Combustion-Derived Organic Aerosols Enhance PM<sub>2.5</sub> Oxidative Potential in a Medium-Sized North China Plain City

Cai, Jing; Liu, Yongchun; Zheng, Feixue; Ding, Dian; Du, Wei; Yuan, Liang; Zou, Yingjie; Suolang, Lazong; Lu, Rongxiang; Li, Wanwei; Liu, Chao; Fan, Xiaolong; Zha, Qiaozhi; Luan, Mengxiao; Wang, Jiandong

doi:10.5194/egusphere-2026-4106

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https://doi.org/10.5194/egusphere-2026-4106

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14 Jul 2026

| 14 Jul 2026

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Combustion-Derived Organic Aerosols Enhance PM_2.5 Oxidative Potential in a Medium-Sized North China Plain City

Jing Cai, Yongchun Liu, Feixue Zheng, Dian Ding, Wei Du, Liang Yuan, Yingjie Zou, Lazong Suolang, Rongxiang Lu, Wanwei Li, Chao Liu, Xiaolong Fan, Qiaozhi Zha, Mengxiao Luan, and Jiandong Wang

Abstract. Organic aerosol (OA) is a major component of PM_2.5 with significant health impacts, particularly in the highly populated NCP. However, characterizing OA composition, toxicity, and sources remains challenging. To address this, we conducted an intensive campaign throughout the 2023–2024 heating season in Weifang, a representative medium-sized NCP city. Despite overall air quality improvements, severe winter PM_2.5 pollution events still occurred, with a maximum concentration reaching 985 µg m^-3. Using a Chemical Ionization Time-of-Flight Mass Spectrometer equipped with a Filter Inlet for Gases and AEROsols (FIGAERO-CIMS), we obtained molecular composition and identified six clusters, linking them to sources resolved by positive matrix factorization (PMF) applied to the online dataset, including secondary inorganics, biomass burning, vehicle emissions, coal combustion, and dust. To evaluate toxicity, we determined the oxidative potential (OP) in Weifang. While volume-normalized OP (OP_v) increased with both the degree of unsaturation and O:C ratios of OA, the compounds most positively correlated with mass-normalized OP (OP_m) exhibited an average carbon oxidation state of -0.5 and O:C of 0.5, indicating that moderately oxygenated OA possesses high OP activity. Multiple linear regression revealed that two combustion-derived groups drive OP: highly unsaturated C_10-15 compounds with ~10 oxygen atoms from solid fuel burning, and C_>15 compounds with <5 oxygen atoms or nitrogen-containing compounds from traffic emissions, which exhibited OP_m up to 23 times higher than the ambient average. Our study highlights the continued need for emission controls, particularly those targeting solid fuel combustion in the NCP region beyond its megacities.

Received: 09 Jul 2026 – Discussion started: 14 Jul 2026

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Jing Cai, Yongchun Liu, Feixue Zheng, Dian Ding, Wei Du, Liang Yuan, Yingjie Zou, Lazong Suolang, Rongxiang Lu, Wanwei Li, Chao Liu, Xiaolong Fan, Qiaozhi Zha, Mengxiao Luan, and Jiandong Wang

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'Comment on egusphere-2026-4106', Anonymous Referee #1, 26 Jul 2026 reply
This manuscript investigates the molecular characteristics of combustion-derived organic aerosols (OA) and their contribution to PM_2.5 oxidative potential (OP) in Weifang, a medium-sized city in the North China Plain. The authors combine FIGAERO-CIMS molecular characterization, PMF source apportionment, hierarchical cluster analysis, and dithiothreitol (DTT)-based oxidative potential measurements to identify OA components associated with coal combustion, biomass burning, vehicle emissions, and secondary formation. The manuscript argues that combustion-related OA, particularly high-DBE aromatic compounds, plays a dominant role in enhancing PM_2.5oxidative potential. The monitoring technique is novel as the authors integrated online CIMS, PMF, and OP measurements. Although the topic is relevant and the dataset contains potentially valuable molecular information, the major scientific claims rely heavily on correlations, indirect source assignments, and assumptions about molecular toxicity without sufficient experimental validation. The work would be improved if further convincing mechanistic evidence can be provided besides the currently represented analytical evidence.

Major comments

The manuscript repeatedly claims that combustion-derived OA enhances PM_2.5 OP and identifies high-DBE compounds as key contributors. However, most evidence is based on statistical correlations between FIGAERO-CIMS signals and DTT-based OP measurements. Correlation between molecular abundance and OP does not establish that these compounds are chemically responsible for oxidative activity. There may be covary issues due to common emission sources or atmospheric processing pathways. For example, the manuscript states that high-DBE compounds, likely aromatic derivatives, are major contributors to OPv based on enhanced correlations between DBE and OP. However, DBE is only a structural indicator and cannot distinguish among redox-active quinones, nitroaromatics, polycyclic aromatic hydrocarbons, sulfur-containing compounds, or thermally generated fragments. Without targeted quantification of representative compounds, authentic standard testing, or fractionation experiments, the proposed molecular drivers remain speculative and may be an overestimation.

The identification of coal combustion-, biomass burning-, and vehicle-related OA compounds is primarily based on correlations between molecular formulas and PMF-derived source factors. This approach has substantial uncertainty because PMF factors themselves are derived from bulk chemical tracers rather than direct OA molecular measurements. The assignment of sulfur-containing compounds to coal combustion is particularly questionable. The manuscript argues that increased sulfur oxidation states indicate coal combustion-related chemistry, but atmospheric oxidation and heterogeneous processing can generate similar sulfur-containing products from multiple sources. Likewise, compounds correlated with biomass burning are interpreted as combustion tracers, but biomass burning and coal combustion are known to co-occur in the North China Plain, making source separation difficult. The moderate correlation of levoglucosan with biomass burning is acknowledged by the authors, but the manuscript does not adequately quantify the uncertainty introduced by mixed combustion sources.

The manuscript combines several established methods, including FIGAERO-CIMS molecular identification, PMF source apportionment, hierarchical clustering, and DTT assays. However, the conceptual advance is unclear. Previous studies have already demonstrated that oxygenated organic aerosols, combustion-derived aromatic compounds, and nitroaromatic species influence aerosol oxidative potential. The manuscript does not introduce a new mechanistic framework, a new molecular toxicity indicator, or a predictive model that substantially improves understanding beyond previous work. The authors concluded that combustion-derived OA contributes to PM toxicity simply based on existing literature. To justify publication in a high-impact journal, the authors need to demonstrate what fundamentally new knowledge is obtained from this dataset.

The interpretation of oxidation state and the role of oxygen on OP is internally inconsistent. The manuscript reports that OPv increases with increasing O ratio, while OPm increases initially and then decreases at higher O ratios. The authors attribute this to highly oxygenated OA becoming less toxic during haze evolution. However, this explanation is not sufficiently supported. Oxidation state alone does not determine oxidative potential. Aging can both generate and remove redox-active structures depending on precursor composition and reaction pathways. The manuscript does not distinguish between increased oxygenation caused by functionalization and oxygenation caused by fragmentation. Additional evidence, such as Van Krevelen analysis combined with molecular-level identification of redox-active species, is required.

The manuscript lacks stronger validation linking ambient OA composition to health relevance. The DTT assay provides a chemical measure of oxidative potential, but the manuscript interprets OP changes as indicators of aerosol toxicity. This connection requires caution. DTT activity represents only one oxidative mechanism and does not directly represent cellular toxicity, inflammatory response, or health impacts. The conclusions should be limited to oxidative potential rather than implying broader toxicological effects. Ideally, the study should include complementary oxidative assays or biological responses to support stronger health-related claims.

Minor comments

Lines 146–150 (Page 6): The authors introduce the integration of FIGAERO-CIMS, OP assays, cluster analysis, and PMF source apportionment as the methodological framework. However, the rationale for combining these approaches is not sufficiently explained. The authors should clarify how each method addresses a distinct scientific question rather than simply providing parallel datasets.

Lines 168–177 (Page 6): The sampling description lacks important QA/QC information. Although the authors state that “all data have undergone strict quality control,” specific details regarding field blank correction, filter storage conditions, extraction recovery, and possible sampling artifacts should be provided.

Lines 204–223 (Page 7): The manuscript identifies 1630 compounds using FIGAERO-CIMS and applies semi-quantitative calibration based on levoglucosan and succinic acid. However, the uncertainty associated with applying only two calibration standards across a broad chemical space (m/z 120–500) should be discussed. The authors should provide an estimate of uncertainty introduced by this calibration approach.

Lines 212–220 (Page 7): The statement that CHOX species can be regarded as oxygenated compounds because I⁻ ionization favors moderately oxygenated OA requires more careful explanation. The authors should clarify the potential bias against highly oxygenated or less oxygenated compounds introduced by the ionization scheme.

Lines 224–250 (Page 8): The DTT assay methodology is described, but the manuscript should clarify whether extraction efficiency was evaluated. Since the authors acknowledge that water-insoluble components, including EC, insoluble organics, and insoluble metals, were excluded (Lines 232–234), the potential impact of this limitation on total PM oxidative potential should be discussed more explicitly.

Lines 252–264 (Page 8): The HCA and PMF methodology requires additional statistical justification. The authors should provide sensitivity tests showing whether the six HCA clusters and selected PMF factor number remain stable under different clustering parameters or factor numbers.

Lines 317–326 (Page 10–11): The comparison among Weifang, Beijing, and Wangdu is potentially valuable, but the authors should clarify whether differences in sampling periods, meteorological conditions, and sampling locations may influence the observed OA composition differences.

Lines 429–432 (Page 14): Figure 3 contains a large number of molecular formulas, but important marker compounds are not explicitly labeled. The authors should annotate representative compounds to improve interpretation and readability.

Lines 480–485 (Page 16): The criteria for selecting high-abundance compounds (>0.05% of total CHOX signal and twice background level) should be justified. The authors should explain whether different thresholds significantly alter the HCA results.

Lines 513–518 (Page 17): The authors interpret Cluster F compounds as thermal fragments based on thermogram behavior. Since thermal decomposition may affect a broader range of FIGAERO-CIMS compounds, the authors should discuss whether other clusters may also be influenced by thermal artifacts.

Lines 815–823 (Page 28): The authors acknowledge that PMF and HCA cannot fully separate primary emissions from aging processes. This limitation should be emphasized earlier in the discussion because it directly affects the interpretation of source-specific OP contributions.

Figures 5 and 7 (Lines 570–575 and 719–722): The figures contain extensive molecular-level information but lack sufficient guidance for readers. The authors should improve figure legends by defining abbreviations, explaining color coding, and identifying representative compounds.

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Citation: https://doi.org/10.5194/egusphere-2026-4106-RC1

Jing Cai, Yongchun Liu, Feixue Zheng, Dian Ding, Wei Du, Liang Yuan, Yingjie Zou, Lazong Suolang, Rongxiang Lu, Wanwei Li, Chao Liu, Xiaolong Fan, Qiaozhi Zha, Mengxiao Luan, and Jiandong Wang

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https://doi.org/10.5194/egusphere-2026-4106-supplement

Jing Cai, Yongchun Liu, Feixue Zheng, Dian Ding, Wei Du, Liang Yuan, Yingjie Zou, Lazong Suolang, Rongxiang Lu, Wanwei Li, Chao Liu, Xiaolong Fan, Qiaozhi Zha, Mengxiao Luan, and Jiandong Wang

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Short summary

Characterizing organic aerosol (OA) molecular composition, sources and toxicity in the North China Plain remains difficult. Through comprehensive online and offline measurements in Weifang, a representative medium-sized city, we identify combustion-derived OA with a moderate oxidation state as the key drivers of oxidative potential (OP). These insights are critical for health risk assessments amidst the region’s ongoing reduction in particulate mass due to emission controls.


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Combustion-Derived Organic Aerosols Enhance PM2.5 Oxidative Potential in a Medium-Sized North China Plain City

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