Spectral absorption properties of organic carbon aerosol during a polluted winter in Beijing, China.

A fraction of organic carbon (OC) is found to exhibit the capability to absorb solar radiation. However, the absorption properties of OC remain poorly characterized partly due to uncertainties in determination methods. In this study, the absorption coefficient (b) of OC (b) in Beijing during a polluted winter was estimated on the basis of the combined measurements of black carbon (BC) size distribution and total aerosol b (b). The bare BC b (b) calculated using Mie theory on the basis of measured size distribution exhibited weak wavelength dependence, with a mean absorption Ångström exponent (AAE) of 0.56 ± 0.04 within the 470-660 nm wavelength range, which was lower than the value of 1 commonly used for freshly emitted BC. The calculated b was compared with b at 950 nm to derive the coating thickness of BC, from which the calculation of coated BC b (b) within 370-660 nm was based using the core-shell Mie model. Given the thick coatings, the AAE of coated BC, with a mean of 0.53 ± 0.12, was slightly lower than that of bare BC. Subsequently, b was obtained by subtracting b from b, accounting for 59.57 ± 4.82% of b at 370 nm on average. The average mass absorption efficiency of OC was estimated to be 1.48 ± 0.36 m g at 370 nm. b significantly decreased as wavelength increased, deriving an AAE of OC with a mean of 2.72 ± 0.32 within the 370-660 nm range. The level of b estimated on the basis of a widely used attribution method assuming a constant BC AAE of 1 was ~60% lower than the currently presented value, probably underestimating OC radiative effect by a factor of >3. More accurate estimations of b based on more advanced measurements and suitable theory calculations are recommended to provide more reliable assessments of OC radiative effects.