Low molecular weight dicarboxylic acids, oxocarboxylic acids and α-dicarbonyls as ozonolysis products of isoprene: Implication for the gaseous-phase formation of secondary organic aerosols

Exploring secondary aerosol formation associated with elemental carbon in the lower free troposphere

The variability of element carbon (EC) mixed with secondary species significantly complicates the assessment of its environmental impact, reflecting the complexity and diversity of EC-containing particles' composition and morphology during their ascent and regional transport. While the catalytic role of EC in secondary aerosol formation is recognized, the effects of heterogeneous chemistry on secondary species formation within diverse EC particle types are not thoroughly understood, particularly in the troposphere. Alpine sites offer a prime environment to explore EC properties post-transport from the ground to the free troposphere. Consequently, we conducted a comprehensive study on the genesis of secondary aerosols in EC-containing particles at Mt. Hua (altitude: 2069 m) from 1 May to 10 July, using a single particle aerosol mass spectrometer (SPAMS). Our analysis identified six major EC particle types, with EC-K, EC-SN, and EC-NaK particles accounting for 27.6 %, 27.0 %, and 19.6 % of the EC particle population, respectively. The concentration-weighted trajectory (CWT) indicated that the lower free troposphere over Mt. Hua is significantly affected by anthropogenic emissions at ground-level, predominantly from northwestern and eastern China. Atmospheric interactions are crucial in generating high sulfate levels in EC-SN and EC-OC particles (> 70 %) and notable nitrate levels in EC-K, EC-BB, and EC-Fe particles (> 80 %). The observed high chloride content in EC-OC particles (56 ± 32 %) might enhance chlorine's reactivity with organic compounds via heterogeneous reactions within the troposphere. Distinct diurnal cycles for sulfate and nitrate are mainly driven by varying transport dynamics and formation processes, showing minimal dependency on EC particle types. Enhanced nocturnal oxalate conversion in EC-Fe particles is likely due to the aqueous oxidation of precursors, with Fe-catalyzed Fenton reactions enhancing OH radical production. This investigation provides critical insights into EC's role in secondary aerosol development during its transport in the lower free troposphere.

Explainable ensemble machine learning revealing the effect of meteorology and sources on ozone formation in megacity Hangzhou, China

Megacity Hangzhou, located in eastern China, has experienced severe O3 pollution in recent years, thereby clarifying the key drivers of the formation is essential to suppress O3 deterioration. In this study, the ensemble machine learning model (EML) coupled with Shapley additive explanations (SHAP), and positive matrix factorization were used to explore the impact of various factors (including meteorology, chemical components, sources) on O3 formation during the whole period, pollution days, and typical persistent pollution events from April to October in 2021-2022. The EML model achieved better performance than the single model, with R2 values of 0.91. SHAP analysis revealed that meteorological conditions had the greatest effects on O3 variability with the contribution of 57 %-60 % for different pollution levels, and the main drivers were relative humidity and radiation. The effects of chemical factors on O3 formation presented a positive response to volatile organic compounds (VOCs) and fine particulate matter (PM2.5), and a negative response to nitrogen oxides (NOx). Oxygenated compounds (OVOCs), alkenes, and aromatic of VOCs subgroups had higher contribution; additionally, the effects of PM2.5 and NOx were also important and increased with the O3 deterioration. The impact of seven emission sources on O3 formation in Hangzhou indicated that vehicle exhaust (35 %), biomass combustion (16 %), and biogenic emissions (12 %) were the dominant drivers. However, for the O3 pollution days, the effects of biomass combustion and biogenic emissions increased. Especially in persistent pollution events with highest O3 concentrations, the magnitude of biogenic emission effect elevated significantly by 156 % compared to the whole situations. Our finding revealed that the combination of the EML model and SHAP analysis could provide a reliable method for rapid diagnosis of the cause of O3 pollution at different event scales, supporting the formulation of control measures.

Comparison of acidity and chemical composition of summertime cloud water and aerosol at an alpine site in Northwest China: Implications for the neutral property of clouds in the free troposphere

Aerosol and cloud acidity are essential to human health, ecosystem health and productivity, as well as climate effects. The main chemical composition of cloud water greatly varies in different regions, resulting in substantial differences in the pH of cloud water. However, the influences of the anthropogenic emissions of acidic gases and substances, alkaline dust components, and dicarboxylic acids (diacids) on the ground concerning the acidity of cloud water in the free troposphere of the Guanzhong Plain, China, remain clear. In this study, cloud water and PM2.5 samples were simultaneously collected in the troposphere (Mt. Hua, 2060 m a.s.l). The results indicated that the cloud water was alkaline (pH = 7.6) and PM2.5 was acidic (pH = 3.2). These results showed the neutral property of clouds collected in the heavily polluted Guanzhong Plain, although most previous studies always considered acidity as a marker of pollution. The sulfate (SO42-), nitrate (NO3-), and ammonium (NH4+) (SNA) of particulate matter and cloud water in the same period were compared. SO42- was dominant in particulate matters (accounting for 63.4 % of the total SNA) but substantially decreased in cloud water (only 30.1 % of the total SNA), whereas NO3- and NH4+ increased from 28.5 % and 8.2 % to 39.8 % and 30.2 %, respectively. This could be attributed to the complex formation mechanism and sources of SO42- and NO3- in the cloud. The results of ion balance indicated that a significant deficit of inorganic anion equivalents was observed in the cloud water samples. The high concentration of diacids in the cloud phase (1237.4 μg L-1) may facilitate the formation of salt complexes with NH4+, thus influencing the acidity of the cloud water. The pH of cloud water has increased in recent decades due to the sustained reduction of sulfur dioxide, which may also affect the acidity of future precipitation.

A year-round observation of δ13C of dicarboxylic acids and related compounds in fine aerosols: Implications from Central European background site

Isotopic analysis of specific compounds in aerosols can be a useful tool when studying atmospheric processes. Here, we present the results of stable carbon isotope ratio (δ13C) measurements performed on a one-year set (n = 96, Sep. 2013-Aug. 2014) of dicarboxylic acids and related compounds in PM1 at a rural Central European background site, Košetice (Czech Republic). The most 13C enriched acid was oxalic (C2, annual average = -16.6 ± 5.0‰) followed by malonic (C3, avg. = -19.9 ± 6.6‰) and succinic (C4, avg. = -21.3 ± 4.6‰) acids. Thus, δ13C values decreased with an increase in carbon numbers. Azelaic acid (C9, avg. = -27.2 ± 3.6‰) was found to be the least 13C enriched. A comparison of δ13C of dicarboxylic acids from other background sites, especially in Asia, shows similar values to those from the European site. This comparison also showed that C2 is more 13C enriched at background sites than at urban ones. In general, we did not observe significant seasonal differences in δ13C values of dicarboxylic acids at the Central European station. We observed statistically significant differences (p value < 0.05) between winter and summer δ13C values solely for C4, glyoxylic acid (ωC2), glutaric acid (C5) and suberic acid (C8). The only significant correlations between δ13C of C2 and δ13C of C3 were found in spring and summer, suggesting that the oxidation of C3 to C2 is significant in these months with a strong contribution from biogenic aerosols. The strongest season-independent annual correlation was observed in δ13C values between C2 and C4, the two dominant dicarboxylic acids. Therefore, C4 appears to be the main intermediate precursor of C2 throughout the whole year.

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.

Influence of vertical transport on chemical evolution of dicarboxylic acids and related secondary organic aerosol from surface emission to the top of Mount Hua, Northwest China

Dicarboxylic acids are strong hygroscopic organic compounds in the atmosphere, and thus significantly affect the cloud formation process and radiative forcing on a regional scale. So far, the evolution of dicarboxylic acids during vertical transport from the surface to the mountaintop has yet to be explicitly understood. In this study, the molecular distribution and stable carbon isotopic (δ¹³C) compositions of dicarboxylic acids and related organic compounds (DCRCs) in PM_2.5 were measured simultaneously at the top (c. 2060 m a.s.l.) and foot (c. 400 m a.s.l.) of Mount (Mt.) Hua during the summer of 2020. Due to the strong anthropogenic emissions at ground level, the concentrations of DCRCs at foot of Mt. Hua were generally higher than those at the top. Oxalic acid (C₂) was the predominant diacid in both sites, whose concentrations at foot and top of Mt. Hua were 87-852 and 40-398 ng m^-3, respectively. Ratios of adipic acid to azelaic acid (C₆/C₉), phthalic aid to azelaic acid (pH/C₉), glyoxal to methylglyoxal (Gly/mGly), and lower δ¹³C values (-21.0 ± 2.3 ‰ and - 21.9 ± 2.7 ‰) of C₂ indicated that the contributions of anthropogenic sources to DCRCs in PM_2.5 in the mountain region are more significant than biogenic sources. Aerosols from the foot of Mt. Hua could affect the atmosphere on the top of the mountain via vertical transport under the influence of daytime valley wind, even though the altitude of Mt. Hua is beyond the boundary layer most of time. The value δ¹³C of C₂ is linearly correlated with C₂/mGly, C₂/pyruvic acid (Pyr), C₂/glyoxylic acid (ωC₂) at the top of the mountain, and C₂/Gly, C₂/ωC₂ at the foot of the mountain, indicating that the formation pathway of C₂ is mGly-Pyr-ωC₂-C₂ at the top of Mt. Hua and Gly-ωC₂-C₂ at the foot of Mt. Hua.

Geochemical studies of low molecular weight organic acids in the atmosphere: sources, formation pathways, and gas/particle partitioning

Low molecular weight monocarboxylic acids (LMW monoacids, C₁-C₁₀) are the most abundant gaseous organic compound class in the atmosphere. Formic or acetic acid is the dominant volatile organic compound (VOC) in Earth's atmosphere. They can largely contribute to rainwater acidity, especially in the tropical forest, and react with alkaline metals, ammonia, and amines, contributing to new particle formation and secondary organic aerosol production. Gaseous and particulate LMW monoacids were abundantly reported in China. They can be directly emitted from fossil fuel combustion and biomass burring; however, the secondary formation is more important than primary emissions via the photochemical oxidation of anthropogenic and biogenic VOCs. In this paper, we review the distributions of LMW monoacids from urban, mountain, and marine sites as well as from rainwater and alpine snow samples and discuss their sources and formation mechanisms in the atmosphere. We also discuss their importance as cloud condensation nuclei (CCN) and provide future perspectives of LMW monoacids study in the warming world.

Large contribution of fossil-derived components to aqueous secondary organic aerosols in China

Incomplete understanding of the sources of secondary organic aerosol (SOA) leads to large uncertainty in both air quality management and in climate change assessment. Chemical reactions occurring in the atmospheric aqueous phase represent an important source of SOA mass, yet, the effects of anthropogenic emissions on the aqueous SOA (aqSOA) are not well constrained. Here we use compound-specific dual-carbon isotopic fingerprints (δ¹³C and Δ¹⁴C) of dominant aqSOA molecules, such as oxalic acid, to track the precursor sources and formation mechanisms of aqSOA. Substantial stable carbon isotope fractionation of aqSOA molecules provides robust evidence for extensive aqueous-phase processing. Contrary to the paradigm that these aqSOA compounds are largely biogenic, radiocarbon-based source apportionments show that fossil precursors produced over one-half of the aqSOA molecules. Large fractions of fossil-derived aqSOA contribute substantially to the total water-soluble organic aerosol load and hence impact projections of both air quality and anthropogenic radiative forcing. Our findings reveal the importance of fossil emissions for aqSOA with effects on climate and air quality.

Four- and Five-Carbon Dicarboxylic Acids Present in Secondary Organic Aerosol Produced from Anthropogenic and Biogenic Volatile Organic Compounds

To better understand precursors of dicarboxylic acids in ambient secondary organic aerosol (SOA), we studied C4–C9 dicarboxylic acids present in SOA formed from the oxidation of toluene, naphthalene, α-pinene, and isoprene. C4–C9 dicarboxylic acids present in SOA were analyzed by offline derivatization gas chromatography–mass spectrometry. We revealed that C4 dicarboxylic acids including succinic acid, maleic acid, fumaric acid, malic acid, DL-tartaric acid, and meso-tartaric acid are produced by the photooxidation of toluene. Since meso-tartaric acid barely occurs in nature, it is a potential aerosol tracer of photochemical reaction products. In SOA particles from toluene, we also detected a compound and its isomer with similar mass spectra to methyltartaric acid standard; the compound and the isomer are tentatively identified as 2,3-dihydroxypentanedioic acid isomers. The ratio of detected C4–C5 dicarboxylic acids to total toluene SOA mass had no significant dependence on the initial VOC/NOx condition. Trace levels of maleic acid and fumaric acid were detected during the photooxidation of naphthalene. Malic acid was produced from the oxidation of α-pinene and isoprene. A trace amount of succinic acid was detected in the SOA produced from the oxidation of isoprene.