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 the 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. 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 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.
Highlights
Organic aerosol (OA) makes up a large fraction of submicron aerosol particles globally (Jimenez et al, 2009)
OA can act as an effective shell of internally mixed black carbon (BC) particles that focuses photons onto the BC core, which leads to so-called light absorption enhancement (Eabs) of BC particles (Xie et al, 2019a, b; Zhang et al, 2018; Liu et al, 2015; Wang et al, 2018)
We presented a series of OA compounds that have the potential to influence OA light absorption in two ways in Beijing: (1) during humid haze, more oxygenated OA, with compounds such as dicarboxylic acids likely formed in aqueous-phase reactions, has the potential to strongly increase the absorption by BC due to the lensing effect; (2) during haze dominated by fresh biomass burning emissions, compounds with a high number of double-bond equivalents (DBEs) and low O numbers, such as aromatics and N-containing aromatics, can act as brown carbon and potentially lead to more absorption at shorter wavelengths
Summary
Organic aerosol (OA) makes up a large fraction of submicron aerosol particles globally (Jimenez et al, 2009). OA can act as an effective shell of internally mixed black carbon (BC) particles that focuses photons onto the BC core (named the “lensing effect”; Jacobson, 2001), which leads to so-called light absorption enhancement (Eabs) of BC particles (Xie et al, 2019a, b; Zhang et al, 2018; Liu et al, 2015; Wang et al, 2018). For all these optical effects, the chemical composition of OA plays a role (Zhang et al, 2011; Fleming et al, 2020; Laskin et al, 2015); OA light absorption can not be fully quantified based on bulk concentrations only. To better understand the impact of OA composition on particle optical properties and to estimate effects on radiative forcing on both regional and global scales, detailed OA chemical composition and BrC–BC optical measurements need to be combined
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
