Influence of organic aerosol composition determined by offline FIGAERO-CIMS on particle absorptive properties in autumn Beijing

Abstract

Abstract. Organic aerosol (OA) is a major component of fine particulate matter (PM) affecting air quality, human health, and the climate. The absorptive and reflective behavior of OA components contributes to determining particle optical properties and thus their effects on the radiative budget of the troposphere. There is limited knowledge on the influence of the molecular composition of OA on particle optical properties in the polluted urban environment. In this study, we characterized the molecular composition of oxygenated OA collected on filter samples in autumn of 2018 in Beijing, China, with a filter inlet for gases and aerosols coupled to a high-resolution time-of-flight chemical ionization mass spectrometer (FIGAERO-CIMS). Three haze episodes occurred during our sampling period with daily maximum concentrations of OA of 50, 30, and 55 µg m−3, respectively. We found that the signal intensities of dicarboxylic acids and sulfur-containing compounds increased during the two more intense haze episodes, while the relative contributions of wood-burning markers and other aromatic compounds were enhanced during the cleaner periods. We further assessed the optical properties of oxygenated OA components by combining the detailed chemical composition measurements with collocated particle light absorption measurements. We show that light-absorption enhancement (Eabs) of black carbon (BC) was mostly related to more oxygenated OA (e.g. dicarboxylic acids), likely formed in aqueous-phase reactions during the intense haze periods with higher relative humidity, and speculate that they might contribute to lensing effects. Aromatics and nitro-aromatics (e.g. nitrocatechol and its derivatives) were mostly related to a high light absorption coefficient (babs) consistent with light-absorbing (brown) carbon (BrC). Our results provide information on oxygenated OA components at the molecular level associated with BrC and BC particle light-absorption and can serve as a basis for further studies on the effects of anthropogenic OA on radiative forcing in the urban environment.

Precursors and Pathways Leading to Enhanced Secondary Organic Aerosol Formation during Severe Haze Episodes

“…Figure shows the relative ion signals grouped by their molecular formulae as well as their carbon and oxygen numbers. A study that was conducted in winter in 2018 in Beijing showed similar molecule compositions except that the relative signal intensities of sulfur-containing ions were smaller in our study (Figure S11 in the Supporting Information) . C x H y O z ·I – ions were the most abundant for all cases, contributing to 60–70% of the total signal intensities in haze-day samples and about 84% in clean-day samples (Figure a–c).…”

“…The campaign-average mass ratio of nitrate to sulfate was 1.6, which was higher than 0.6–1.4 reported previously for winter Beijing . An even greater ratio of 2.1 was observed in winter in 2018 in Beijing . The increasing nitrate-to-sulfate ratio was consistent with the significant reduction of sulfate and less efficient reduction of nitrate in the NCP …”

“…3 An even greater ratio of 2.1 was observed in winter in 2018 in Beijing. 35 The increasing nitrate-to-sulfate ratio was consistent with the significant reduction of sulfate and less efficient reduction of nitrate in the NCP. 36 The PMF analysis identified six OA factors for PM 2.5 , including four factors related to primary OA (POA) and two OOA factors.…”

“…A study that was conducted in winter in 2018 in Beijing showed similar molecule compositions except that the relative signal intensities of sulfur-containing ions were smaller in our study (Figure S11 in the Supporting Information). 35 C x H y O z •I − ions were the most abundant for all cases, contributing to 60−70% of the total signal intensities in haze-day samples and about 84% in clean-day samples (Figure 2a−c). Over 80% of the C x H y O z •I − signal intensities were contributed by compounds with carbon numbers of less than 10 (Figure 2d−f).…”

“…Nitrogen-containing ions (C x H y O z N 1–2 ·I – ) accounted for 15–35% of the total ion intensities and their relative signal contributions were much greater in haze-day samples than in clean-day samples (Figure a–c). These compounds might be nitrated phenols (−NO 2 ), ONs such as nonperoxy (−ONO 2 ), peroxy (−OONO 2 ), and peroxyacyl (−(O)­OONO 2 ) nitrates, or nitrogen-containing products from the carbonyl and ammonia (or amine) reactions (e.g., −CON−). ,,,, C 5–7 H y O 3–4 N 1–2 ·I – was the major group of detected nitrogen-containing ions (Figure g–i), which was similar to the findings of the later study in 2018 in Beijing . In this group, major ions such as C 6 H 5 NO 3 , C 7 H 7 NO 3 , C 6 H 5 NO 4 , and C 7 H 7 NO 4 were likely to be contributed predominantly by nitrated phenols.…”