Improved estimation of PM2.5 brown carbon contributions to filter light attenuation

Abstract

Multiwavelength light attenuation measurements have been acquired as part of thermal/optical carbon analysis in the U.S. Chemical Speciation Network (CSN) and the Interagency Monitoring of PROtected Visual Environments (IMPROVE) network beginning in 2016. These are used to estimate PM2.5 brown carbon (BrC) contributions to light absorption at various wavelengths, a useful method for separating biomass burning contributions from other sources. Attenuation of light transmitted through the filter deviates from Beers Law as the mass of light absorbing materials increase. This study estimates the effects of these deviations with empirical adjustment factors applied to samples for CSN from 2016 to 2017 and for IMPROVE from 2016 to 2019. Accounting for the filter loading effect results in an annual average increase of ∼6–7% BrC contribution to light attenuation: from 3.6% to 10.7% for the urban, more heavily loaded CSN samples; and from 23.7% to 29.5% for the non-urban IMPROVE samples. An alternative method is examined for BrC and black carbon (BC) adjustments by calculating the Absorption Ångström Exponent (AAE) for BC (i.e., AAEBC) based on the ratios of 635nm/780nm light attenuation rather than assuming AAEBC of unity. These paired-wavelength calculations result in a median AAEBC of 0.76 for CSN and 0.8 for IMPROVE, with the majority of samples (i.e., 91% of CSN and 70% of IMPROVE) showing AAEBC<1. By assuming negligible contributions from BrC to AAE at longer wavelengths, the amount of light attenuation at shorter wavelengths (e.g., 405nm) where BrC is dominant can be calculated. The paired-wavelength method applied to the filter loading adjusted data has a greater effect on urban (fresh) than on non-urban (aged) aerosols, resulting in a factor of two increase in annual averaged BrC light attenuation (from 10.7% to 21.6%) for CSN and by a factor of 1.11 (from 29.5% to 32.7%) for IMPROVE samples. This result demonstrates the importance of particle loading and AAE correction on quantifying BrC light attenuation from multi-wavelength thermal/optical analysis.