Comparison of BC and BrC Absorption from AERONET and In Situ Apportionment at Wintertime Fresno
The significant radiative impacts of black carbon (BC) and brown carbon (BrC) have been increasingly recognized but remain highly uncertain. The Aerosol Robotic Network (AERONET) provides measurements of aerosol absorption optical depths (AAOD) and other parameters which could be used to estimate BC and BrC absorption across its global network, but the uncertainty of the apportionment method is not well understood. Surface measurements of aerosol optical properties were collected from January 13 to February 10, 2013, and from December 25, 2014, to January 13, 2015, at Fresno, California. This work compares the BC and BrC fractions from AERONET measurements apportioned by the absorption angstrom exponent (AAE) method and from the in situ measurements apportioned by the mass absorption coefficients (MAC) and thermodenuder (TD) method and scaled to planetary boundary layer height (PBLH). During the 2013 campaign, the AERONET apportionment shows that BC contributes 67% and BrC contributes 33% of the absorption at 440 nm while the in situ apportionment shows that BC contributes 89% and BrC contributes 11% of the absorption at 405 nm. During the 2014 campaign, the fraction is 72% BC and 28% BrC from the AERONET apportionment, and 68% BC and 32% BrC from the in situ apportionment. The comparisons show stronger correlations between AERONET and in situ measurements in 2014. These results show that the estimates of BC and BrC absorption from AERONET measurements using the AERONET-AAE method have good agreement with in situ estimates when the surface measurements are representative of the column properties.