Secondary Organic Aerosol Tracers Research Articles

Measurements of particulate methanesulfonic acid above the remote Arctic Ocean using a high resolution aerosol mass spectrometer

Methanesulfonic acid (MSA) is an important product from the oxidation of dimethyl sulfide (DMS), and thus is often used as a tracer for marine biogenic sources and secondary organic aerosol. MSA also contributes to aerosol mass and potentially to the formation of cloud condensation nuclei and new particles. However, measurements of MSA at high temporal resolution in the remote Arctic are scarce, which limits our understanding of its formation, climate change impact and regional transport. Here, we applied a validated quantification method to determine the mass concentration of MSA and non-sea salt sulfate (nss-SO4) in PM2.5 in the marine boundary layer, using a high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) during a research cruise to the Arctic and North Atlantic Ocean, between 55 °N and 68 °N (26th May to June 23, 2022). With this method, the concentrations of MSA in the remote Arctic marine boundary layer were determined for the first time. Results show that the average MSA concentration was 0.025 ± 0.03 μg m−3, ranging from <0.01 to 0.32 μg m−3. The lowest MSA level was found towards the northern leg of the cruise (near Sisimut (67 °N)) with air masses from sea ice over the northern polar region, and the highest MSA concentrations were observed over the Atlantic open ocean. The diurnal cycles of gas MSA, particulate MSA and nss-SO4 peaked in the afternoon, about one hour later than that of peak of solar radiation, which suggests that photochemical process is an important mechanism for the conversion of DMS into MSA above the remote ocean. The mass ratio of MSA to nss-SO4 (MSA/nss-SO4) presents a temperature dependence, which indicates that the addition branching pathway favors MSA formation, while thermal decay of intermediate radicals could be a possible pathway for sulfate formation. Finally, we found that the MSA/nss-SO4 ratio is around 0.22-0.25 in the remote northern marine atmosphere.

Molecular characteristics and formation mechanisms of biogenic secondary organic aerosols in the mountainous background atmosphere of southern China

Biogenic secondary organic aerosols (BSOA) play a crucial role in atmospheric chemistry, with significant implications for air quality and human health. However, the mechanisms by which anthropogenic emissions influence the formation of BSOA in the real atmosphere have not been fully understood. In this study, fine aerosol (PM2.5) samples were collected during both the wet and dry seasons at a high-altitude (1690 m) background site in the Nanling Mountains. Eleven typical secondary organic aerosols (SOA) tracers derived from isoprene (SOAI), α/β-pinene (SOAM), β-caryophyllene (SOAS), and toluene (SOAA), as well as water-soluble inorganic ions and the ambient concentrations of isoprene and its two primary oxidation products, were measured. The results showed that the concentrations and composition of SOA tracers exhibited significant seasonal variations. The total concentration of SOA tracers in the wet season (159 ± 100 ng m−3) was almost three times higher than that in the dry season (61 ± 68 ng m−3). SOAI dominated in both seasons, followed by SOAM, SOAS, and SOAA. Compared with other studies, the concentrations of SOAI and SOAA in the wet season were relatively high, which was mainly attributed to the rapid oxidation of their precursors due to the higher atmospheric oxidative capacity. C5-alkene triols had a significantly higher proportion in SOAI species than in other mountaintop sites. Temperature was the main factor causing the seasonal difference in the concentration of SOAI. Anthropogenic pollutants significantly promoted the formation of SOAI in the wet season. The influence of anthropogenic pollutants on the reaction pathway of IEPOX was consistent in both seasons, favoring the formation of C5-alkene triols. Our study provides vital insights into the molecular characteristics and formation mechanisms of BSOA in a high-altitude background region. It also highlights that the complex air pollution not only affects BSOA concentrations but also the reaction pathways of biogenic volatile organic compounds (BVOCs) transforming into BSOA.

Advances in the Separation and Detection of Secondary Organic Aerosol Produced by Decamethylcyclopentasiloxane (D5) in Laboratory-Generated and Ambient Aerosol.

Decamethylcyclopentasiloxane (D5), a common ingredient in many personal care products (PCPs), undergoes oxidation in the atmosphere, leading to the formation of secondary organic aerosol (SOA). Yet, the specific contributions of D5-derived SOA on ambient fine particulate matter (PM2.5) have not been characterized. This study addresses this knowledge gap by introducing a new analytical method to advance the molecular characterization of oxidized D5 and its detection in ambient aerosol. The newly developed reversed phase liquid chromatography method, in conjunction with high-resolution mass spectrometry, separates and detects D5 oxidation products, enabling new insights into their molecular and isomeric composition. Application of this method to laboratory-generated SOA and urban PM2.5 in New York City expands the number of D5 oxidation products observed in ambient aerosol and informs a list of molecular candidates to track D5-derived SOA in the atmosphere. An oxidation series was observed in which one or more methyl groups in D5 (C10H30O5Si5) is replaced by a hydroxyl group, which indicates the presence of multistep oxidation products in ambient PM2.5. Because of their specificity to PCPs and demonstrated detectability in ambient PM2.5, several oxidation products are proposed as molecular tracers for D5-derived SOA and may prove useful in assessing the impact of PCPs-derived SOA in the atmosphere.

Chemical and optical characterization of aqueous secondary organic aerosol generated by reaction of pyruvaldehyde with sodium sulfite

Organic sulfonates are major constituents and tracers for secondary organic aerosol (SOA). Based on sodium sulfite (Na2SO3) and pyruvaldehyde are main components in aqueous environments, their mixed solution is atomized to simulate atmospheric processes of formation aquous SOA (aqSOA) from liquid aerosol particles rich in Na2SO3 and pyruvaldehyde after water evaporation. Constituents and optics of aqSOA are characterized using mass and spectroscopic instruments in this study. Negative ion mass spectra of aqSOA detected by aerosol laser time-of-flight mass spectrometer appeare strong organic sulfonates' characteristic cleavage peaks of m/z = 32(S−), 48(SO−), 64 (SO2−) and 80(SO3−). UV–Vis spectra of extraction solution for aqSOA existe characteristic absorption peak of organic sulfonate' SO at 210 nm. Peaks of 1140 and 1034 cm−1 in infrared absorption spectra correspond to antisymmetric and symmetric stretching vibration peaks of SO3 group, while, peaks of 830 and 760 cm−1 refer to asymmetric and symmetric stretching vibration absorption peaks of C–S–O. Also, peaks at m/z = 125, 153, 169, 185, 235 and 243 are presented in electrospray ionization negative ion mass spectra of extraction solution. These indicate that 1-hydroxyethane sulfonate and other organic sulfonates are major products of aqueous reaction for Na2SO3 and pyruvaldehyde. Light absorption capacity of aqSOA is remarkably enhanced, mass absorption coefficient averaged in 200–600 nm (<MAC>) for aqSOA is reach to that of biomass burning particles, and gradually raises with increment concentration of Na2SO3. These results can offer experimental basis to study components and optics of organic sulfonates in aerosols under high humidity conditions in polluted areas.

Long‐Emission‐Wavelength Humic‐Like Component (L‐HULIS) as a Secondary Source Tracer of Brown Carbon in the Atmosphere

AbstractThe optical properties of secondary brown carbon (BrC) aerosols are poorly understood, hampering quantitative assessments of their impact. We propose a new method for estimating secondary source of BrC using excitation‐emission matrix (EEM) fluorescence spectroscopy, combined with parallel factor analysis (PARAFAC) and partial least squares regression (PLSR). Experiments were conducted on a collection of PM2.5 samples from urban areas in five Chinese cities during winter and summer. The humic‐like component with long‐emission wavelengths (L‐HULIS) was identified as a secondary source tracer of BrC. This was confirmed by correlating PARAFAC components with secondary organic aerosol tracers and molecular oxidation indices obtained from Fourier transform ion cyclotron resonance mass spectrometry analysis. Using L‐HULIS as a secondary tracer of BrC, it was determined that the contribution of secondary sources to water‐soluble BrC (WS‐BrC) in source emission samples is significantly smaller than in PM2.5 from five Chinese cities, supporting our method. In the five cities, secondary source derived via L‐HULIS contributes a dominant potion (80% ± 3.5%) of WS‐BrC at 365 nm during the summer, which is approximately twice as high as during the winter (45% ± 4.9%). Radiocarbon isotope (14C) analysis provides additional constraints to the sources of L‐HULIS‐derived secondary WS‐BrC in urban PM2.5, suggesting that aged biomass burning is the dominant contributor to secondary WS‐BrC in winter, and biogenic emission is dominant during summer. This study is the first report on identification of secondary sources of BrC using the fluorescence technique. It demonstrates the potential of this method in characterizing non‐fossil source secondary BrC in the atmosphere.

Source apportionment of PM2.5 in Montréal, Canada, and health risk assessment for potentially toxic elements

Abstract. Source apportionment of PM2.5 was performed using positive matrix factorization (PMF) based on detailed chemical composition data from 24 h filter samples collected over a 3-month period (August–November 2020) at an urban site in Montréal, a Canadian city with a population of approximately 4×106 people. This source apportionment study, which examined the main contributing sources to PM2.5 using a larger suite of organic molecular markers than other Canadian studies, is the first of its sort in Canada. A focus of this study was on quantifying previously unresolved sources of PM2.5 through the inclusion in the PMF analysis of additional organic molecular markers beyond those measured typically by the Canadian government's National Air Pollution Surveillance Program (NAPS). The organic species included in the PMF model were comprised of six n-alkanes, two fatty acids, one dicarboxylic acid, two biogenic secondary organic aerosol (SOA) tracers, and hopane. Secondary inorganic aerosols (SIAs) and SOAs were the dominant components and constituted 39 % of the measured PM2.5 mass, while the local primary anthropogenic sources, namely traffic exhaust, road dust, industrial, and cooking emissions, contributed 23 %. The chemical transport model GEOS-Chem revealed that ammonium sulfate concentrations in Montréal are strongly influenced by both local sources in Quebec and transboundary input from the United States, with the transboundary input exceeding the local emissions for SOA. Co and Cr(VI) presented an elevated cancer risk, highlighting that more attention should be given to these trace metals, which were associated with industrial emissions by the PMF analysis. Furthermore, the results showed that industrial emissions were minor contributors to the total PM2.5 mass concentration but were the largest contributors to Co and Cr(VI) concentrations. Thus, the health hazards associated with this source cannot be entirely established by the PM2.5 mass concentration alone. This study highlights that, when evaluating air quality in Montréal and other urban regions, the prioritization of sources for mitigation strategies will diverge if one considers total PM2.5 mass concentration or the concentration of individual particulate-bound contaminants. Furthermore, the large transboundary contribution from the United States to total PM2.5 levels suggests that future municipal, provincial, and federal monitoring and regulations would be more effective if they focus on specific high-risk contaminants (e.g., Co and Cr(VI) rather than total PM2.5).

Molecular compositions of marine organic aerosols over the Bohai and Yellow Seas: Influence of primary emission and secondary formation

To understand the influences of continental emissions on marine aerosols from East Asia, the total suspended particle (TSP) samples were collected from the Yellow Sea (YS) and Bohai Sea (BS) of China through a cruise campaign during the spring of 2018. >100 individual organic compounds were identified and determined in the samples, and they were classified into 6 major groups according to the functionality and sources. The mass concentrations of total quantified organic compounds in BS (307.6 ± 268.3 ng m−3) was approximately 3 times those of YS (111.0 ± 97.3 ng m−3). Organic compounds from biomass burning (23.2 ± 34.2 ng m−3), marine/microbial source (23.1 ± 6.2 ng m−3) and fossil fuel combustion (21.4 ± 11.3 ng m−3) were the most abundant in the YS samples, whereas primary biological origins (90.0 ± 107.1 ng m−3), biomass burning (52.1 ± 39.1 ng m−3) and fossil fuel combustion (41.9 ± 20.1 ng m−3) were the most important organic species in BS marine aerosols. The concentrations of total biogenic secondary organic aerosol (BSOA) tracers including six isoprene SOA (SOAI), three monoterpene SOA (SOAM), and one β-caryophyllene SOA (SOAC) were 2.4 ± 1.5 ng m−3 and 7.3 ± 4.4 ng m−3 in the YS and BS samples, respectively. A strong correlation between BSOA tracers and levoglucosan was shown in the marine aerosol samples, indicating that biomass burning emissions could also promote BSOA formations. The spatial distribution of chemical and source characteristics suggests a relatively large impacts of continental emissions to the BS aerosols. Based on the tracer-based method, the biomass burning (YS: 4.9 ± 6.8%; BS: 8.8 ± 8.0%) was the largest contributor to OC, followed by primary biogenic OC (YS: 0.59 ± 0.44%; BS: 2.9 ± 3.0%) and biogenic secondary OC (YS: 0.45 ± 0.23%; BS: 1.0 ± 0.59%). This study suggests important contributions of biomass burning emissions to organic aerosols in the YS and BS marine atmosphere, although marine source, fossil fuel combustion and secondary formation was also substantial.

Relative humidity-dependent evolution of molecular composition of α-pinene secondary organic aerosol upon heterogeneous oxidation by hydroxyl radicals

Heterogeneous oxidation by gas-phase oxidants is an important chemical transformation pathway of secondary organic aerosol (SOA) and plays an important role in controlling the abundance, properties, as well as climate and health impacts of aerosols. However, our knowledge on this heterogeneous chemistry remains inadequate. In this study, the heterogeneous oxidation of α-pinene ozonolysis SOA by hydroxyl (OH) radicals was investigated under both low and high relative humidity (RH) conditions, with an emphasis on the evolution of molecular composition of SOA and its RH dependence. It is found that the heterogeneous oxidation of SOA at an OH exposure level equivalent to 12 hr of atmospheric aging leads to particle mass loss of 60% at 25% RH and 95% at 90% RH. The heterogeneous oxidation strongly changes the molecular composition of SOA. The dimer-to-monomer signal ratios increase dramatically with rising OH exposure, in particular under high RH conditions, suggesting that aerosol water stimulates the reaction of monomers with OH radicals more than that of dimers. In addition, the typical SOA tracer compounds such as pinic acid, pinonic acid, hydroxy pinonic acid and dimer esters (e.g., C17H26O8 and C19H28O7) have lifetimes of several hours against heterogeneous OH oxidation under typical atmospheric conditions, which highlights the need for the consideration of their heterogeneous loss in the estimation of monoterpene SOA concentrations using tracer-based methods. Our study sheds lights on the heterogeneous oxidation chemistry of monoterpene SOA and would help to understand their evolution and impacts in the atmosphere.

Contrasting molecular characteristics and formation mechanisms of biogenic and anthropogenic secondary organic aerosols at the summit and foot of Mt. Huang, East China

Secondary organic aerosol (SOA) exerts a considerable influence on atmospheric chemistry. However, little information about the vertical distribution of SOA in the alpine setting is available, which limited the simulation of SOA using chemical transport models. Here, a total of 15 biogenic and anthropogenic SOA tracers were measured in PM2.5 aerosols at both the summit (1840 m a.s.l.) and foot (480 m a.s.l.) of Mt. Huang during the winter of 2020 to explore their vertical distribution and formation mechanism. Most of the determined chemical species (e.g., BSOA and ASOA tracers, carbonaceous components, major inorganic ions) and gaseous pollutants at the foot of Mt. Huang were 1.7–3.2 times higher concentrations than those at the summit, suggesting the relatively more significant effect of anthropogenic emissions at the ground level. The ISORROPIA-II model showed that aerosol acidity increases as altitude decreases. Air mass trajectories, potential source contribution function (PSCF), and correlation analysis of BSOA tracers with temperature revealed that SOA at the foot of Mt. Huang was mostly derived from the local oxidation of volatile organic compounds (VOCs), while SOA at the summit was mainly influenced by long-distance transport. The robust correlations of BSOA tracers with anthropogenic pollutants (e.g., NH3, NO2, and SO2) (r = 0.54–0.91, p < 0.05) indicated that anthropogenic emissions could promote BSOA productions in the mountainous background atmosphere. Moreover, most of SOA tracers (r = 0.63–0.96, p < 0.01) and carbonaceous species (r = 0.58–0.81, p < 0.01) were correlated well with levoglucosan in all samples, suggesting that biomass burning played an important role in the mountain troposphere. This work demonstrated that daytime SOA at the summit of Mt. Huang was significantly influenced by the valley breeze in winter. Our results provide new insights into the vertical distributions and provenance of SOA in the free troposphere over East China.

Distribution characteristics of secondary organic aerosol tracers in PM2.5 in Jinzhong

In this study, 13 secondary organic aerosol (SOA) tracers in PM2.5 were measured in Jinzhong, Shanxi, to investigate the distribution characteristics of SOAs. According to the results, isoprene SOA tracers had the highest annual average concentration at 7.35 ± 14.23 ng/m3, accounting for 57.8% of all tracers by concentration, followed by β-caryophyllene SOA tracer (3.53 ± 2.67 ng/m3, 22.6%), α/β-pinene SOA tracers (2.68 ± 2.23 ng/m3, 18.9%) and toluene SOA tracer (0.11 ± 0.08 ng/m3, 0.7%). The concentrations of isoprene and α/β-pinene SOA tracers in summer and autumn were higher than those in winter and spring, whereas the concentration of β-caryophyllene SOA tracer was higher in autumn and winter than in spring and summer. However, no statistically significant seasonal variation was observed in the concentration of the toluene SOA tracer. According to results for pinolic acid and 3-methyl-1,2,3-butanetricarboxylic acid, α/β-pinene SOA tracers in Jinzhong were fresh in winter and the contribution ratio of β-pinene to SOA formation was higher than that of α-pinene. A correlation analysis also indicated that the high concentrations of isoprene and α/β-pinene SOA tracers in summer were correlated with temperature, whereas the high concentration of the β-caryophyllene SOA tracer in autumn and winter was primarily correlated with biomass burning. According to tracer yield estimation results, plant-derived volatile organic compounds were the main source of SOAs in PM2.5 in Jinzhong, with a contribution ratio of 94%. Isoprene contributed the most to secondary organic carbon concentrations in spring and summer, whereas β-caryophyllene contributed the most in autumn and winter.

Secondary organic aerosols in PM2.5 on a coastal island in southeastern China: Impact of atmospheric process and biomass burning

To better understand the formation and characteristics of secondary organic aerosols (SOA) in coastal areas, a four-season observation of PM2.5-bound SOA tracers was conducted on a coastal island in southeastern China. Typical organic tracers, including the tracer for biomass burning (levoglucosan) and tracers from isoprene (SOAI), α/β pinene (SOAM), β-caryophyllene (SOAC), and toluene (SOAA), were analyzed by GC-MS. The total concentrations of SOA tracers were found to be higher in the fall and summer compared to spring and winter. The highest concentration among the SOA tracers was observed for SOAI (12.96 ± 17.89 ng m−3), followed by SOAM (6.66 ± 6.03 ng m−3), SOAA (1.76 ± 1.73 ng m−3), and SOAC (1.52 ± 1.11 ng m−3). The study revealed that the production of SOAI was significantly influenced by the HO2 channel. Compared with the first generation products and the later generation products of α/β-pinene, we found the aerosols in the study area were relatively fresh. In summer, the majority of SOA tracers showed strong positive correlations with SO42−, H+, and Ox (O3+NO2), indicating the influence of acid sulfate and photochemical oxidation. However, based on the LEV/K+ ratio, it was concluded that crop residue burning has a substantial influence on SOA in winter and mostly comes from regional transportation. During biomass burning transport episodes, the concentration of β-caryophyllenic acid (CPA) increased along with LEV and K+. The average concentration of total secondary organic carbon (SOC) estimated from the SOA-tracer method was found to be 0.399 μg C m−3, with SOCA constituting the majority (0.220 ± 0.216 μg C m−3). Isoprene contributed the most to biogenic SOA (BSOA) in summer (42.1%), whereas sesquiterpene had the largest contribution (66.8%) in winter. This study provides valuable insights into the characteristics and formation mechanisms of SOA in coastal areas with low anthropogenic emissions.

PM2.5-bound biogenic secondary organic aerosol tracers over a regionally representative site in central India: Characteristics and sources

Biogenic secondary organic aerosol (BSOA) in PM2.5 over Bhopal, central India were quantified. Fine particles were collected on Teflon filters for measuring PM2.5 mass and on quartz fiber filter for organic carbon (OC), elemental carbon (EC), and BSOA tracers, including oxidation products of isoprene, monoterpene and sesquiterpene between November 2017 and May 2018. Isoprene SOA tracers were present in the highest mean concentration (86.5 ng m−3 ± 93.5 ng m−3) amongst all quantified tracers, accounting for 64% ± 23% of the BSOA tracers. Isoprene oxidation products, including C5-alkene triols and 2-methyltetrols, and sesquiterpene oxidation products (β-caryophyllinic acid), were found to be the most abundant compound class over the study location. Correlations in the temporal variation of tracers of isoprene and monoterpene, with ozone (O3) suggested that photochemical oxidation influenced their concentrations over central India. The temporal variation of β-caryophyllinic acid and comparison with the previous suggested that their concentration was potentially influenced by biomass burning. Overall, the findings suggest that oxidation products of isoprene, monoterpene and sesquiterpene can be utilized to understand the characteristics and apportionment of BSOA sources over central India.

Seasonal characteristics of biogenic secondary organic aerosol tracers in a deciduous broadleaf forest in northern Japan

We collected total suspended particulate (TSP) samples from January 2010 to December 2010 at Sapporo deciduous forest to understand the oxidation processes of biogenic volatile organic compounds (BVOCs). The gas chromatography-mass spectrometric technique was applied to determine biogenic secondary organic aerosols (BSOAs) in the TSP samples. We found the predominance of the isoprene SOA (iSOA) tracers (20.6 ng m−3) followed by α/β-pinene SOA (pSOA) tracers (8.25 ng m−3) and β-caryophyllene SOA (cSOA) tracer (1.53 ng m−3) in the forest aerosols. The results showed large isoprene fluxes and relatively high levels of oxidants in the forest atmosphere. The iSOA and pSOA tracers showed a clear seasonal trend with summer and autumn maxima and winter and spring minima. Their seasonal trends were mainly controlled by BVOCs emission from the local broadleaf deciduous forest. Additionally, the regional level of isoprene emissions from the oceanic sources may also be contributed during summertime aerosols. cSOA tracer showed high concentrations in the winter and spring, possibly due to an additional contribution of biomass burning (BB) aerosols from the local or regional BB activities. The biogenic secondary organic carbon (BSOC) was contributed mainly by the oxidation products of isoprene (136 ngC m−3) followed by β-caryophyllene (63.0 ngC m−3) and α/β-pinene (35.9 ngC m−3). The mass concentration ratio (0.92) of pinonic acid + pinic acid and 3-methyl-1,2,3-butanetricarboxylic acid ((PNA + PA)/3-MBTCA) indicates the photochemical transformation of first-generation oxidation products to the higher generation oxidation products. The average ratios of isoprene to α/β-pinene (1.64) and β-caryophyllene (18.6) oxidation products suggested a large difference in the emissions of isoprene and α/β-pinene compared to β-caryophyllene. The cSOA tracers in the forest aerosols are also contributed by BB during the winter and spring. Positive matrix factorization analyses of the BSOA tracers confirmed that organic aerosols of deciduous forests are mostly related to isoprene emissions. This study suggests that isoprene is a more significant precursor for the BSOA than α/β-pinene and β-caryophyllene in a broadleaf deciduous forest.

Molecular characteristics, sources and influencing factors of isoprene and monoterpenes secondary organic aerosol tracers in the marine atmosphere over the Arctic Ocean

Biogenic secondary organic aerosols (BSOA) are important components of the remote marine atmosphere. However, the response of BSOA changes to sea ice reduction over the Arctic Ocean remains unclear. Here we investigated isoprene and monoterpenes secondary organic aerosol (SOAI and SOAM) tracers in three years of summer aerosol samples collected from the Arctic Ocean atmosphere. The results indicated that methyltetrols were the most abundant SOAI tracers, while the main oxidation products of monoterpenes varied over the years owing to different aerosol aging. The results of the principal component analysis (PCA)-generalized additive model (GAM) combined with correlation analysis suggested that SOAI tracers were mainly generated by the oxidation of isoprene from marine emissions, while SOAM tracers were probably more influenced by terrestrial transport. Estimation of secondary organic carbon (SOC) indicated that monoterpenes oxidation contributed more than isoprene and that sea ice changes had a relatively small effect on biogenic SOC concentration levels. Our study quantified the contribution of influencing factors to the atmospheric concentration of BSOA tracers in the Arctic Ocean, and showed that there were differences in the sources of precursors for different BSOA. Hence, our findings have contributed to a better understanding of the characteristics, sources and formation of SOA in the atmosphere of the Arctic Ocean.

Chemical evolution of secondary organic aerosol tracers during high-PM2.5 episodes at a suburban site in Hong Kong over 4 months of continuous measurement

Abstract. Secondary organic aerosol (SOA) makes a sizable contribution to fine-particulate-matter (PM2.5) pollution, especially during high-PM episodes. Past studies of SOA evolution at the episode scale mainly rely on measurements of bulk SOA mass, with few studies probing individual SOA molecular tracers. In this study, we continuously monitored (at a bi-hourly resolution) SOA tracers specific to a few common volatile organic compound (VOC) precursors at a suburban site in Hong Kong for a 4-month period from the end of August to December 2020. The SOA molecules include tracers for SOA derived from biomass burning (BB) emissions, monoaromatics, naphthalene/methylnaphthalenes, and three biogenic VOCs (isoprene, monoterpene, and sesquiterpene). Generally, the SOA tracers showed regional characteristics for both anthropogenic and biogenic SOA as well as for the BB-derived SOA. This work focused on the seasonal variation and evolution characteristics of SOA tracers during 11 city-wide PM2.5 episodes, which are defined as periods with PM2.5 concentrations exceeding 35 µg m−3 at 3 or more of the 15 general air quality monitoring stations cross the city. Mass increment ratios (MIR), calculated as the ratio of the mass concentration prior to an episode to that during an episode, were examined for individual species during each episode. During most episodes, the SOA tracer concentrations were enhanced (i.e. MIR &gt;1), and the maximum MIR values were in the range of 5.5–11.0 for SOA tracers of different precursors. Episodes on summer and fall days showed notably larger MIR values than those falling on winter days, indicating the higher importance of SOA to the formation of summer/fall PM2.5 episodes. Simultaneous monitoring of six tracers for isoprene SOA revealed the dominance of the low-NOx pathway in forming isoprene SOA in our study region. The multiple monoterpene SOA products suggested fresher SOA in winter, evidenced by the increased presence of the early-generation products. Thus, the current study has shown by example the precursor-specific SOA chemical evolution characteristics during PM2.5 episodes in different seasons. This study also suggests the necessity to apply high-time-resolution organic marker measurement at multiple sites in order to fully capture the spatial heterogeneity of haze pollution at the city scale.

Seasonal characteristics and provenance of organic aerosols in the urban atmosphere of Liaocheng in the North China Plain: Significant effect of biomass burning

To better understand the seasonal characteristics of urban organic aerosol (OA) in the North China Plain (NCP), PM2.5 samples in the urban atmosphere of Liaocheng were collected and analyzed. The molecular distribution of the organic markers in the urban atmosphere of Liaocheng reveals that n-alkanes (39.3%) was the most abundant species all year round, followed by saccharides (28.2%), phthalic acids (Ph, 20.8%), biogenic secondary organic aerosol (BSOA) tracers (9.4%), and polycyclic aromatic hydrocarbon (PAHs, 2.3%). PM2.5, organic carbon (OC), elemental carbon (EC), and primary organic markers exhibit the highest concentrations in winter, due largely to the increased biomass burning and coal combustion for house heating in local and surrounding regions. However, the concentration and relative abundance of BSOA are significantly higher in summer than other seasons, induced by the more favorable meteorological conditions that would promote the emissions of biogenic volatile organic compounds (BVOCs) and the secondary production of BSOA. The ratios of OC/EC and 3-methyl-1,2,3-butanetricarboxylic acid to cis-pinic acid plus cis-pinonic acid (MBTCA/(PA + PNA) are higher in the warm seasons than those in the cold seasons, indicating that the oxidation of OA is sensitive to air temperature. Compared to 2017, the concentration level of PAHs during wintertime decreased by 40.8%, confirming that the stringent regulation of coal burning is effective. The highest concentration of high molecular weight (HMW) n-alkanes and three anhydrosugars in winter, and the close correlation of levoglucosan with HMW n-alkanes suggests that the impact of biomass burning was more significant in winter. The same seasonal characteristic of the ratios of high-/low-NOx products with NOx and the strong correlation of high-/low-NOx products with levoglucosan indicate that the formation of isoprene SOA (SOAI) tracers was significantly influenced by anthropogenic emissions. The molecular compositions, the distributions of fire spots, backward trajectories of air masses, and correlation analysis suggest that air pollution events in spring were primarily resulted from biomass burning and secondary oxidation, while pollution events in winter were largely driven by the increased combustion sources, and promoted aqueous secondary formation. Our results suggest that the reduction of biomass and coal combustion should be taken into account to improve the urban air quality in the NCP.

Seasonal variation, source contribution, and impact factors of biogenic organic aerosols in PM2.5 in Nanjing, China

In this study, a liquid chromatography–mass spectrometry (HPLC-ESI-MS/MS) was used to develop a rapid method to detect carbohydrates (one anhydrosugar, five sugars, and four sugar alcohols) and monoterpene secondary organic aerosol (SOAM) tracers in the atmosphere, successfully applied to 139 atmospheric aerosol samples in Nanjing in 2019. Anhydrosugar (levoglucosan) is the most abundant carbohydrate in the atmospheric environment, with an annual mean concentration of 103.96 ng m−3. The measured compounds exhibited distinct seasonal variation, with a higher concentration in spring and summer and a lower concentration in autumn and winter. The distinct seasonal variation in these Bioorganic aerosol (BOA) tracers may be caused by the different dominant sources in different seasons. Their seasonal trends are also similar to other worldwide observations. In Nanjing, the yield of SOAM tracers is affected by meteorological conditions and anthropogenic pollutants (e.g., NO2 and O3). Furthermore, five source factors (secondary formation, biomass burning, soil resuspension, plant debris, and fungal spores) of BOA and their geographic origin were obtained using the Positive Matrix Factor (PMF) and Potential Source Contribution Factor (PSCF) models. The contribution of these factors to BOA also varies, with biomass burning (77.4 %) contributing the most, followed by fungal spores (12.2 %) and soil suspension (2.1 %).

Measurement report: Effects of anthropogenic emissions and environmental factors on the formation of biogenic secondary organic aerosol (BSOA) in a coastal city of southeastern China

Abstract. To better understand the formation of biogenic secondary organic aerosol (BSOA), aerosol samples with a 4 h time resolution were collected during summer and winter in the southeast of China, along with online measurements of trace gases, aerosol chemical compositions, and meteorological parameters. The samples were analyzed by gas chromatography–mass spectrometry for PM2.5-bound secondary organic aerosol (SOA) tracers, including isoprene (SOAI), α/β-pinene (SOAM), β-caryophyllene (SOAC), and toluene (ASOA). The average concentrations of total SOA tracers in winter and summer were 38.8 and 111.9 ng m−3, respectively, with the predominance of SOAM (70.1 % and 45.8 %), followed by SOAI (14.0 % and 45.6 %), ASOA (11.0 % and 6.2 %) and SOAC (4.9 % and 2.3 %). Compared to those in winter, the majority of BSOA tracers in summer showed significant positive correlations with Ox (O3+NO2) (r = 0.443–0.808), HONO (r = 0.299–0.601), ultraviolet (UV) (r = 0.382–0.588) and temperature (T) (r = 0.529–0.852), indicating the influence of photochemical oxidation under relatively clean conditions. However, in winter, BSOA tracers were significantly correlated with PM2.5 (r = 0.407–0.867), NO3- (r = 0.416–0.884), SO42- (r = 0.419–0.813), and NH3 (r = 0.440–0.757), attributed to the contributions of anthropogenic emissions. Major BSOA tracers in both seasons were linearly correlated with aerosol acidity (pH) (r = 0.421–0.752), liquid water content (LWC) (r = 0.403–0.876) and SO42- (r = 0.419–0.813). The results indicated that acid-catalyzed reactive uptake onto sulfate aerosol particles enhanced the formation of BSOA. In summer, the clean air mass originated from the ocean, and chlorine depletion was observed. We also found that concentrations of the total SOA tracers were correlated with HCl (R2=0.545) and chlorine ions (r = 0.280–0.639) in PM2.5, reflecting the contribution of Cl-initiated volatile organic compound (VOC) oxidations to the formation of SOA. In winter, the northeast dominant wind direction brought continental polluted air mass to the monitoring site, affecting the transformation of BSOA tracers. This implied that anthropogenic emissions, atmospheric oxidation capacity and halogen chemistry have significant effects on the formation of BSOA in the southeast coastal area.

Tracer-based characterization of fine carbonaceous aerosol in Beijing during a strict emission control period

Strict emission controls were implemented in Beijing and the surrounding regions in the North China Plain to guarantee good air quality during the 2014 Asia-Pacific Economic Cooperation (APEC) summit. Thus, the APEC period provides a good opportunity to study the sources and formation processes of atmospheric organic aerosol. Here, fine particles (PM2.5, particulate matter with a diameter of 2.5 μm or less) collected in urban Beijing before and during the APEC period were analyzed for molecular tracers of primary and secondary organic aerosol (SOA). The primary organic carbon (POC) and secondary organic carbon (SOC) were also reconstructed using a tracer-based method. The concentrations of biogenic SOA tracers ranged from 1.09 to 34.5 ng m−3 (mean 10.3 ± 8.51 ng m−3). Monoterpene oxidation products were the largest contributor to biogenic SOA, followed by isoprene- and sesquiterpene-derived SOA. The concentrations of biogenic SOA tracers decreased by 50 % during the APEC, which was largely attributed to the implementation of emission controls by the Chinese government. The increasing mass fractions of biogenic SOA tracers from isoprene and sesquiterpene during the pollution episodes implied that their photooxidation processes contributed to the poor air quality in urban Beijing. The reconstructed biogenic and anthropogenic SOC and POC concentrations were 89.6 ± 96.8 ng m−3, 570 ± 611 ng m−3, and 2.49 ± 2.08 μg m−3, respectively, accounting for 21.9 ± 11.4 % of OC in total. Biomass-burning derived OC was the largest contributor to carbonaceous aerosol over the North China Plain. By comparing the results before and during the APEC, the emission controls effectively mitigated about 34 % of the estimated OC and were more effective at reducing SOC than POC. This suggests that the reduction of the primary organic aerosol loading is harder than SOA over the North China Plain.

Diurnal Variations of Isoprene, Monoterpenes, and Toluene Oxidation Products in Aerosols at a Rural Site of Guanzhong Plain, Northwest China

In this study, the characteristics and formation mechanism of summertime isoprene, monoterpene, and toluene-derived secondary organic aerosols (SOAs) were investigated in a rural area of Guanzhong Plain, Northwest China. The variations in key indicators of primary sources indicated a significant influence of biomass burning on PM2.5 during the observation period. The concentrations of total measured SOA tracers from isoprene, monoterpene, and toluene were 40.85 ± 17.31, 24.27 ± 7.50, and 10.61 ± 0.33 ng/m3, respectively. The average ratio of cis-pinonic and pinic acids to 3-Methyl-1,2,3-butanetricarboxylic acid (MBTCA)(P/M) were 0.45 and 0.85 by day and by night, respectively. The low ratio in the daytime was mainly due to the stronger photo-degradation and particle-to-gas distribution of semi-volatile cis-pinonic and pinic acids. The monoterpene SOA tracers were significantly correlated with levoglucosan at night (R2 = 0.51, p &lt; 0.01), as were toluene SOA tracers and levoglucosan (R2 &gt; 0.67, p &lt; 0.01), indicating the significant contribution of biomass combustion to these SOAs. The mass concentration of isoprene-, monoterpenes-, and toluene-derived SOC was estimated by using the tracer yield method. The total calculated SOCs by day and by night were 0.25–0.71 (average: 0.46) and 0.26–0.78 (average: 0.42) µgC/m3, accounting for 3.35–10.58% and 3.87–13.51% of OC by day and by night, respectively.