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

The study addresses the sources and formation mechanisms of aqueous secondary organic aerosol (aqSOA), a major component of oxygenated organic aerosol that influences air quality and climate. While models and experiments increasingly point to aqueous-phase reactions in cloud droplets and wet aerosols as an important SOA pathway, the extent to which aqSOA is driven by anthropogenic (fossil) versus biogenic emissions remains unclear. Prior modeling often assumes aqSOA is mostly biogenic (e.g., from isoprene oxidation), yet field observations in East Asia suggest substantial fossil contributions to water-soluble organic aerosol (WSOC). This work applies compound-specific dual-carbon isotopic analysis (δ13C and Δ14C) of key aqSOA tracers (notably oxalic acid and related acids) to quantify precursor sources and to distinguish aqueous-phase processing from other pathways. The research tests the hypothesis that fossil-derived precursors significantly contribute to aqSOA formation in China, particularly under conditions of high aerosol liquid water driven by anthropogenic inorganic salts and humid meteorological regimes.

Previous studies have highlighted that global models underpredict SOA, with aqueous-phase chemistry identified as a missing pathway. aqSOA formation involves interactions between anthropogenic and biogenic emissions, but the relative contributions are poorly constrained. Modeling has emphasized biogenic precursors (e.g., isoprene) for aqSOA, assuming fossil precursors are less hydrophilic. However, recent East Asian observations indicate rapid aqueous oxidation of fossil-derived PAHs during winter haze and radiocarbon evidence of ~50% fossil WSOC in East Asia outflow, in contrast to <30% in Europe, the U.S., and South Asia. Prior field and laboratory work in other regions often suggested that WSOC has limited fossil contribution and that fossil-derived OA is more water-insoluble. These disparate findings motivate molecular-level isotopic source apportionment to directly trace aqSOA sources and pathways.

Field sampling and site characterization: Year-round PM2.5 aerosol samples (n=32; 48 h each at 1 m3/min) were collected June 2017–May 2018 at the Heshan Atmospheric Environmental Monitoring Superstation (rural receptor 50 km SW of Guangzhou; 22.711°N, 112.927°E, 60 m asl). Back trajectories (HYSPLIT, 3-day, 100 m, every 12 h) and meteorology identified two regimes: coastal background (South China Sea air; n=11) and continental outflow (Pearl River Delta-influenced, anthropogenic; n=21). Additional PM2.5 samples were collected in five megacities (Beijing, Shanghai, Guangzhou, Chengdu, Wuhan) for one week in winter (Jan 2018) and one week in summer (Jul 2018); the seven daily filters were combined per season. Chemical analyses: Concentrations of diacids, oxoacids, and α-dicarbonyls were measured following established protocols. Filter aliquots were extracted with Milli-Q water, concentrated, derivatized with 10% BF3 in 1-butanol at 100 °C for ~1 h, extracted with n-hexane, and quantified by GC-MS (Agilent 7890A/5975C). Carbonaceous fractions (OC, EC, WSOC), water-soluble inorganic constituents (Na+, NH4+, K+, Mg2+, Ca2+, Cl−, NO3−, SO42−), biogenic SOA tracers, and sugars were analyzed per Supplementary Method 1. Aerosol liquid water (ALW): Inorganic ALW was estimated using ISORROPIA-II with measured inorganic ions and meteorology (T, RH). Organic ALW was calculated via the Zdanovskii-Stokes-Robinson mixing rule. Internal mixing of particles was assumed. Isotope analyses: Compound-specific δ13C of derivatized diacids and related compounds were determined by GC-IRMS (Thermo GC IsoLink/IRMS); δ13C of free acids was back-calculated via isotopic mass balance using the δ13C of 1-butanol (−30.21‰). Replicate precision was generally <1‰ (diacids) and <2‰ (Pyr, ωC2). For Δ14C, microgram quantities of C2, C3, C4, ωC2, and MeGly were isolated via preparative capillary GC across ~50 injections, combusted (920 °C), reduced to graphite, and measured by AMS (0.5 MV). Results reported as fraction modern (Fm) normalized to δ13C=−25‰, corrected for bomb 14C (factor 1.06), derivatization carbon (butanol Fm=0.0029±0.001), and procedural blanks. WSOC δ13C (EA-IRMS) and Δ14C (AMS) were measured after ultrasonication extraction; method validity was verified against soaking extraction. Data analysis: Samples were grouped by air mass regime. Relationships among oxalic acid, ALW, WSOC, and precursors (Gly, MeGly) were examined. Δ14C-derived fractions of fossil vs non-fossil sources were computed, including partitioning of oxalic acid into fossil, biomass burning (bb), and biogenic components using levoglucosan and nss-K as tracers. Ratios such as C6/C9 were used to infer source influence. Dual-isotope (δ13C–Δ14C) patterns were evaluated seasonally and spatially across cities.

- Air mass regimes: Continental outflow samples had 2–5× higher gas-phase pollutants and major PM2.5 components than coastal background; OM, anthropogenic WSIC, and ALW were ~3×, 5×, and 3× higher, respectively. Nitrate was ~20× higher and drove ALW increases. - Aqueous processing indicators: C3/C4 (malonic/succinic) ratios were higher in coastal background (1.5±0.4) than continental outflow (0.9±0.3), indicating more photochemical aging in the coastal background. Despite this, WSOC/OC ratios were similar between regimes (44±12% vs 39±10%), implicating additional formation pathways (aqSOA). - Precursor availability: Continental outflow had sixfold higher (Gly+MeGly) mass fractions in OC than coastal background, consistent with enhanced aqSOA formation. - δ13C evidence of aqueous formation: Oxalic acid δ13C averaged −24.6±2.7‰ (continental outflow) and −19.9±2.3‰ (coastal background), with significant correlation between oxalic acid and ALW (r2=0.78, P<0.001) and between δ13C-oxalic acid and WSOC (negative trend; r=0.70, P<0.001), indicating gas-to-liquid transfer and aqueous-phase processing. Oxalic acid correlated with glyoxylic acid (r=0.65, P<0.001). δ13C of intermediates (ωC2, Pyr) were significantly lower during continental outflow, supporting an aqueous pathway AVOCs/BVOCs → SVOCs/WSOC → Pyr → ωC2 → oxalic acid. - Δ14C source apportionment: In coastal background, oxalic acid was 67±9% non-fossil (fossil 33±9%, range 19–48%). In continental outflow, fossil fraction was 55±10% (range 42–78%), exceeding non-fossil contribution. A strong relationship existed between C6/C9 ratio and f_fossil of oxalic acid (r=0.63, P<0.001). Anthropogenic contributors (traffic, plastic burning; abundant terephthalic acid ~5% of total diacids) were implicated. - Fossil dominance in aqSOA precursors: In continental outflow, fossil fractions were oxalic acid 55±10%, glyoxylic acid 69±4%, and methylglyoxal 67±5%. Higher diacids had lower fossil fractions (C3: 31±4%; C4: 36±2%), indicating C3/C4 mainly from biogenic fatty acid breakdown, while C2 formed strongly from fossil-derived aqueous processing. - Partitioning of oxalic acid sources: Using levoglucosan and nss-K constraints, anthropogenic oxalic acid (fossil + bb) accounted for 78% in continental outflow vs 39% in coastal background. Source breakdown: coastal background ≈ biogenic 61%, fossil 33%, biomass 6%; continental outflow ≈ fossil 55%, biomass 23%, biogenic 22%. - Seasonal and spatial consistency: Across five megacities, in winter oxalic acid was more depleted in both 13C and 14C than bulk WSOC (consistent with fossil-dominated aqSOA under high ALW). In summer, oxalic acid tended to be more enriched in 13C and 14C than WSOC in several cities, consistent with greater biogenic influence and oxidative aging of WSOC to small acids. - Atmospheric implications: Enhanced nitrate-driven ALW promotes uptake of soluble oxidation products, increasing aqSOA mass. Oxalic acid contributes measurably to organic ALW (3–38% of organic-contributed ALW; mean 10%). Dual-isotope constraints challenge the paradigm that aqSOA is mostly biogenic, revealing substantial fossil-derived aqSOA in China.

The dual carbon isotope approach demonstrates that aqueous-phase chemistry substantially converts fossil-derived precursors into water-soluble organic aerosol, especially under continental outflow conditions with high nitrate and ALW. The δ13C patterns (depletion with increasing WSOC and ALW) and strong Δ14C fossil fractions for oxalic acid and its aqueous intermediates provide robust evidence for an anthropogenic, fossil-driven aqSOA pathway. This pathway coexists with biogenic and biomass sources but can dominate aqSOA mass under polluted, humid conditions, altering aerosol hygroscopicity and potential CCN activity. The findings reconcile high WSOC levels and model underestimation by identifying ALW-limited aqueous processing as a key driver. Spatial and seasonal analyses across Chinese megacities align with the Heshan results: in winter, fossil-derived aqSOA is prominent; in summer, enhanced biogenic contributions and oxidative aging shift isotopic signatures. The positive feedback among inorganic salts, ALW, and aqSOA suggests that anthropogenic emissions (NOx, SO2, VOCs) influence both inorganic and organic aerosol water, amplifying aqSOA formation and its climate-relevant properties.

Compound-specific dual-carbon isotope evidence reveals that fossil anthropogenic precursors contribute over half of aqSOA in China, particularly oxalic acid and its aqueous precursors (glyoxylic acid, methylglyoxal). Aqueous-phase processing, facilitated by anthropogenic nitrate-driven ALW, is a major formation route for water-soluble organic aerosols in polluted, humid environments. These results challenge the prevailing assumption of predominantly biogenic aqSOA and imply that models must better represent aqueous processing of fossil-derived VOC oxidation products. The study underscores the need for comprehensive control strategies targeting NOx, SO2, and VOCs to mitigate both inorganic and organic aerosol burdens. Future research should extend dual-isotope analyses to additional aqSOA constituents (e.g., glyoxal), further resolve source isotopic signatures, and quantify the impacts of energy transitions (e.g., increased combustion-derived water vapor) on ALW and aqSOA formation under changing climate and emission scenarios.

- Attribution complexity: δ13C signatures reflect both sources and processing; while kinetic isotope effects were inferred to dominate, source isotopic variability (e.g., differing AVOC/BVOC δ13C) could also contribute and was not fully constrained. - Pathway separation: Differentiating aqueous-phase formation from gas-phase photochemical aging and biomass burning contributions remains challenging in field conditions, especially during intensive burning or strong atmospheric aging. - Seasonal and biosource variability: Changes in C3/C4 vegetation and seasonality may affect isotopic baselines, complicating interpretation. - Measurement pooling: Monthly pooling for several compounds (owing to low mass) may mask short-term variability. - Generalizability: While extended to several megacities, broader global applicability requires additional multi-region studies with similar molecular-level isotope resolution.