10th International Aerosol Conference
September 2 - September 7, 2018
America's Center Convention Complex
St. Louis, Missouri, USA

Abstract View


Estimation of Brown Carbon in PM2.5 Samples from Long-term Networks

XIAOLIANG WANG, Judith Chow, Brandon Daub, Steven Gronstal, L.W. Antony Chen, Mark Green, John Watson, Desert Research Institute

     Abstract Number: 184
     Working Group: Carbonaceous Aerosol

Abstract
Black carbon (BC) and brown carbon (BrC) aerosols are light-absorbing substances that affect the Earth’s radiative balance. While BC absorbs light across the visible spectrum, BrC primarily absorbs light at wavelengths less than 500 nm. The emission sources and climate effects of BC are widely studied and relatively well understood. In contrast, despite increasing interest in BrC aerosols, there is a large degree of uncertainty about their abundance, source attribution, and environmental/climate impacts, partially owing to a lack of BrC quantification at many different times and locations.
Since 2016, BrC is being measured in U.S. chemical speciation networks using a thermal/optical carbon analyzer with seven wavelengths ranging from 405 to 980 nm. Outputs include estimates of aerosol absorption and charring corrections at each wavelength. A power-law fit through the light attenuation spectra yields the Absorption Ångström Exponent (AAE). BrC light absorption is estimated by subtracting the BC contribution with an AAE of 1 for each of the shorter wavelengths. This study evaluates the multiwavelength thermal/optical carbon measurements from the U.S. Interagency Monitoring of PROtected Visual Environments (IMPROVE) and Chemical Speciation Network (CSN) (~300 sites) for 2016 (~30,000 samples) to better understand the abundance and temporal variation of carbonaceous aerosols in the U.S. It is shown that the analyses maintain continuity with the long-term OC and EC databases for these networks. On average, non-urban IMPROVE samples show higher fractional BrC absorption than urban CSN samples, owing to greater influence from biomass burning and aged aerosols, as well as to higher primary BC contributions from engine exhaust at urban sites. The AAEs are higher at the IMPROVE than at the CSN sites. Sequential samples taken during an Everglades National Park wildfire illustrate the evolution from flaming to smoldering, with the BrC fraction increasing as smoldering begins to dominate the fire event. Characterization of light absorption properties of laboratory-generated BrC surrogate compounds demonstrate the feasibility of using the surrogate compounds’ mass absorption efficiency to convert BrC light attenuation to BrC mass contributions to the carbonaceous fraction of PM2.5.