Sensitivity and Contribution of Organic Aerosols to Aerosol Optical Properties Based on Their Refractive Index and Hygroscopicity

Abstract

Organic carbon (OC) accounts for a large fraction of particulate matter. Since many atmospheric organic compounds have different optical properties, it is difficult to determine the optical properties of OC accurately. In particular, hygroscopicity and light absorption of OC are important factors in understanding the aerosol optical properties. In this study, the sensitivity of organic carbon (OC) to aerosol optical properties was tested. Both the refractive index and the hygroscopicity of OC were considered. Based on the filter-based monthly averaged sampling measurement data from an intensive observation site in Seoul, Korea, the contribution of each component on the aerosol optical properties was estimated. The aerosol optical properties were simulated by combining the aerosol dynamic model for polydispersed aerosols with an optical properties model based on Mie code. The optical properties were compared with the AERONET Aerosol Optical Thickness (AOT) measurement data. In order to estimate the contribution of the light absorption and hygroscopicity of organic carbon (OC) on the optical properties of the aerosols, a sensitivity test was conducted with different imaginary refractive indices and OC hygroscopic growth factors. The results show that mass absorption efficiency can be fitted linearly as the imaginary refractive index increases. This means that one can estimate the mass absorption efficiency of OC as a function of the imaginary refractive index. The results also show that mass extinction and absorption efficiency decrease as the hygroscopic factor of OC increases because of the increase in water content. The contribution of OC to the mass extinction efficiency, however, depends on the chemical composition of other aerosol mixtures and hence, more comprehensive studies are required in this regard.

Source-Based Size-Resolved Optical Properties of Carbonaceous Aerosols

“…The aerosol extinction coefficient (b ext ) can be calculated using the Mie theory based on the mass concentration, chemical composition, and size distribution [7,14]. Figure A2 shows the composition-based extinction coefficients (b ext ) of carbonaceous aerosols with different geometric mean diameters (d g0 ) and imaginary refractive indices (IRIs) of HULIS of 0.006 and 0.3.…”

“…In general, humic-like substances are weak absorbers. However, the results of several studies have showed that the imaginary refractive index of absorbing aerosols, such as humic-like substances and brown carbon, is much higher and its contribution to absorption may therefore be important [5][6][7][8][9][10]. Receptor models are used to infer source contributions by determining the linear combination of emission source chemical compositions that best fits the chemical composition of ambient samples [11].…”

“…For example, optical properties, such as radiative forcing, have been characterized using direct measurements [12,13]. Chemical composition-based optical properties are also important for understanding the contribution of aerosol species and their effects on climate [7,14,15]. In addition, source-based aerosol optical properties can provide information about their effects on the climate [16].…”

Estimation of Source-Based Aerosol Optical Properties for Polydisperse Aerosols from Receptor Models

“…Nine sources from both transported and local sources were identified: (1) transported secondary sources (TS), (2) local secondary sulfates (LS), (3) local secondary nitrates (LN), (4) gasoline (G), (5) diesel (D), (6) transported biomass burning (TBB), (7) local biomass burning (LBB), (8) fugitive soil dust (FSD), and (9) waste burning (WB). TS is closely related to NH 4 + , NO 3 − , SO 4 2− , and OM; LS to NH 4 + and SO 4 2− ; LN to NH 4 + , NO 3 − ; G to OM (water-soluble) and EC; D to OM (water-insoluble) and…”

“…In this study, we assumed OM to be a non-absorbing compound. However, some OM (e.g., HULIS) is known to have weakly to moderate light-absorbing ability depending on its characteristics [8,9]. In order to identify the effects of the source-resolved MEEs on HULIS light-absorbing property, HULIS imaginary refractive index was tested.…”