Natural and Anthropogenically Influenced Isoprene Oxidation in Southeastern United States and Central Amazon

Tower-based profiles of wintertime secondary organic aerosols in the urban boundary layer over Guangzhou

Secondary organic aerosol (SOA) accounts for a large fraction of fine particulate matter (PM2.5), but the lack of vertical observations of SOA in the urban boundary layer (UBL) limits a comprehensive understanding of its sources and formation mechanisms. In this study, PM2.5 samples were simultaneously collected at 3 m, 118 m, and 488 m on the Canton Tower in Guangzhou during winter. Typical SOA tracers, including oxidation products of isoprene (SOAI), monoterpene (SOAM), sesquiterpene (SOAS), and toluene (ASOA), were investigated alongside meteorological parameters and gaseous/particulate pollutants. Total concentrations of SOA tracers showed an increasing trend with height, with daytime levels exceeding nighttime levels. C5-alkene triols and 2-methylglyceric acid displayed a significant increase with height, potentially affected by nighttime chemistry in the residual layer, determining the overall vertical trend of SOAI tracers. Concentrations of later-generation SOAM (SOAM_S) tracers also increased with height, while those of first-generation SOAM (SOAM_F) tracers decreased, indicating relatively aged SOAM in the upper layers. SOAS and ASOA tracers exhibited higher enhancement under polluted conditions, likely impacted by biomass burning and anthropogenic emissions. The yields of SOAI tracers varied with temperature in the vertical profile. The formation of SOAM_F tracers was negatively correlated with relative humidity, liquid water content, and pH, affecting their vertical distributions. The effect of O3 on SOA formation enhanced significantly with height, influenced by air mass transport, and likely contributed to the higher yields of SOA in the upper layer. However, at ground level, SOA formation was primarily driven by high local emissions of both NOx and volatile organic compounds. We also observed the roles of SO2 in SOA generation, particularly at 118 m. This study demonstrates the vertical diurnal characteristics of SOA tracers in the UBL, highlighting the varying effects of meteorological conditions and anthropogenic pollutants on SOA formation at different heights.

Differences in aerosol chemistry at a regional background site in Hong Kong before and during the COVID-19 pandemic

Restrictions on human-related activities implemented in Hong Kong to curb the spread of the coronavirus disease 2019 (COVID-19) pandemic provided an opportunity to investigate the anthropogenic impact on organic aerosols (OA) composition. In this study, we conducted a comparative analysis of online measurements of non-refractory submicron particulate matters (NR-PM1) at a regional background site in Hong Kong, covering the periods before the COVID-19 control (November 2018) and during the COVID-19 control (October to November 2020), to investigate changes in OA sources and formation mechanisms. Among the measured NR-PM1 components, organics were the most dominant species with an average percentage of 51.0 ± 0.5 %, exceeding pre-control levels of 44.0 ± 0.7 %. Moreover, 88 % of the organics were attributed to oxygenated OA (OOA). Diurnal variations of all bulk components in NR-PM1 consistently showed afternoon peaks, indicating photochemical processes during COVID-19 control. Similar to the pre-restriction period, the positive matrix factorization (PMF) model showed that OOA was composed of three factors, including two less-oxidized oxygenated factors (LO-OOA1 and LO-OOA2) and one more-oxidized oxygenated factor (MO-OOA). The contribution of the LO-OOA2 factor remained small and stable during both sampling campaigns, which might imply background levels of OOA at this site. The formation of the two predominant components of organics (e.g., LO-OOA1 and MO-OOA) was further discussed. Compared with before control, observational evidence showed that the levels of MO-OOA exceeded LO-OOA1 during the control period, and the average concentration of odd oxygen (Ox = ozone + nitrogen dioxide) increased by 53 % during the COVID-19 control. Besides, the results showed that both LO-OOA1 and MO-OOA exhibited similar diurnal variations to Ox, and their concentrations generally enhanced with increasing Ox levels. This suggested that the formation of OOA was closely related to the photochemical oxidation processes when anthropogenic emissions were reduced. By correlating LO-OOA1 and MO-OOA with speciated OA markers, we found that the formation of LO-OOA1 remained associated with anthropogenic sources, while biogenic emissions contributed to the formation of MO-OOA during the COVID-19 control. Our findings highlight the interplay between emissions, atmospheric conditions, and aerosol composition, providing valuable insights to guide strategic decisions for future air quality improvement.

Identification of volatile organic compounds and their sources driving ozone and secondary organic aerosol formation in NE Spain

Volatile organic compounds (VOCs) play a crucial role in the formation of ozone (O3) and secondary organic aerosol (SOA). We conducted measurements of VOC ambient mixing ratios during both summer and winter at two stations: a Barcelona urban background station (BCN) and the Montseny rural background station (MSY). Subsequently, we employed positive matrix factorization (PMF) to analyze the VOC mixing ratios and identify their sources. Our analysis revealed five common sources: anthropogenic I (traffic & industries); anthropogenic II (traffic & biomass burning); isoprene oxidation; monoterpenes; long-lifetime VOCs. To assess the impact of these VOCs on the formation of secondary pollutants, we calculated the ozone formation potential (OFP) and secondary organic aerosol formation potential (SOAP) associated with each VOC. In conclusion, our study provides insights into the sources of VOCs and their contributions to the formation of ozone and SOA in NE Spain. The OFP was primarily influenced by anthropogenic aromatic compounds from the traffic & industries source at BCN (38-49 %) and during winter at MSY (34 %). In contrast, the summer OFP at MSY was primarily driven by biogenic contributions from monoterpenes and isoprene oxidation products (45 %). Acetaldehyde (10-35 %) and methanol (13-14 %) also made significant OFP contributions at both stations. Anthropogenic aromatic compounds originating from traffic, industries, and biomass burning played a dominant role (88-93 %) in SOA formation at both stations during both seasons. The only exception was during the summer at MSY, where monoterpenes became the primary driver of SOA formation (41 %). These findings emphasize the importance of considering both anthropogenic and biogenic VOCs in air quality management strategies.

Analytical methodologies for oxidized organic compounds in the atmosphere

Oxidized compounds in the atmosphere can occur as emitted primary compounds or as secondary products when volatile emitted precursors react with various oxidants. Due to the presence of polar functional groups, their vapor pressures decrease, and they condense onto small particles. Thereby, they have an effect on climate change by the formation of clouds and scattering solar radiation. The particles and oxidized compounds themselves can cause serious health problems when inhaled. Therefore, it is of utmost importance to study oxidized compounds in the atmosphere. Much ongoing research is focused on the discovery of new oxidized substances and on the evaluation of their sources and factors influencing their formation. Monitoring biogenic and anthropogenic primary oxidized compounds or secondary oxidized products in chamber experiments or field campaigns is common. New discoveries have been reported, including various oxidized compounds and a new group of compounds called highly oxidized organic molecules (HOMs). Analytics of HOMs are mainly focused on chromatography and high-resolution mass spectrometry employing chemical ionization for identifying and quantifying compounds at low concentrations. Oxidized compounds can also be monitored by spectrophotometric methods in which the determinations of total amounts are based on functional groups. This review highlights recent findings on oxidized organic compounds in the atmosphere and analytical methodologies used for their detection and quantification. The discussion includes gas and liquid chromatographic methods, sampling, extraction, concentration, and derivatization procedures involved, as well as mass spectrometric and spectrophotometric methods.

Size distributions of molecular markers for biogenic secondary organic aerosol in urban Beijing

To understand the source, formation, and seasonality of biogenic secondary organic aerosol (BSOA), a nine-stage cascade impactor was utilized to collect size-segregated particulate samples from April 2017 to January 2018 in Beijing, China. BSOA tracers derived from isoprene, monoterpene, and sesquiterpene were measured with gas chromatography-mass spectrometry. Isoprene and monoterpene SOA tracers exhibited significant seasonal variations, with a summer maximum and a winter minimum. Dominance of 2-methyltetrols (isoprene SOA tracers) with a good correlation with levoglucosan (a biomass burning tracer), which was combined with the detection of methyltartaric acids (possible indicators for aged isoprene) in summer, implies possible biomass burning and long-range transport. In contrast, sesquiterpene SOA tracer (β-caryophyllinic acid) was dominant in winter and was probably associated with the local burning of biomass. Bimodal size distributions were observed for most isoprene SOA tracers, consistent with previous laboratory experiments and field studies showing that they can be formed not only in the aerosol phase but also in the gas phase. Monoterpene SOA tracers cis-pinonic acid and pinic acid showed a coarse-mode peak (5.8-9.0 μm) in four seasons due to their volatile nature. Sesquiterpene SOA tracer β-caryophyllinic acid showed a unimodal pattern with a major fine-mode peak (1.1-2.1 μm), which is linked to local biomass burning. The tracer-yield method was used to quantify the contributions of isoprene, monoterpene, and sesquiterpene to secondary organic carbon (SOC) and SOA. The highest isoprene SOC and SOA concentrations occurred in summer (2.00 μgC m^-3 and 4.93 μg m^-3, respectively), contributing to 1.61% of OC and 5.22% of PM_2.5, respectively. These results suggest that BSOA tracers are promising tracers for understanding the source, formation, and seasonality of BSOA.

Isoprene in urban Atlantic forests: Variability, origin, and implications on the air quality of a subtropical megacity

Biosphere-atmosphere interactions play a key role in urban chemistry because of biogenic volatile organic compound (BVOC) emissions. Of the BVOC, isoprene is the most emitted compound; however, it also has anthropogenic origins in urban areas. In this study, we aimed to investigate the spatio-temporal variability and atmospheric impacts of biogenic and anthropogenic isoprene in the subtropical megacity of São Paulo (MASP), Brazil. Several measurement campaigns were conducted in three different urban Atlantic forests (Matão, PEFI, and RMG), and an urban background site (IAG); this equated to a total of 268 samples for the 2018-2019 period. For all sampling points, daytime average concentrations of isoprene were two to three times higher during the rainy season (IAG: 1.75 ± 0.93 ppb; Matão: 0.87 ± 0.35 ppb; PEFI: 0.50 ± 0.30 ppb; RMG: 0.37 ± 0.18 ppb), than those observed during the dry season (IAG: 0.46 ± 0.24 ppb; Matão: 0.31 ± 0.17 ppb; PEFI: 0.17 ± 0.11 ppb; RMG: 0.11 ± 0.07 ppb). Average isoprene concentrations were similar to those observed in other places worldwide, with the exception of the Amazon forest. Our results indicate differences in isoprene concentrations between sites, suggesting that environmental conditions such as the urban heat island and vegetation types, may play a role in spatial variability. Estimates of the isoprene fraction indicated that the biogenic fraction (85%) surpassed the anthropogenic fraction during the rainy season. By contrast, the anthropogenic fraction (52%) exceeded the biogenic fraction during dry periods. These fractions have an impact on potentially forming secondary pollutants gaseous (ozone formation potential: 7.19-33.32 μg m^-3), and aerosols (secondary organic aerosols formation potential: 0.41-1.88 μg m^-3). These results highlight the role of biogenic isoprene and its potential impact on urban air quality in subtropical megacities; this requires further investigation under future climate change scenarios.

Characteristics of secondary organic aerosols tracers in PM2.5 in three central cities of the Yangtze river delta, China

Secondary organic aerosols (SOA) are important atmospheric pollutants that affect air quality, radiation, and human health. In this study, 14 typical SOA tracers were measured in PM_2.5 collected from three central cities of the Yangtze River Delta (YRD) region in the winter of 2014 and the summer of 2015. Among the determined SOA tracers, α/β-pinene SOA tracers contributed 55.9%, followed by isoprene SOA tracers (33.7%), anthropogenic benzene SOA tracer (6.4%) and β-caryophyllene SOA tracer (4.0%). There was no significant difference in the concentration of individual SOA tracers among the three cities (p > 0.05), indicating a high degree of regional consistency. The concentrations of isoprene, α/β-pinene, and toluene SOA tracers were significantly higher in summer than in winter. A correlation of SOA tracers with temperature implies that the isoprene and α/β-pinene SOA tracers in summer were greatly boosted by plant emissions and the high DHOPA in summer could be attributed to evaporation of paint and solvent. In contrast, the elevated β-caryophyllene SOA tracer in winter was likely associated with active biomass burning. Furthermore, we observed a close correlation of summer isoprene and α/β-pinene SOA tracers with sulfate only in Shanghai, which verifies that biogenic SOA formation was facilitated by high concentration of sulfate. The ratios of MGA/MTLs and P/M were applied to reveal the impact of NO_x on SOA formation and the aging degree of SOA, respectively. The MGA/MTLs ratios were comparable for the three cities, but much higher than the background value of this region as expected. The P/M ratios suggest that the aging degree of SOA in the YRD region was generally low, but the winter SOA were fresher than the summer SOA. Our research helps to understand pollution characteristics of SOA tracers in the urban agglomeration.

Observational insights into the compound environmental effect for 2-methyltetrols formation under humid ambient conditions

Laboratory experiments suggest acid-catalyzed aqueous-phase production can promote the formation of isoprene SOA, i.e., 2-methyltetrols. In this study we use ambient observations of the 2-methyltetrols along with other chemical measurements, as well as meteorological factors to investigate the relative importance of environmental influence for isoprene epoxydiols (IEPOX) SOA formation under atmospheric humidity conditions. The 2-Methyltetrols revealed good relationships with temperature and total solar radiation, but were weakly correlated with aerosol acidity and SO₄^2-. EC-scaled 2-methyltetrols were observed to vary in a narrow pH range (1.5-2.0), indicating aerosol acidity was not a limiting factor for 2-methyltetrols formation. High values of 2-methyltetrols were consistently observed at high total solar radiation, the strong dependence of total solar radiation demonstrated that photochemical processes dominated 2-methyltetrols formation in humid environments. Although 2-methyltetrols can be enhanced by acid-catalyzed aqueous-phase reactions, it is not sufficient to compensate the synchronously weakened photochemical activity influence, leading to an obvious net decrease in the formation of 2-methyltetrols in the ambient. Moreover, aerosol droplet acidity was reduced under high liquid water content (LWC) condition, subsequently diminishing the enhancement of SOA formation by acidity. Overall, our results highlight that the environmental impact factors are highly variable and interplay, influencing the production of 2-methyltetrols, and suggest that the formation pathway of 2-methyltetrols is insensitive to aerosol acidity but dominated by photochemical production process in humid environments.

Second-Order Kinetic Rate Coefficients for the Aqueous-Phase Hydroxyl Radical (OH) Oxidation of Isoprene-Derived Secondary Organic Aerosol Compounds at 298 K

The hydroxyl radical (OH) oxidation of the most abundant nonmethane volatile organic compound emitted to the atmosphere, isoprene (C₅H₈), produces a number of chemical species that partition to the condensed phase via gas-particle partitioning or form condensed-phase compounds via multiphase/heterogeneous chemistry to generate secondary organic aerosols (SOA). The SOA species in aerosol water or cloud/fog droplets may oxidize further via aqueous reaction with OH radicals, among other fates. Rate coefficients for compounds in isoprene's photochemical cascade are well constrained in the gas phase; however, a gap of information exists for the aqueous OH rate coefficients of the condensed-phased products, precluding the atmospheric modeling of the oxidative fate of isoprene-derived SOA. This work investigated the OH-initiated oxidation kinetic rate coefficients (k_OH) for six major SOA compounds formed from the high-NO and low-NO channels of isoprene's atmospheric oxidation and one analog, most of which were synthesized and purified for study: (k₁) 2-methyltetrol [MT: 1.14 (±0.17) × 10⁹ M^-1 s^-1], (k₂) 2-methyl-1,2,3-trihydroxy-4-sulfate [MT-4-S: 1.52 (±0.25) × 10⁹ M^-1 s^-1], (k₃) 2-methyl-1,2-dihydroxy-3-sulfate [MD-3-S: 0.56 (±0.15) × 10⁹ M^-1 s^-1], (k₄) 2-methyl-1,2-dihydroxy-but-3-ene [MDE: 4.35 (±1.16) × 10⁹ M^-1 s^-1], (k₅) 2-methyl-2,3-dihydroxy-1,4-dinitrate [MD-1,4-DN: 0.24 (±0.04) × 10⁹ M^-1 s^-1], (k₆) 2-methyl-1,2,4-trihydroxy-3-nitrate [MT-3-N: 1.12 (±0.15) × 10⁹ M^-1 s^-1], and (k₇) 2-methylglyceric acid [MGA: pH 2:1.41 (±0.49) × 10⁹ M^-1 s^-1; pH 5:0.97 (±0.42) × 10⁹ M^-1 s^-1]. The second-order rate coefficients are determined against the known k_OH of erythritol in pure water. The decays of each reagent were measured with nuclear magnetic resonance (NMR) and high-performance liquid chromatography-high resolution mass spectrometry (HPLC-HRMS). The aqueous photooxidation fates of isoprene-derived SOA compounds are substantial and may impact the SOA budget when implemented into global models.

Anthropogenic-Biogenic Interactions at Night: Enhanced Formation of Secondary Aerosols and Particulate Nitrogen- and Sulfur-Containing Organics from β-Pinene Oxidation

Mixing of anthropogenic gaseous pollutants and biogenic volatile organic compounds impacts the formation of secondary aerosols, but still in an unclear manner. The present study explores secondary aerosol formation via the interactions between β-pinene, O₃, NO₂, SO₂, and NH₃ under dark conditions. Results showed that aerosol yield can be largely enhanced by more than 330% by NO₂ or SO₂ but slightly enhanced by NH₃ by 39% when the ratio of inorganic gases to β-pinene ranged from 0 to 1.3. Joint effects of NO₂ and SO₂ and SO₂ and NH₃ existed as aerosol yields increased with NO₂ but decreased with NH₃ when SO₂ was kept constant. Infrared spectra showed nitrogen-containing aerosol components derived from NO₂ and NH₃ and sulfur-containing species derived from SO₂. Several particulate organic nitrates (MW 215, 229, 231, 245), organosulfates (MW 250, 264, 280, 282, 284), and nitrooxy organosulfates (MW 295, 311, 325, 327, and 343) were identified using high-resolution orbitrap mass spectrometry in NO₂ and SO₂ experiments, and their formation mechanism is discussed. Most of these nitrogen- and sulfur-containing species have been reported in ambient particles. Our results suggest that the complex interactions among β-pinene, O₃, NO₂, SO₂, and NH₃ during the night might serve as a potential pathway for the formation of particulate nitrogen- and sulfur-containing organics, especially in polluted regions with both anthropogenic and biogenic influences.

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.

Biogenic secondary organic aerosol formation in an urban area of eastern central India: Seasonal variation, size distribution and source characterization

Samples of ambient aerosols were collected at an urban site of eastern central India from monsoon to summer 2016-17 for the characterization of biogenic secondary organic aerosols (BSOA). The BSOA tracers derived from isoprene, α/β-pinene and β-caryophyllene in size-distributed aerosols were studied. Concentrations of total SOAI (Isoprene secondary organic aerosols) were found more abundant than α/β-pinene in summer, while contradictory trends were found in the winter season, where SOAM (monoterpene derived SOA) and SOAS (sesquiterpenes derived SOA) were dominated. Size-distribution study revealed that most of the BSOA were formed in the aerosol phase and dominated in fine mode, except cis-pinonic acid. They were formed in the gaseous phase and partitioned onto the aerosol phase. The alkaline nature of mineral dust particles that triggered the adsorption of gaseous species onto pre-existing particles could be the reason for bimodal size distribution with major coarse mode peak and miner fine mode peak. Temporal variations suggest that the BSOA must be derived from terrestrial vegetation and biomass burning. The isoprene SOC (secondary organic carbon) contributed 0.91%, 1.38%, 0.88% and 1.04% to OC during winter, summer, post-monsoon and monsoon season, respectively. The isoprene SOC in fine mode was found to be higher than the coarse mode.