Optical properties and cloud condensation nuclei activity of brown carbon containing α–dicarbonyls and reduced nitrogen compounds

The aqueous–phase reaction of carbonyls with reduced nitrogen compounds (RNCs) is considered an important pathway for the formation of secondary BrC, and these particles could greatly impact the global climate. However, our knowledge of their properties is still limited. In this study, we measured the optical properties, cloud condensation nuclei (CCN) activity (represented as hygroscopicity parameter κCCN), and effective density (ρeff) under varied reaction conditions for BrC mixtures formed by reactions between two α–dicarbonyls (glyoxal GX and methylglyoxal MG) and three RNCs (ammonium sulfate AS, glycine GC, and dimethylamine DA). Our results showed a wide range of particle ρeff from 1.08 to 1.66 g cm−3, depending on the type of RNCs reactant. At 532 nm wavelength, the real part n of complex refractive index ranged from 1.41 to 1.60 and the imaginary part k of BrC ranged from 0.024 to 0.043, and the absorption cross section for MG mixtures were 20.3–51.0% higher than those of GX mixtures. These values did not appear to vary significantly with mixing molar ratios between RNCs and α–dicarbonyls (10:1, 1:1, and 1:3) for mixtures with the same reactants, but the mass absorption cross section for mixtures after a reaction time of 14 days were 1.57–74.2% higher than those of mixtures after reaction times of 0.5 and 7 days. The κCCN values of different BrC were consistent with the order of κCCN for single RNCs, i.e., AS–containing (0.349–0.428) > GC–containing (0.221–0.352) > DA–containing (0.090–0.131), and showed an overall increasing trend (1.47–59.3%) with the RNCs content. Our results highlight the importance of reaction conditions on secondary BrC properties and are helpful for a more accurate assessment of aerosol radiative forcing.