Biomass burning (BB) contributes large amounts of black carbon (BC) and particulate organic matter (POM) to the atmosphere and contributes significantly to the earth's radiation balance. BB particles can be a complicated optical system, with scattering and absorption contributions from BC, internal mixtures of BC and POM, and wavelength-dependent absorption of POM. Large amounts of POM can also be externally mixed. We report on the unique ability of multi-wavelength photo-acoustic measurements of dry and thermal-denuded absorption to deconstruct this complicated wavelength-dependent system of absorption and mixing. Optical measurements of BB particles from the Four Mile Canyon fire near Boulder, Colorado, showed that internal mixtures of BC and POM enhanced absorption by up to 70%. The data supports the assumption that the POM was very weakly absorbing at 532 nm. Enhanced absorption at 404 nm was in excess of 200% above BC absorption and varied as POM mass changed, indicative of absorbing POM. Absorption by internal mixing of BC and POM contributed 19ðAE8Þ% to total 404-nm absorption, while BC alone contributed 54ðAE16Þ%. Approximately 83% of POM mass was externally mixed, the absorption of which contributed 27ðAE15Þ% to total particle absorption (at 404 nm). The imaginary refractive index and mass absorption efficiency (MAE) of POM at 404 nm changed throughout the sampling period and were found to be 0.007 AE 0.005 and 0.82 AE 0.43 m 2 g −1 , respectively. Our analysis shows that the MAE of POM can be biased high by up to 50% if absorption from internal mixing of POM and BC is not included.forest fire | climate | tar balls P article emissions from biomass burning (BB) are a significant component of global combustion-sourced black carbon (BC) and primary particulate organic matter (POM), contributing approximately 63% and 94%, respectively (1). The radiative impact of BB emissions at regional and global scales are significant (2) and can create instantaneous top-of-atmosphere cooling or warming (up to AE10 s of Wm −2 ), depending on surface albedo (3, 4). BB emissions from northern Eurasia and North America are often efficiently transported into the Arctic (5, 6) and contribute to climate change in that region (7), including snow and ice melt following BC deposition (8).The radiative impacts of BB particle emissions are very different than those of particles emitted from relatively efficient (i.e., internal) fossil fuel (FF) combustion. Co-emission of strongly absorbing BC and non-or mildly absorbing POM contrast to the predominantly absorbing BC emissions of FF combustion (2, 9) and can lead to a dominance of light scattering from the POM.The co-emission of BC and POM can additionally lead to internal mixing, which can enhance BC absorption by serving as a radiation lens. Such lensing may increase absorption by a factor of two (10-12). BC particles can be associated with significant amounts of non-BC material in the atmosphere and enhancement of ambient BC absorption is expected (13-15). Studies of direct and i...

Abstract. The absorption Ångström exponent (AAE) of externally mixed black carbon (BC Ext ), or BC internally mixed with non-absorbing material (BC Int ), is often used to determine the contribution of brown carbon (BrC) light absorption at short visible wavelengths. This attribution method contains assumptions with uncertainties that have not been formally assessed. We show that the potential range of AAE for BC Ext (or BC Int ) in the atmosphere can reasonably lead to +7 % to −22 % uncertainty in BC Ext (or BC Int ) absorption at short wavelengths derived from measurements made at longer wavelengths, where BrC is assumed not to absorb light. These uncertainties propagate to errors in the attributed absorption of BrC. For uncertainty in attributed BrC absorption to be ≤ ± 33 %, 23 % to 41 % of total absorption must be sourced from BrC. These uncertainties would be larger if absorption by dust were also to be considered due to additional AAE assumptions. For data collected during a biomass-burning event, the mean difference between measured and AAE attributed BrC absorption was found to be 34 % -an additional uncertainty in addition to the theoretical uncertainties presented. In light of the potential for introducing significant and poorly constrained errors, we caution against the universal application of the AAE method for attributing BrC absorption.

This paper describes the design and performance of a photoacoustic aerosol absorption spectrometer (PAS) built for operation on a research aircraft platform. The PAS instrument is capable of measuring dry absorption at 659 nm, 532 nm, and 404 nm, and absorption enhancement due to coatings at 532 nm and 404 nm. The measurement accuracy for all channels is < = 10% and inflight 1 Hz sensitivities lie within the range of 0.5-1.5 Mm −1 . PAS measurements of calibrated absorbing aerosol samples are shown to be consistent with measurements made by a previous generation single channel photo-acoustic instrument. Aircraft data collected during a recent field campaign in California are used to demonstrate the capabilities of the PAS. In combination with an aircraft cavity ring down aerosol extinction spectrometer described in a companion paper, the new PAS instrument provides a sensitive airborne in-situ characterization of aerosol optics.