Seasonal characteristics of oxalic acid and related SOA in the free troposphere of Mt. Hua, central China: implications for sources and formation mechanisms

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.

Temporal variations and chemical characteristics of marine PM2.5 at Dongsha Islands, South China Sea: Three-year measurement

The study of long-range transport effects on marine fine particles (PM2.5), particularly in remote sites such as the Dongsha Islands, is pivotal for advancing our understanding of air pollution dynamics on a regional scale and for formulating effective environmental policies. PM2.5 concentrations were examined over three consecutive years and grouped based on their transport routes. The backward trajectory simulation revealed that high PM2.5 concentrations were observed in the West Channel, originating from North and Central China, the Korean Peninsula, and the Japanese Islands, opposed to the East Channel. High PM2.5 concentrations, commonly observed in winter and spring, were mainly attributed to the Asian Northeastern Monsoons. Water-soluble inorganic ions constituted the major components, accounting for 37.8-48.7% of PM2.5, and followed by metal elements (15.5-20.0%), carbons (7.5-13.3%), levoglucosan (0.01-0.17%), and organic aerosols (0.2-2.2%). Secondary inorganic aerosols as the dominant source accounted for 8.3-24.7% of PM2.5, while sea salts were the secondary major contributor. High levoglucosan contribution (3.8-7.2%) in winter and spring was attributed to biomass burning, mainly from the Indochina Peninsula. Chemical mass balance receptor modeling resolved that major sources of PM2.5 were secondary sulfate, sea salts, fugitive dust, and industrial boilers. This study concluded that the long-range transport of PM2.5 gradually increased since fall, contributing 52.1-74.3%, highlighting its substantial impact on PM2.5 in all seasons except summer.

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.

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.

Particle characterization and quantification of organic and inorganic compounds from Chinese and Iranian aerosol filter samples using scanning laser desorption/ionization mass spectrometry

Besides their influence on climate and cloud formation, many organic and inorganic substances in aerosol particles pose a risk to human health. Namely, polycyclic aromatic hydrocarbons (PAH) and heavy metals are suspected to be carcinogenic or acutely toxic. The detection and quantification of such compounds is difficult if only small amounts of particulate matter (PM) are available. In addition, filter samples are often complex and time-consuming to prepare for chromatographic measurements and elemental analysis. Here, we present a method based on high-resolution atmospheric pressure laser desorption ionization mass spectrometry imaging (AP-LDI-MSI) and statistical analysis which allows the analysis and characterization of very small sample quantities (< 30 µg) without any sample preparation. The power and simplicity of the method is demonstrated by two filter samples from heavily polluted mega cities. The samples were collected in Tehran (Iran) and Hangzhou (China) in February 2018. In the course of the measurement, more than 3200 sum formulae were assigned, which allowed a statistical evaluation of colocalized substances within the particles on the filter samples. This resulted in a classification of the different particle types on the filters. Finally, both megacities could be distinguished based on characteristic compounds. In the samples from Tehran, the number of sulphur-containing organic compounds was up to 6 times as high as the samples from Hangzhou, possibly due to the increasing efforts of the Chinese government to reduce sulphur emissions in recent years. Additionally, quantification of 13 PAH species was carried out via standard addition. Especially, the samples from Tehran showed elevated concentrations of PAHs, which in the case of higher-molecular-weight species (> m/z 228) were mostly more than twice as high as in Hangzhou. Both cities showed high levels of heavy metals and potentially harmful organic compounds, although their share of total particulate matter was significantly higher in the samples from Tehran. The pre-treatment of the samples was reduced to a minimum with this method, and only small amounts of particles were required to obtain a comprehensive picture for a specific filter sample. The described method provides faster and better control of air pollution in heavily polluted megacities.

Molecular characteristics and stable carbon isotope compositions of dicarboxylic acids and related compounds in wintertime aerosols of Northwest China

Dicarboxylic acids are one of the important water-soluble organic compounds in atmospheric aerosols, causing adverse effects to both climate and human health. More attention has therefore been paid to organic acids in aerosols. In this study, the molecular distribution and diurnal variations of wintertime dicarboxylic acids in a rural site of Guanzhong Plain, Northwest China, were explored. Oxalic acid (C₂, day: 438.9 ± 346.8 ng m⁻³, night: 398.8 ± 392.3 ng m⁻³) is the most abundant compound followed by methylglyoxal (mGly, day: 207.8 ± 281.1 ng m⁻³, night: 222.9 ± 231.0 ng m⁻³) and azelaic (C₉, day: 212.8 ± 269.1 ng m⁻³, night: 211.4 ± 136.7 ng m⁻³) acid. The ratios of C₉/C₆ and C₉/Ph indicating that atmospheric dicarboxylic acids in winter in the region mainly come from biomass burning. Furthermore, secondary inorganic ions (NO₃⁻, SO₄²⁻, and NH₄⁺), relative humidity, liquid water content, and in-situ pH of aerosols are highly linearly correlated with C₂, suggesting that liquid phase oxidation is an important pathway for the formation of dicarboxylic acids. The δ¹³C analysis of C₂ suggested that lighter carbon isotope compositions tend to be oxidized to form aqueous-phase secondary organic aerosols (aqSOA), leading to the decay of ¹³C in aqSOA products rather than aerosol aging. This study provides a theoretical basis for the mechanism of formation of dicarboxylic acid.

Molecular characterization and spatial distribution of dicarboxylic acids and related compounds in fresh snow in China

Low molecular weight organic compounds are ubiquitous in the atmosphere. However, knowledge on their concentrations and molecular distribution in fresh snow remains limited. Here, twelve fresh snow samples collected at eight sites in China were investigated for dicarboxylic acids and related compounds (DCRCs) including oxocarboxylic acids and α-dicarbonyls. Dissolved organic carbon (DOC) concentrations in the snow samples ranged from 0.99 to 14.6 mg C L^-1. Concentrations of total dicarboxylic acids were from 225 to 1970 μg L^-1 (av. 650 μg L^-1), while oxoacids (28.3-173, av. 68.1 μg L^-1) and dicarbonyls (12.6-69.2, av. 31.3 μg L^-1) were less abundant, accounting for 4.6-8.5% (6.2%), 0.45-1.4% (0.73%), and 0.12-0.88% (0.46%) of DOC, respectively. Molecular patterns of dicarboxylic acids are characterized by a predominance of oxalic acid (C₂) (95.0-1030, av. 310 μg L^-1), followed by phthalic (Ph) (9.69-244, av. 69.9 μg L^-1) or succinic (C₄) (23.8-163, av. 63.7 μg L^-1) acid. Higher concentrations of Ph in snow from Beijing and Tianjin than other urban and rural regions suggest significant emissions from vehicular exhausts and other fossil fuel combustion sources in megacities. C₂ constituted 40-54% of total diacids, corresponding to 1.5-2.6% of snow DOC. The total measured DCRCs represent 5.5-10% of snow DOC, which suggests that there are large amounts of unknown organics requiring further investigations. The spatial distributions of diacids exhibited higher loadings in megacities than rural and island sites. Molecular distributions of diacids indicated that the photochemical modification was restrained under the weak solar radiation during the snow events, while anthropogenic primary sources had a more significant influence in megacities than rural areas and islands.

Impacts of primary emissions and secondary aerosol formation on air pollution in an urban area of China during the COVID-19 lockdown

Restrictions on human activities were implemented in China to cope with the outbreak of the Coronavirus Disease 2019 (COVID-19), providing an opportunity to investigate the impacts of anthropogenic emissions on air quality. Intensive real-time measurements were made to compare primary emissions and secondary aerosol formation in Xi'an, China before and during the COVID-19 lockdown. Decreases in mass concentrations of particulate matter (PM) and its components were observed during the lockdown with reductions of 32-51%. The dominant contributor of PM was organic aerosol (OA), and results of a hybrid environmental receptor model indicated OA was composed of four primary OA (POA) factors (hydrocarbon-like OA (HOA), cooking OA (COA), biomass burning OA (BBOA), and coal combustion OA (CCOA)) and two oxygenated OA (OOA) factors (less-oxidized OOA (LO-OOA) and more-oxidized OOA (MO-OOA)). The mass concentrations of OA factors decreased from before to during the lockdown over a range of 17% to 58%, and they were affected by control measures and secondary processes. Correlations of secondary aerosols/ΔCO with Ox (NO2 + O3) and aerosol liquid water content indicated that photochemical oxidation had a greater effect on the formation of nitrate and two OOAs than sulfate; however, aqueous-phase reaction presented a more complex effect on secondary aerosols formation at different relative humidity condition. The formation efficiencies of secondary aerosols were enhanced during the lockdown as the increase of atmospheric oxidation capacity. Analyses of pollution episodes highlighted the importance of OA, especially the LO-OOA, for air pollution during the lockdown.

Summertime atmospheric dicarboxylic acids and related SOA in the background region of Yangtze River Delta, China: Implications for heterogeneous reaction of oxalic acid with sea salts

The diacid chemistry of summertime PM_2.5 and the size-segregated aerosols (9-stages) in Chongming Island, a coastal site in the Yangtze River Delta (YRD), China, were investigated. Our results showed that oxalic acid (C₂) was the dominant dicarboxylic acid, followed by succinic acid (C₄), malonic acid (C₃), adipic acid (C₆) and phthalic acid (Ph). Two types of haze pollution events were identified during the sampling period, i.e., Event I, which was mainly caused by the local biomass burning emission, and Event II, which was caused by a long-distance transport of the YRD urban pollution. C₂ linearly correlated with SO₄^2- and NO₃^- in Event I but only with O₃ in Event II, indicating that oxalic acid formation was dominated by the aerosol aqueous phase oxidation in Event I and by the gaseous phase oxidation in Event II, respectively. 65.5% of Cl^- in sea salts at the site in the clean period was depleted and robustly correlated with oxalic acid (R² = 0.74). We proposed a mechanism to explain such a significant Cl^- depletion, in which anthropogenic VOC oxidize into oxalic acid and its precursors such as glyoxal and methyglyoxal by a photochemical oxidation, and then oxalic acid and the related compounds subsequently react with sea salts and release HCl into the troposphere. The significant Cl^- depletion of sea salts related with the organic acid (C₂) in coastal China was found for the first time and should be considered in future studies, because oxalic acid and related SOA in the country are abundant and the released HCl may effectively enhance the oxidation capacity of the atmosphere by photolytically producing Cl radicals.

Insight into the photochemistry of atmospheric oxalate through hourly measurements in the northern suburbs of Nanjing, China

Oxalate-iron is an integral part of the photochemical system in the atmosphere. Here, we combined high-resolution online observations and laboratory simulations to discuss the distribution of oxalate and oxalate-iron photochemical system in Nanjing atmosphere at the molecular level. The results show that the oxidation state of iron in the oxalate-iron photochemical system changes significantly and regularly. Among them, Fe (II)/Fe (III) is 3.82 during the day and 0.76 at night. At the same time, Cl^- may accelerate the generation of hydroxyl radicals in the system and promote the photooxidation rate of oxalate. Oxalate can be converted into formate (C1) and acetate (C2) in the photochemical system, but <4% of degraded oxalate is converted, which means that the photochemical system may not be the main source of formate and acetate in the atmosphere. Besides, the ratio of C1/C2 < 1 in the conversion is opposite to the ratio of C1/C2 > 1 in the general secondary conversion, which means that not all ratio of C1/C2 in the photochemical pathway is >1. These results are beneficial for us to understand the effect of the oxalate-iron photochemical system on the distribution of oxalate in the atmosphere, and also help us to analyze the conversion of organics in the atmospheric aqueous phase.

Molecular characteristics and stable carbon isotope compositions of dicarboxylic acids and related compounds in the urban atmosphere of the North China Plain: Implications for aqueous phase formation of SOA during the haze periods

In the past five years, Chinese government has promulgated stringent measures to mitigate air pollution. However, PM_2.5 levels in the China North Plain (NCP), which is one of the regions with the heaviest air pollution in the world, are still far beyond the World Health Organization (WHO) standard. To improve our understanding on the sources and formation mechanisms of haze in the NCP, PM_2.5 samples were collected during the winter of 2017 on a day/night basis at the urban site of Liaocheng, which is one of the most polluted cities in the NCP. The samples were determined for molecular distributions and stable carbon isotope compositions of dicarboxylic acids and their precursors (ketocarboxylic acids and α-dicarbonyls), levoglucosan, elemental carbon (EC), organic carbon (OC) and water-soluble organic carbon (WSOC). Our results showed that oxalic acid (C₂) is the dominant dicarboxylic acid, followed by succinic acid (C₄) and malonic acid (C₃), and glyoxylic acid (ωC₂) is the most abundant ketocarboxylic acids. Concentrations of C₂, glyoxal (Gly) and methylglyoxal (mGly) presented robust correlations with levoglucosan, suggesting that biomass burning is a significant source of PM_2.5 in the NCP. Moreover, C₂ and Gly and mGly linearly correlated with SO₄^2-, relative humidity (RH), aerosol liquid water content (LWC) as well as particle in-situ pH (pH_is), indicating that aqueous-phase oxidation is the major formation pathway of these SOA, and is driven by acid-catalyzed oxidation. Concentrations and relative abundances of secondary species including SNA (SO₄^2-, NO₃^- and NH₄⁺), dicarboxylic acids, and aerosol LWC in PM_2.5 are much higher in the haze periods than in the clean periods, suggesting that aqueous reaction is a vital role in the haze formation. In comparison with those in the clean periods, stable carbon isotopic compositions (δ¹³C) of major dicarboxylic acids and related SOA and the mass ratios of C₂/diacids, C₂/Gly and C₂/mGly are higher in the haze periods, indicating that haze particles were more aged and enriched in secondary species.

Enhanced aqueous-phase formation of secondary organic aerosols due to the regional biomass burning over North China Plain

This study reveals the impact of biomass burning (BB) on secondary organic aerosols (SOA) formation in the North China Plain (NCP). Filter samples were analyzed for secondary inorganic aerosols (SIA), oxalic acid (C₂) and related aqueous-phase SOA compounds (aqSOA), stable carbon isotope composition of C₂ (δ¹³C(C₂)) and aerosol liquid water content (ALWC). Based on the PM_2.5 loadings, BB tracer concentrations, wildfire spots and air-mass back trajectories, we distinguished two episodes from the whole campaign, Episode I and Episode II, which were characteristic of regional and local BB, respectively. The abundances of PM_2.5 and organic matter in the two events were comparable, but concentrations and fractions of SIA, aqSOA during Episode I were much higher than those during Episode II, along with heavier δ¹³C(C₂), suggesting an enhanced aqSOA formation in the earlier period. We found that the enhancement of aqSOA formation during Episode I was caused by an increased ALWC, which was mainly driven by SIA during the regional BB event. Our work showed that intensive burning of crop residue in East Asia can sharply enhance aqSOA production on a large scale, which may have a significant impact on the regional climate and human health.

Oxalate Formation Enhanced by Fe-Containing Particles and Environmental Implications

We used a single particle mass spectrometry to online detect chemical compositions of individual particles over four seasons in Guangzhou. Number fractions (Nfs) of all the measured particles that contained oxalate were 1.9%, 5.2%, 25.1%, and 15.5%, whereas the Nfs of Fe-containing particles that were internally mixed with oxalate were 8.7%, 23.1%, 45.2%, and 31.2% from spring to winter, respectively. The results provided the first direct field measurements for the enhanced formation of oxalate associated with Fe-containing particles. Other oxidized organic compounds including formate, acetate, methylglyoxal, glyoxylate, purivate, malonate, and succinate were also detected in the Fe-containing particles. It is likely that reactive oxidant species (ROS) via Fenton reactions enhanced the formation of these organic compounds and their oxidation product oxalate. Gas-particle partitioning of oxalic acid followed by coordination with Fe might also partly contribute to the enhanced oxalate. Aerosol water content likely played an important role in the enhanced oxalate formation when the relative humidity is >60%. Interactions with Fe drove the diurnal variation of oxalate in the Fe-containing particles. The study could provide a reference for model simulation to improve understanding on the formation and fate of oxalate, and the evolution and climate impacts of particulate Fe.

Stepwise Oxidation of Aqueous Dicarboxylic Acids by Gas-Phase OH Radicals

A leading source of uncertainty in predicting the climate and health effects of secondary organic aerosol (SOA) is how its composition changes over their atmospheric lifetimes. Because dicarboxylic acid (DCA) homologues are widespread in SOA, their distribution provides an ideal probe of both aerosol age and the oxidative power of the atmosphere along its trajectory. Here we report, for the first time, on the oxidation of DCA(aq) by ·OH(g) at the air-water interface. We found that exposure of aqueous HOOC-Rn-COOH (Rn = C2H4, C3H6, C4H8, C5H10, and C6H12) microjets to ∼10 ns ·OH(g) pulses from the 266 nm laser photolysis of O3(g)/O2(g)/H2O(g) mixtures yields the corresponding (n-1) species O═C(H)-Rn-1-COO(-)/HOOC-Rn-1-COO(-), in addition to an array of closed-shell HOOC-Rn(-H)(OOH)-COO(-), HOOC-Rn(-2H)(═O)-COO(-), HOOC-Rn(-H)(OH)-COO(-), and radical HOOC-Rn(-H)(OO·)-COO(-) species. Oxalic and malonic acids, which are shown to be significantly less hydrophobic and reactive than their higher homologues, will predictably accumulate in SOA, in accordance with field observations.