Measurement report: Optical properties of carbonaceous aerosols modulated by source variations of spring haze

Carbonaceous aerosols exert unique impacts on earth energy balance, but are not well constrained in air quality and climate models yet. In this study, a field campaign was conducted in a Chinese megacity during a spring season to explore the optical properties of carbonaceous aerosols modulated by dramatic variations of particulate matter sources. We first integrated the light absorption results derived from different methods, including both on-line and off-line approaches. Then by synthesizing a series of source-relevant signatures, three types of haze episodes were identified. In general, agricultural fire emissions exerted strong influences on brown carbon (BrC), by effectively increasing the mass absorption efficiency (MAE) of bulk BrC, and emitting chromophores with a characteristic absorption peak at ∼ 365 nm. Specifically, fires with low combustion efficiencies were more capable of enhancing the impacts of BrC on bulk aerosol absorption, and were more favorable for the emissions of organic compounds with relatively high molecular weights and aromatic contents. In addition, the agricultural fires, especially those with relatively low efficiencies, resulted in overestimations of black carbon (BC) mass by thermal-optical method due to unique emitted species. BC concentrations were also overestimated for the dust episodes. By excluding the impacts from agricultural fires and dusts, we unfolded the influence of secondary aerosol formation on the MAE of BC (MAE_BC). With the enhancement of secondary aerosols, MAE_BC was found to exhibit a clear increasing trend, which could be translated into the light absorption enhancement (_Eabs) factors of up to ∼ 2.3.