Mechanisms Enhancing Absorbing Aerosol Forcing from ASR’s FLAME-4 and ClearFlo Detling Campaigns

Description

Biomass and fossil fuel combustion emits black (BC) and brown carbon (BrC) aerosols that absorb sunlight to warm climate and organic carbon (OC) aerosols that scatter sunlight to cool climate. The net forcing depends strongly on the initial composition, mixing state and transformations of these carbonaceous aerosols that are uncertain. We present laboratory fuel combustion studies and wintertime field measurements in the UK to uncover fundamental mechanism that control the optical properties of carbonaceous aerosols indicating additional light absorption as elaborated below: 1. Wavelength dependence of absorption and the single scattering albedo (ω) of fresh biomass burning aerosols produced from many fuels during FLAME-4 was analysed to determine the factors that control the variability in ω. Results show that ω varies strongly with fire-integrated modified combustion efficiency (MCEFI)—higher MCEFI results in lower ω values and greater spectral dependence of ω (Liu et al GRL 2014). A parameterization of ω as a function of MCEFI for fresh BB aerosols is derived from the laboratory data and is evaluated by field data. We also demonstrate that BrC production correlates with BC indicating that that they are produced by a common mechanism that is driven by MCEFI (Saleh et al NGeo 2014) and BrC absorption is concentrated in the extremely low volatility component. 2. Theoretical modeling and laboratory experiments have demonstrated that light absorption by BC can be enhanced when BC is internally mixed with other materials. However, recent field observations in urban-influenced regions have shown negligible absorption enhancement (Cappa et al. Science 2012), even for BC-containing particles with substantial coatings. Here we report the first direct evidence of substantial absorption enhancement for BC emitted from a mixture of sources in wintertime London, UK. The absorption enhancement, 1.3-1.4 on average at 405 nm and 781 nm, strongly depends on the amount of coatings on BC. Increases in BC coating are shown to be due to a combination of changing BC sources and photochemical aging processes and Mie theory calculations constrained by SP-AMS data reproduce the enhancement. We further show that the absorption enhancement at 405 nm is affected by low-volatility BrC. We examine the impact of our observational findings on enhanced absorption by BC coatings and by BrC on radiative forcing estimates of absorbing aerosols in climate models.