More water-soluble brown carbon after the residential “coal-to-gas” conversion measure in urban Beijing

Haze pollution with high PM₂.₅ is a major environmental and health issue in North China, especially in winter heating seasons. Multiple policies (e.g., 2013 Action Plan, 2016 Law, traffic and industrial controls) lowered annual PM₂.₅ in North China from 86 μg m⁻³ (2013) to 37 μg m⁻³ (2021), yet winter haze persists. Residential coal combustion has been identified as a key wintertime source of organic aerosol and BrC. A regional residential “coal-to-gas” conversion replaced coal with gas for heating/cooking by end of 2018, reducing Beijing’s coal for heating by >70% (3.47 Mt in 2015 to 0.97 Mt in 2019) and winter PM₂.₅ (84.0 to 47.9 μg m⁻³). Because BrC’s light absorption depends on chemical composition and residential coal is a major BrC source, the study asks: how did the “coal-to-gas” conversion alter the chemical composition of water-soluble (HULIS-BrC) and water-insoluble (WI-BrC) brown carbon in Beijing’s PM₂.₅? Using HPLC-PDA and high-resolution Orbitrap MS (ESI−/ESI+) on 2015 vs 2019 winter samples, the work quantifies changes in number, intensity, and oxidation state of chromophores by elemental classes (CHO, CHN, CHON, S-containing).

Sampling: Daily PM₂.₅ high-volume filter samples were collected on a rooftop (~20 m AGL; 39.99°N, 116.32°E) at the National Center for Nanoscience and Technology, Beijing, from 17 Jan to 15 Mar in 2015 (pre-conversion) and 2019 (post-conversion). Quartz-fiber filters (Whatman QM-A, 20.3 × 25.4 cm) were prebaked (780 °C, 3 h). Flow: 1.05 m³ min⁻¹. Samples stored at −20 °C. Fractionation and extraction: For chemical characterization, combined filters from six high light-absorption samples (Abs365 above campaign average) were used per period. Water-soluble fraction extracted twice with 5 mL ultrapure water (18.2 MΩ cm) in ultrasonic bath (30 min each), filtered (PVDF 0.45 μm), and processed to isolate HULIS per Lin et al. The residual filters were dried and extracted twice with 5 mL methanol (HPLC grade) in ultrasonic bath (30 min each), filtered (PTFE), yielding water-insoluble OC (WISOC). HULIS and WISOC concentrates were reduced to 300 μL for analysis. Instrumentation: HPLC with photodiode array (PDA) coupled to high-resolution Orbitrap mass spectrometer operated in ESI− and ESI+ modes (HPLC-PDA-HRMS). PDA used to identify chromophores; HRMS for elemental formula assignment. Data processing: Raw data in Thermo Xcalibur 4.0; peak detection and chromatogram construction via MZmine 2.52. Molecular formula constraints: C1–30H0–60O0–15N0–3S0–1Na0–1; mass tolerance ±3 ppm (ESI+) and ±2 ppm (ESI−). Chemical plausibility filters: 0.3 ≤ H/C ≤ 3.0, O/C ≤ 3.0, N/C ≤ 0.5, S/C ≤ 0.2; nitrogen rule enforced; non-integer or negative DBE excluded. Chromophore candidates required DBE/C ≥ 0.5. DBE computed as (2C + 2 − H + N)/2. Aromaticity equivalent Xc computed as Xc = [3(DBE − (mO + nS)) − 2]/[DBE − (mO + nS)], with m = n = 0.5 in ESI− and m = n = 1 in ESI+. Quantification: Peak intensity used as a proxy for relative abundance. Results blank-corrected. Note: Ionization/transmission efficiency differs by compound class; the study assumes uniform response across chromophores for comparisons. Contextual air quality metrics: Post-conversion (2019) vs pre (2015) winter Beijing averages: PM₂.₅ 52 vs 84 μg m⁻³, NO₂ 38 vs 52 μg m⁻³, CO 0.8 vs 1.4 mg m⁻³; O₃ 67 vs 60 μg m⁻³ (source: local environmental authority).

- Total chromophores: HULIS-BrC increased from 9708 (2015) to 11,032 (2019) (~14%); WI-BrC (WISOC) decreased from 11,150 to 10,239 (~8%). - Category-level changes (HULIS-BrC): Numbers rose for CHON (~14%), CHO (~15%), CHN (~7%), S-containing (~3%); intensities rose strongly: CHON +80%, CHO +81%, CHN +24%, S-containing +51%. - Category-level changes (WI-BrC): Numbers declined: CHON −8%, CHO −6%, CHN −12%, CH −9%, S-containing −13%; intensities declined: CHON −34%, CHO −35%, CHN −44%, CH −52%, S-containing −19%. - Oxidation state trends: • HULIS CHO−: O/Cavg increased from 0.36 to 0.39; majority of chromophores had higher intensity in 2019, with O/C increasing with intensity ratio (2019/2015). Only 5 CHO− present only in 2015 (avg O/C 0.18), whereas 229 appeared only in 2019 (avg O/C 0.34). • WI-BrC CHO−: O/Cavg slightly increased 0.29 to 0.31; most chromophores decreased in intensity; extra-only 2015 chromophores (n=97) had low O/C (avg 0.16); extra-only 2019 chromophores were few (n=15) and more oxygenated (avg O/C 0.44). • CHON (HULIS): Average formulas became more oxygen- and nitrogen-rich post-conversion; many new CHON− chromophores in 2019 (n=302, >50% with O/N ≥ 3) vs few lost (n=12, 75% with O/N < 3), indicating increased nitroaromatics/organonitrates (secondary origin). MFavg (HULIS): CHON− C7.67H7.21O3.50N1.11 → C7.62H7.06O3.59N1.18; CHON+ C10.21H9.96O1.95N1.30 → C10.13H10.12O2.24N1.38. • CHON (WI-BrC): Many less-oxidized CHON decreased; O/N=2 dominated; most had O/N < 3 both years (57%→53%). MFavg (WI-BrC): CHON− C8.80H8.81O3.47N1.10 → C8.56H8.33O3.62N1.21; CHON+ C13.06H12.14O1.78N1.35 → C12.52H12.12O1.95N1.44. - CHN+ chromophores: Increased in HULIS-BrC (603→643 formulas; intensity generally higher in 2019), decreased in WI-BrC (1811→1587; intensity generally higher in 2015). HULIS CHN+ mostly polycyclic aromatics with C < 15 and DBE 5–12 (consistent with alkaloids); WI-BrC CHN+ included larger fused aromatics (C > 15, DBE 12–24), consistent with N-heterocyclic fused structures. - S-containing chromophores (CHOS, CHONS): More prevalent in HULIS than WI; intensities higher in 2019 (total CHOS intensity ~7.0×10⁷ vs 5.5×10⁷ in 2015). CHOS largely aliphatic with O/S 4–11 (organosulfates). CHONS included subsets with O ≥ 7 (≈24–26% in HULIS; ≈20–21% in WI), suggesting nitrooxy-organosulfates formed under high-NOx photochemistry. - CH+ (PAH-like) chromophores in WI-BrC: 564 (2015) → 512 (2019); >95% decreased in intensity after conversion, indicating reduced combustion emissions. - Mode-specific patterns: HULIS-BrC showed more CHON in ESI+ (number) and more CHO in ESI− (number), with intensities higher in ESI−; WI-BrC had higher numbers and intensities in ESI+ than ESI− overall. - Source/process interpretation: Post-conversion, water-soluble, more oxygenated BrC increased (consistent with secondary formation under maintained or higher oxidant levels, e.g., O₃), while water-insoluble, less oxygenated, combustion-related BrC decreased (consistent with reduced residential coal emissions).

The residential “coal-to-gas” conversion led to contrasting shifts in BrC composition: decreased numbers and intensities of water-insoluble, less oxygenated chromophores (often associated with primary combustion, including PAH-like CH+ and low-oxidation CHON/CHN species) and increased numbers and intensities of water-soluble, more oxygenated chromophores (CHO and CHON) indicative of enhanced secondary production (e.g., nitroaromatics, organonitrates, organosulfates). These compositional changes likely stem from both reduced primary emissions from residential coal and changes in atmospheric oxidative capacity (e.g., similar or higher O₃). The findings imply that emission control policies can shift BrC from less soluble, strongly absorbing WI components toward more soluble, oxygenated HULIS, potentially altering optical properties, atmospheric processing (e.g., aqueous-phase reactions), mixing state with black carbon, particle size distribution, and radiative effects. Understanding these shifts is crucial for accurate estimation of BrC’s radiative forcing and for optimizing air quality and climate co-benefits of energy transitions.

This study provides molecular-level evidence that Beijing’s residential “coal-to-gas” conversion decreased water-insoluble, low-oxygenated BrC chromophores and increased water-soluble, highly oxygenated BrC chromophores in wintertime PM₂.₅. The dominant increases in HULIS CHO/CHON species and declines in WI-BrC PAH-like and low-oxidation N-containing species indicate reduced primary coal-related BrC and relatively enhanced secondary BrC contributions. These results highlight the complex impacts of emission controls on BrC chemistry and the need to consider shifts in solubility, oxidation state, and source contributions when assessing radiative forcing. Future research should investigate: (i) impacts on BC–BrC mixing state and resultant absorption enhancement; (ii) roles of aqueous-phase processing and water content in sustaining secondary BrC; (iii) size-resolved compositional changes and optical consequences; and (iv) source-resolved laboratory and field studies to disentangle residential coal vs secondary BrC contributions across seasons and regions.

- Analytical assumption: Comparisons of chromophore abundances rely on HRMS peak intensities, assuming uniform ionization and transmission response across compounds, despite known class-dependent sensitivities; this introduces uncertainty in inter-class intensity comparisons. - Sample selection: Only six high light-absorption (Abs365 above average) samples per period were combined for detailed molecular analysis, which may bias results toward more chromophoric compositions and limit representativeness. - Temporal and seasonal scope: Winter heating period (Jan–Mar) only, in a single urban Beijing site; findings may not generalize to other seasons or locations. - Formula assignment constraints: Elemental formula assignment within specified bounds (C/H/O/N/S/Na) and mass tolerances can misassign isomers/isobars; DBE and aromaticity metrics rely on assumptions (m, n values for π-bond participation). - Mode dependence: ESI−/ESI+ ionization efficiencies differ, affecting detectability and apparent composition between modes.