Determination of isoprene and alpha-/beta-pinene oxidation products in boreal forest aerosols from Hyytiälä, Finland: diel variations and possible link with particle formation events

Organic synthesis in the study of terpene-derived oxidation products in the atmosphere

Covering: 1997 up to 2022Volatile biogenic terpenes involved in the formation of secondary organic aerosol (SOA) particles participate in rich atmospheric chemistry that impacts numerous aspects of the earth's complex climate system. Despite the importance of these species, understanding their fate in the atmosphere and determining their atmospherically-relevant properties has been limited by the availability of authentic standards and probe molecules. Advances in synthetic organic chemistry directly aimed at answering these questions have, however, led to exciting discoveries at the interface of chemistry and atmospheric science. Herein we provide a review of the literature regarding the synthesis of commercially unavailable authentic standards used to analyze the composition, properties, and mechanisms of SOA particles in the atmosphere.

Molecular and Structural Characterization of Isomeric Compounds in Atmospheric Organic Aerosol Using Ion Mobility-Mass Spectrometry

Secondary organic aerosol (SOA) formed through multiphase atmospheric chemistry makes up a large fraction of airborne particles. The chemical composition and molecular structures of SOA constituents vary between different emission sources and aging processes in the atmosphere, which complicates their identification. In this work, we employ drift tube ion mobility spectrometry with quadrupole time-of-flight mass spectrometry (IM-MS) detection for rapid gas-phase separation and multidimensional characterization of isomers in two biogenic SOAs produced from ozonolysis of isomeric monoterpenes, d-limonene (LSOA) and α-pinene (PSOA). SOA samples were ionized using electrospray ionization (ESI) and characterized using IM-MS in both positive and negative ionization modes. The IM-derived collision cross sections in nitrogen gas (^DTCCS_N2 ) for individual SOA components were obtained using multifield and single-field measurements. A novel application of IM multiplexing/high-resolution demultiplexing methodology was employed to increase sensitivity, improve peak shapes, and augment mobility baseline resolution, which revealed several isomeric structures for the measured ions. For LSOA and PSOA samples, we report significant structural differences of the isomer structures. Molecular structural calculations using density functional theory combined with the theoretical modeling of CCS values provide insights into the structural differences between LSOA and PSOA constituents. The average ^DTCCS_N2 values for monomeric SOA components observed as [M + Na]⁺ ions are 3-6% higher than those of their [M - H]^- counterparts. Meanwhile, dimeric and trimeric isomer components in both samples showed an inverse trend with the relevant values of [M - H]^- ions being 3-7% higher than their [M + Na]⁺ counterparts, respectively. The results indicate that the structures of Na⁺-coordinated oligomeric ions are more compact than those of the corresponding deprotonated species. The coordination with Na⁺ occurs on the oxygen atoms of the carbonyl groups leading to a compact configuration. Meanwhile, deprotonated molecules have higher ^DTCCS_N2 values due to their elongated structures in the gas phase. Therefore, ^DTCCS_N2 values of isomers in SOA mixtures depend strongly on the mode of ionization in ESI. Additionally, PSOA monomers and dimers exhibit larger ^DTCCS_N2 values (1-4%) than their LSOA counterparts owing to more rigid structures. A cyclobutane ring is present with functional groups pointing in opposite directions in PSOA compounds, as compared to noncyclic flexible LSOA structures, forming more compact ions in the gas phase. Lastly, we investigated the effects of direct photolysis on the chemical transformations of selected individual PSOA components. We use IM-MS to reveal structural changes associated with aerosol aging by photolysis. This study illustrates the detailed molecular and structural descriptors for the detection and annotation of structural isomers in complex SOA mixtures.

Secondary Organic Aerosol Formation from Isoprene: Selected Research, Historic Account and State of the Art

In this review, we cover selected research on secondary organic aerosol (SOA) formation from isoprene, from the beginning of research, about two decades ago, to today. The review begins with the first observations of isoprene SOA markers, i.e., 2-methyltetrols, in ambient fine aerosol and focuses on studies dealing with molecular characterization, speciation, formation mechanisms, and source apportionment. A historic account is given on how research on isoprene SOA has developed. The isoprene SOA system is rather complex, with different pathways being followed in pristine and polluted conditions. For SOA formation from isoprene, acid-catalyzed hydrolysis is necessary, and sulfuric acid enhances SOA by forming additional nonvolatile products such as organosulfates. Certain results reported in early papers have been re-interpreted in the light of recent results; for example, the formation of C5-alkene triols. Attention is given to mass spectrometric and separation techniques, which played a crucial role in molecular characterization. The unambiguous structural characterization of isoprene SOA markers has been achieved, owing to the preparation of reference compounds. Efforts have also been made to use air quality data to estimate the influence of biogenic and pollution aerosol sources. This review examines the use of an organic marker-based method and positive matrix factorization to apportion SOA from different sources, including isoprene SOA.

Seasonal characteristics of biogenic secondary organic aerosols at Mt. Wuyi in Southeastern China: Influence of anthropogenic pollutants

Thirteen secondary organic aerosol (SOA) tracers of isoprene, monoterpenes and sesquiterpenes were measured for PM_2.5 aerosols collected at the summit of Mt. Wuyi (1139 m, a.s.l.), to investigate their seasonality and formation mechanism. Concentrations of the isoprene and monoterpene SOA tracers were much higher in summer than those in other seasons. In contrast, β-caryophyllinic acid was found to be the lowest in summer. Concentrations of those BSOA tracers showed a positive correlation with temperature (R² = 0.52-0.70), and a negative correlation with relative humidity (R² = 0.43-0.78). Moreover, thermodynamic model (i.e., ISORROPIA-II) calculation results showed that acidity conditions are favorable for BSOA formation. Robust linear correlations between the BSOA tracers and anthropogenic pollutants such as SO₂ (R² = 0.53-0.7) and NO₂ (R² = 0.37-0.54) were observed for all the samples, suggesting that SO₂ and NOx can enhance BSOA production in the remote mountain area of southeast China, which is related to an acid-catalyzed heterogeneous chemistry. Moreover, we also found a significant correlation between the concentrations of the BSOA tracers and levoglucosan especially for β-caryophyllinic acid, indicating that biomass burning plumes from the distant lowland regions could influence the production of BSOA in the mountain free troposphere. Our results clearly demonstrated that anthropogenic emissions in China could enhance BSOA formation in the distant mountain regions.

Seasonal variation and size distribution of biogenic secondary organic aerosols at urban and continental background sites of China

Size-resolved biogenic secondary organic aerosols (BSOA) derived from isoprene and monoterpene photooxidation in Qinghai Lake, Tibetan Plateau (a continental background site) and five cities of China were measured using gas chromatography/mass spectrometry (GC/MS). Concentrations of the determined BSOA are higher in the cities than in the background and are also higher in summer than in winter. Moreover, strong positive correlations (R²=0.44-0.90) between BSOA and sulfate were found at the six sites, suggesting that anthropogenic pollution (i.e., sulfate) could enhance SOA formation, because sulfate provides a surface favorable for acid-catalyzed formation of BSOA. Size distribution measurements showed that most of the determined SOA tracers are enriched in the fine mode (<3.3μm) except for cis-pinic and cis-pinonic acids, both presented a comparable mass in the fine and coarse (>3.3μm) modes, respectively. Mass ratio of oxidation products derived from isoprene to those from monoterpene in the five urban regions during summer are much less than those in Qinghai Lake region. In addition, in the five urban regions relative abundances of monoterpene oxidation products to SOA are much higher than those of isoprene. Such phenomena suggest that BSOA derived from monoterpenes are more abundant than those from isoprene in Chinese urban areas.

Phyllostachys edulis forest reduces atmospheric PM2.5 and PAHs on hazy days at suburban area

This study is aim to illustrate Phyllostachys edulis' role in affecting air quality under hazy day and solar day. P. edulis is a crucial plants growing well at suburban area at China Southern. In this manuscript, on 2 weather conditions (hazy day; solar day), changes in atmospheric particulate matter (PM), polycyclic aromatic hydrocarbons (PAHs), associated volatile organic compounds (VOCs), and PAHs in leaves and soils were measured, with PM-detection equipment and the GC-MC method, in a typical bamboo forest at suburban areas. The results showed that: (1) Bamboo forest decreased atmospheric PM_2.5 and PM₁₀ concentrations significantly by 20% and 15%, respectively, on the hazy day nightfall time, when they were times higher than that on any other time. Also, similar effects on atmospheric PAHs and VOCs were found. (2) Significant increases in PAHs of leaves and soil were found inside the forest on the hazy day. (3) Bamboo forest also reduced the atmospheric VOC concentrations, and changed the compounds of 10 VOCs present in the highest concentration list. Thus, bamboo forests strongly regulate atmospheric PM_2.5 through capture or retention, for the changes in atmospheric VOCs and increase in PAHs of leaves and soil.

Impacts of springtime biomass burning in the northern Southeast Asia on marine organic aerosols over the Gulf of Tonkin, China

Fine particles (PM_2.5) samples, collected at Weizhou Island over the Gulf of Tonkin on a daytime and nighttime basis in the spring of 2015, were analyzed for primary and secondary organic tracers, together with organic carbon (OC), elemental carbon (EC), and stable carbon isotopic composition (δ¹³C) of total carbon (TC). Five organic compound classes, including saccharides, lignin/resin products, fatty acids, biogenic SOA tracers and phthalic acids, were quantified by gas chromatography/mass spectrometry (GC/MS). Levoglucosan was the most abundant organic species, indicating that the sampling site was under strong influence of biomass burning. Based on the tracer-based methods, the biomass-burning-derived fraction was estimated to be the dominant contributor to aerosol OC, accounting for 15.7% ± 11.1% and 22.2% ± 17.4% of OC in daytime and nighttime samples, respectively. In two episodes E1 and E2, organic aerosols characterized by elevated concentrations of levoglucosan as well as its isomers, sugar compounds, lignin products, high molecular weight (HMW) fatty acids and β-caryophyllinic acid, were attributed to the influence of intensive biomass burning in the northern Southeast Asia (SEA). However, the discrepancies in the ratios of levoglucosan to mannosan (L/M) and OC (L/OC) as well as the δ¹³C values suggest the type of biomass burning and the sources of organic aerosols in E1 and E2 were different. Hardwood and/or C₄ plants were the major burning materials in E1, while burning of softwood and/or C₃ plants played important role in E2. Furthermore, more complex sources and enhanced secondary contribution were found to play a part in organic aerosols in E2. This study highlights the significant influence of springtime biomass burning in the northern SEA to the organic molecular compositions of marine aerosols over the Gulf of Tonkin.

Tropospheric volatile organic compounds in China

Photochemical smog, characterized by high concentrations of ozone (O₃) and fine particles (PM_2.5) in the atmosphere, has become one of the top environmental concerns in China. Volatile organic compounds (VOCs), one of the key precursors of O₃ and secondary organic aerosol (SOA) (an important component of PM_2.5), have a critical influence on atmospheric chemistry and subsequently affect regional and global climate. Thus, VOCs have been extensively studied in many cities and regions in China, especially in the North China Plain, the Yangtze River Delta and the Pearl River Delta regions where photochemical smog pollution has become increasingly worse over recent decades. This paper reviews the main studies conducted in China on the characteristics and sources of VOCs, their relationship with O₃ and SOA, and their removal technology. This paper also provides an integrated literature review on the formulation and implementation of effective control strategies of VOCs and photochemical smog, as well as suggestions for future directions of VOCs study in China.

Molecular Markers of Secondary Organic Aerosol in Mumbai, India

Biogenic secondary organic aerosols (SOA) are generally considered to be more abundant in summer than in winter. Here, polar organic marker compounds in urban background aerosols from Mumbai were measured using gas chromatography-mass spectrometry. Surprisingly, we found that concentrations of biogenic SOA tracers at Mumbai were several times lower in summer (8-14 June 2006; wet season; n = 14) than in winter (13-18 February 2007; dry season; n = 10). Although samples from less than 10% of the season are extrapolated to the full season, such seasonality may be explained by the predominance of the southwest summer monsoon, which brings clean marine air masses to Mumbai. While heavy rains are an important contributor to aerosol removal during the monsoon season, meteorological data (relative humidity and T) suggest no heavy rains occurred during our sampling period. However, in winter, high levels of SOA and their day/night differences suggest significant contributions of continental aerosols through long-range transport together with local sources. The winter/summer pattern of SOA loadings was further supported by results from chemical transport models (NAQPMS and GEOS-Chem). Furthermore, our study suggests that monoterpene- and sesquiterpene-derived secondary organic carbon (SOC) were more significant than those of isoprene- and toluene-SOC at Mumbai.

Natural sources of atmospheric aerosols influencing air quality across Europe

Atmospheric aerosols are emitted by natural and anthropogenic sources. Contributions from natural sources to ambient aerosols vary widely with time (inter-annual and seasonal variability) and as a function of the distance to source regions. This work aims to identify the main natural sources of atmospheric aerosols affecting air quality across Europe. The origin, frequency, magnitude, and spatial and temporal variability of natural events were assessed for the years 2008 and 2009. The main natural sources of atmospheric aerosols identified were African dust, sea spray and wildfires. Primary biological particles were not included in the present work. Volcanic eruptions did not affect air quality significantly in Europe during the study period. The impact of natural episodes on air quality was significant in Southern and Western Europe (Cyprus, Spain, France, UK, Greece, Malta, Italy and Portugal), where they contributed to surpass the PM10 daily and annual limit values. In Central and Northern Europe (Germany, Austria and Latvia) the impact of these events was lower, as it resulted in the exceedance of PM daily but not annual limit values. Contributions from natural sources to mean annual PM10 levels in 2008 and 2009 ranged between 1 and 2 μg/m(3) in Italy, France and Portugal, between 1 and 4 μg/m(3) in Spain (10 μg/m(3) when including the Canary Islands), 5 μg/m(3) in UK, between 3 and 8 μg/m(3) in Greece, and reached up to 13 μg/m(3) in Cyprus. The evaluation of the number of monitoring stations per country reporting natural exceedances of the daily limit value (DLV) is suggested as a potential tool for air quality monitoring networks to detect outliers in the assessment of natural contributions. It is strongly suggested that a reference methodology for the identification and quantification of African dust contributions should be adopted across Europe.

Isoprene synthase genes form a monophyletic clade of acyclic terpene synthases in the TPS-B terpene synthase family

Many plants emit significant amounts of isoprene, which is hypothesized to help leaves tolerate short episodes of high temperature. Isoprene emission is found in all major groups of land plants including mosses, ferns, gymnosperms, and angiosperms; however, within these groups isoprene emission is variable. The patchy distribution of isoprene emission implies an evolutionary pattern characterized by many origins or many losses. To better understand the evolution of isoprene emission, we examine the phylogenetic relationships among isoprene synthase and monoterpene synthase genes in the angiosperms. In this study we identify nine new isoprene synthases within the rosid angiosperms. We also document the capacity of a myrcene synthase in Humulus lupulus to produce isoprene. Isoprene synthases and (E)-β-ocimene synthases form a monophyletic group within the Tps-b clade of terpene synthases. No asterid genes fall within this clade. The chemistry of isoprene synthase and ocimene synthase is similar and likely affects the apparent relationships among Tps-b enzymes. The chronology of rosid evolution suggests a Cretaceous origin followed by many losses of isoprene synthase over the course of evolutionary history. The phylogenetic pattern of Tps-b genes indicates that isoprene emission from non-rosid angiosperms likely arose independently.

Source apportionment of organic compounds in Berlin using positive matrix factorization - assessing the impact of biogenic aerosol and biomass burning on urban particulate matter

Source apportionment of 13 organic compounds, elemental carbon and organic carbon of ambient PM(10) and PM(1) was performed with positive matrix factorization (PMF). Samples were collected at three sites characterized by different vegetation influences in Berlin, Germany in 2010. The aim was to determine organic, mainly biogenic sources and their impact on urban aerosol collected in a densely populated region. A 6-factor solution provided the best data fit for both PM-fractions, allowing the sources isoprene- and α-pinene-derived secondary organic aerosol (SOA), bio primary, primarily attributable to fungal spores, bio/urban primary including plant fragments in PM(10) and cooking and traffic emissions in PM(1), biomass burning and combustion fossil to be identified. With mean concentrations up to 2.6 μg Cm(-3), biomass burning dominated the organic fraction in cooler months. Concentrations for α-pinene-derived SOA exceeded isoprene-derived concentrations. Estimated secondary organic carbon contributions to total organic carbon (OC) were between 7% and 42% in PM(10) and between 11% and 60% in PM(1), which is slightly lower than observed for US- or Asian cities. Primary biogenic emissions reached up to 33% of OC in the PM(10)-fraction in the late summer and autumn months. Temperature-dependence was found for both SOA-factors, correlations with ozone and mix depth only for the α-pinene-derived SOA-factor. Latter indicated input of α-pinene from the borders, highlighting differences in the origin of the precursors of both factors. Most factors were regionally distributed. High regional distribution was found to be associated with stronger influence of ambient parameters and higher concentrations at the background station. A significant contribution of biogenic emissions and biomass burning to urban organic aerosol could be stated. This indicates a considerable impact on PM concentrations also in cities in a densely populated area, and should draw the attention concerning health aspects not only to cardio-vascular diseases but also to allergy issues.

The use of polar organic compounds to estimate the contribution of domestic solid fuel combustion and biogenic sources to ambient levels of organic carbon and PM2.5 in Cork Harbour, Ireland

PM(2.5) samples collected at Cork Harbour, Ireland during summer, autumn, late autumn and winter, 2008-2009 were analyzed for polar organic compounds that are useful markers for aerosol source characterization. The determined compounds include tracers for biomass burning primary particles, fungal spores, markers for secondary organic aerosol (SOA) from isoprene, α-/β-pinene, and d-limonene. Seasonal and temporal variations and other characteristic features of the detected tracers are discussed in terms of aerosol sources and processes. The biogenic species were detected only during the summer period where the contributions of isoprene SOA and fungal spores to the PM(2.5) organic carbon (OC) were estimated to be 1.6% and 1% respectively. The biomass burning markers, and in particular levoglucosan, were present in all samples and attributed to the combustion of cellulose-containing fuels including wood, peat, bituminous and smokeless coal. The contribution of domestic solid fuel (DSF) burning to the measured OC mass concentration was estimated at 10.8, 50, 66.4 and 74.9% for summer, autumn, late autumn and winter periods, respectively, based on factors derived from a series of burning experiments on locally available fuels. Application of an alternative approach, namely principal component analysis-multiple linear regression (PCA-MLR), to the measured concentrations of the polar organic marker compounds used in conjunction with real-time air quality data provided similar trends and estimates for DSF combustion during all seasons except summer. This study clearly demonstrates that, despite the ban on the sale of bituminous coal in Cork and other large urban areas in Ireland, DSF combustion is still the major source of OC during autumn and winter periods and also makes a significant contribution to PM(2.5) levels. The developed marker approach for estimating the contribution of DSF combustion to ambient OC concentrations can, in principle, also be applied to other locations.

Cardiopulmonary response to inhalation of biogenic secondary organic aerosol

An irradiation chamber designed for reproducible generation of inhalation test atmospheres of secondary organic aerosol (SOA) was used to evaluate cardiopulmonary responses in rodents exposed to SOA derived from the oxidation of alpha-pinene. SOA atmospheres were produced with 10:1 ratios of alpha-pinene:nitrogen oxides (NO(x)) and 10:1:1 ratios of alpha-pinene:nitrogen oxides:sulfur dioxide (SO(2)). SOA atmospheres were produced to yield 200 microg m(-3) of particulate matter (PM). Exposures were conducted downstream of honeycomb denuders employed to remove the gas-phase precursors and reaction products. Nose-only exposures were conducted with both rats (pulmonary effects) and mice (pulmonary and cardiovascular effects). Composition of the atmospheres was optimized to ensure that the SOA generated resembled SOA observed in previous irradiation studies, and contained specific SOA compounds of interest (e.g., organosulfates) identified in ambient air. Pulmonary and cardiovascular toxicity were measured in two different rodent species. In situ chemiluminescence and thiobarbituric acid- reactive substances (TBARS) were used to evaluate oxidative reactions in the F344 rats. ApoE(-/-) mice were exposed for 7 days and measurements of TBARS and gene expression of heme oxygenase-1 (HO-1), endothelin-1 (ET-1), matrix metalloproteinase-9 (MMP-9) were made in aorta. Pulmonary inflammatory responses in both species were measured by bronchoalveolar lavage fluid (BALF) cell counts. No pulmonary inflammation was observed in either species. A mild response was observed in mouse aorta for the upregulation of HO-1 and MMP-9, but was not seen for ET-1. Overall, alpha-pinene-derived SOA, including SOA that included organosulfate compounds, revealed limited biological response after short-term inhalation exposures.

Composition, source, mass closure of PM2.5 aerosols for four forests in eastern China

PM2.5 aerosols were collected in forests along north latitude in boreal-temperate, temperate, subtropical and tropical climatic zones in eastern China, i.e., Changbai Mountain Nature Reserve (CB), Dongping National Forest Park in Chongming Island (CM), Dinghu Mountain Nature Reserve (DH), Jianfengling Nature Reserve in Hainan Island (HN). The mass concentrations of PM2.5, organic carbon (OC), elemental carbon (EC), water soluble organic carbon (WSOC) as well as concentrations of ten inorganic ions (F-, Cl-, NO3-, SO4(2-), C2O4(2-), NH4+, Na+, K+, Ca2+, Mg2+) were determined. Aerosol chemical mass closures were achieved. The 24-hr average concentrations of PM2.5 were 38.8, 89.2, 30.4, 18 Cig/m3 at CB, CM, DH and HN, respectively. Organic matter and EC accounted for 21%-33% and 1.3%-2.3% of PM2.5 mass, respectively. The sum of three dominant secondary ions (SO4(2-), NO3-, NH4+) accounted for 44%, 50%, 45% and 16% of local PM2.5 mass at CB, CM, DH and HN, respectively. WSOC comprised 35%-65% of OC. The sources of PM2.5 include especially important regional anthropogenic pollutions at Chinese forest areas.

Diaterebic acid acetate and diaterpenylic acid acetate: atmospheric tracers for secondary organic aerosol formation from 1,8-cineole oxidation

Detailed organic speciation of summer time PM10 collected in Melbourne, Australia, indicated the presence of numerous monoterpene oxidation products that have previously been reported in the literature. In addition, two highly oxygenated compounds with molecular formulas C9H14O6 (MW 218) and C10H16O6 (MW 232), previously unreported, were detected during a period associated with high temperatures and bushfire smoke. These two compounds were also present in laboratory-produced secondary organic aerosol (SOA) through the reaction of OH radicals with 1,8-cineole (eucalyptol), which is emitted by Eucalyptus trees. The retention times and mass spectral behavior of the highly oxygenated compounds in high-performance liquid chromatography (LC) coupled to electrospray ionization-time-of-flight mass spectrometry (MS) in parallel to ion trap MS of agree perfectly between the ambient samples and the laboratory-produced SOA samples, suggesting that 1,8-cineole is the precursor of the highly oxygenated compounds. The proposed structure of the compound with molecular formula C10H16O6 was confirmed by synthesis of a reference compound. The two novel compounds were identified as diaterebic acid acetate (2-[1-(acetyloxy)-1-methylethyl]succinic acid, C9H14O6) and diaterpenylic acid acetate (3-[1-(acetyloxy)-1-methylethyl]glutaric acid, C10H16O6) based on the consideration of reaction mechanisms, the structure of a reference compound, and the interpretation of mass spectral data. Depending on the experimental conditions, the SOA yields determined in chamber experiments ranged between 16 and 20% for approximately 25 ppb of hydrocarbon consumed. The concentrations of these compounds were as high as 50 ng m(-3) during the summertime in Melbourne. This study demonstrates the importance and influence of local vegetation patterns on SOA chemical composition.

Polar organic marker compounds in PM2.5 aerosol from a mixed forest site in western Germany

The molecular composition of PM2.5 (particulate matter with an aerodynamic diameter <2.5 microm) aerosol samples collected during a very warm and dry 2003 summer period at a mixed forest site in Jülich, Germany, was determined by gas chromatography/mass spectrometry in an effort to evaluate photooxidation products of biogenic volatile organic compounds (BVOCs) and other markers for aerosol source characterization. Six major classes of compounds represented by twenty-four individual organic species were identified and measured, comprising tracers for biomass combustion, short-chain acids, fatty acids, sugars/sugar alcohols, and tracers for the photooxidation of isoprene and alpha-/beta-pinene. The tracers for the photooxidation of alpha-/beta-pinene include two compounds, 3-hydroxyglutaric acid and 3-methyl-1,2,3-butanetricarboxylic acid, which have only recently been elucidated. The characteristic molecular distribution of the fatty acids with a strong even/odd number carbon preference indicates a biological origin, while the presence of isoprene and terpene secondary organic aerosol products suggests that the photooxidation of BVOCs contributes to aerosol formation at this site. The sum of the median concentrations of the isoprene oxidation products was 21.2 ng m(-3), while that of the terpene oxidation products was 19.8 ng m(-3). On the other hand, the high median concentration of malic acid (37 ng m(-3)) implies that photooxidation of unsaturated fatty acids should also be considered as an important aerosol source process. In addition, the occurrence of levoglucosan and pyrogallol indicates that the site is affected by biomass combustion. Their median concentrations were 30 and 8.9 ng m(-3), respectively.