O/C and OM/OC Ratios of Primary, Secondary, and Ambient Organic Aerosols with High-Resolution Time-of-Flight Aerosol Mass Spectrometry

TLDR

O/C ratios indicate the oxidation state of organic aerosols, with ambient urban values ranging 0.2–0.8 that vary diurnally and increase with photochemical processing, while regional averages approach 0.9. The study improves a rapid HR‑ToF‑AMS method for elemental composition of bulk organic aerosols and applies it to ambient measurements, also presenting an approximation to predict O/C from unit‑mass‑resolution data. The authors use an enhanced HR‑ToF‑AMS technique combined with positive matrix factorization to quantify elemental ratios and identify distinct OA factors, and provide an approximation for predicting O/C from unit‑mass‑resolution data. The study finds that ambient OA exhibits H/C and N/C ratios that rise with primary emissions, OM/OC correlates tightly with O/C (R² = 0.997), and aged oxygenated OA shows the highest O/C (1.0) and OM/OC (2.5) values, surpassing chamber SOA, while primary biomass‑burning OA remains at O/C ≈ 0.3–0.4 and hydrocarbon‑like OA has the lowest O/C (0.06–0.10).

Abstract

A recently developed method to rapidly quantify the elemental composition of bulk organic aerosols (OA) using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) is improved and applied to ambient measurements. Atomic oxygen-to-carbon (O/C) ratios characterize the oxidation state of OA, and O/C from ambient urban OA ranges from 0.2 to 0.8 with a diurnal cycle that decreases with primary emissions and increases because of photochemical processing and secondary OA (SOA) production. Regional O/C approaches ∼0.9. The hydrogen-to-carbon (H/C, 1.4–1.9) urban diurnal profile increases with primary OA (POA) as does the nitrogen-to-carbon (N/C, ∼0.02). Ambient organic-mass-to-organic-carbon ratios (OM/OC) are directly quantified and correlate well with O/C (R2 = 0.997) for ambient OA because of low N/C. Ambient O/C and OM/OC have values consistent with those recently reported from other techniques. Positive matrix factorization applied to ambient OA identifies factors with distinct O/C and OM/OC trends. The highest O/C and OM/OC (1.0 and 2.5, respectively) are observed for aged ambient oxygenated OA, significantly exceeding values for traditional chamber SOA, while laboratory-produced primary biomass burning OA (BBOA) is similar to ambient BBOA, O/C of 0.3–0.4. Hydrocarbon-like OA (HOA), a surrogate for urban combustion POA, has the lowest O/C (0.06–0.10), similar to vehicle exhaust. An approximation for predicting O/C from unit mass resolution data is also presented.