. Genuine atmospheric HULIS were extracted from aerosol samples collected in Chamonix (France) in winter and used in coated flow tube experiments to evaluate heterogeneous uptake of O₃ on such mixtures. The uptake coefficient (γ) was investigated as a function of pH (from 2.5 to 10), O₃ concentration (from 8 to 33 × 10¹¹ molecules cm⁻³), relative humidity (20 to 65%) and photon flux (from 0 to 1.66 × 10¹⁵ photons cm⁻² s⁻¹). Reactive uptake was found to increase in the irradiated experiment with pH, humidity and photon flux. The extract was characterized before and after exposure to O₃ and/or UV light in the attempt to elucidate the effect of the photochemical aging. Carbon content measurements, UV-vis spectroscopy and functional groups analysis revealed a decrease of the UV absorbance as well as of the carbon mass content, while the functionalization rate (COOH and C═O) and therefore the polarity increased during the simulated photochemical exposure.

Characterization of atmospheric aerosols at a forested site in central Europe

Mass concentrations, mass size distributions, time series, and diel variations for organic tracers and major inorganic ions in aerosols from K-puszta, Hungary, during a 2003 summer period are reported. Emphasis was placed on alpha-beta-pinene secondary organic aerosol (SOA) tracers comprising cis-pinic acid, 3-hydroxyglutaric acid, and 3-methyl-1,2,3-butanetricarboxylic acid. Only cis-pinic acid and the d-limonene SOA tracer 3-carboxyheptanedioic acid exhibited diel variations with highest concentrations at night Malic acid was fairly well correlated with succinic and oxalic acid, pointing to a similar SOA formation process. No day-night variations were observed for the latter acids, suggesting that they are formed over relatively longtime scales. Of the ionic species sulfate, ammonium, and nitrate, only nitrate showed clear diel variations with highest concentrations at night. As to the size-segregated samples, the 2-methyltetrols were present in both the fine and coarse modes, while the C5-alkene triols and the alpha-/beta-pinene SOA tracers were only associated with the fine mode. The ionic species sulfate, ammonium, and nitrate made up for, on average, 24, 10, and 26% of the PM2.5 mass, while organic matter was responsible for 47% of that mass. Isoprene and alpha-pinene secondary organic carbon (SOC) accounted, on average, for, respectively, 6.8 and at least 4.8% of the PM2.5 organic carbon, but the contribution of isoprene SOC was more pronounced during daytime (9.6%), whereas that of alpha-pinene SOC was largest at night (at least 6.0%).

Intercomparison of measurement techniques for black or elemental carbon under urban background conditions in wintertime: influence of biomass combustion

A generally accepted method to measure black carbon (BC) or elemental carbon (EC) still does not exist. An earlier study in the Vienna area comparing practically all measurement methods in use in Europe gave comparable BC and EC concentrations under summer conditions (Hitzenberger et al., 2006a). Under summer conditions, Diesel traffic is the major source for EC or BC in Vienna. Under winter conditions, space heating (also with biomass as fuel) is another important source (Caseiro et al., 2007). The present study compares the response of thermal methods (a modified Cachier method, Cachier et al., 1989; a thermal-optical method, Schmid et al., 2001; and two thermal-optical (TOT) methods using Sunset instruments, Birch and Cary, 1996 and Schauer et al., 2003) and optical methods (a light transmission method, Hansen et al., 1984; the integrating sphere method, Hitzenberger et al., 1996; and the multiangle absorption photometer MAAP, Petzold and Schönlinner, 2004). Significant differences were found between the TOT methods on the one hand and all other methods on the other. The TOT methods yielded EC concentrations that were lower by 44 and 17% than the average of all measured concentrations (including the TOT data). The largest discrepancy was found when the contribution of brown carbon (measured with the integrating sphere method) was largest.