Articles | Volume 16, issue 19
Atmos. Chem. Phys., 16, 12733–12752, 2016
Atmos. Chem. Phys., 16, 12733–12752, 2016

Research article 13 Oct 2016

Research article | 13 Oct 2016

Deriving brown carbon from multiwavelength absorption measurements: method and application to AERONET and Aethalometer observations

Xuan Wang1, Colette L. Heald1,2, Arthur J. Sedlacek3, Suzane S. de Sá4, Scot T. Martin4, M. Lizabeth Alexander5, Thomas B. Watson3, Allison C. Aiken6, Stephen R. Springston3, and Paulo Artaxo7 Xuan Wang et al.
  • 1Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA
  • 2Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
  • 3Environmental and Climate Sciences Department, Brookhaven National Laboratory, Upton, NY, USA
  • 4School of Engineering and Applied Science, Harvard University, Cambridge, MA, USA
  • 5Pacific Northwest National Laboratory, Richard, WA, USA
  • 6Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM, USA
  • 7Institute of Physics, University of São Paulo, São Paulo, Brazil

Abstract. The radiative impact of organic aerosols (OA) is a large source of uncertainty in estimating the global direct radiative effect (DRE) of aerosols. This radiative impact includes not only light scattering but also light absorption from a subclass of OA referred to as brown carbon (BrC). However, the absorption properties of BrC are poorly understood, leading to large uncertainties in modeling studies. To obtain observational constraints from measurements, a simple absorption Ångström exponent (AAE) method is often used to separate the contribution of BrC absorption from that of black carbon (BC). However, this attribution method is based on assumptions regarding the spectral dependence of BC that are often violated in the ambient atmosphere. Here we develop a new AAE method which improves upon previous approaches by using the information from the wavelength-dependent measurements themselves and by allowing for an atmospherically relevant range of BC properties, rather than fixing these at a single assumed value. We note that constraints on BC optical properties and mixing state would help further improve this method. We apply this method to multiwavelength absorption aerosol optical depth (AAOD) measurements at AERONET sites worldwide and surface aerosol absorption measurements at multiple ambient sites. We estimate that BrC globally contributes up to 40 % of the seasonally averaged absorption at 440 nm. We find that the mass absorption coefficient of OA (OA-MAC) is positively correlated with the BC ∕ OA mass ratio. Based on the variability in BC properties and BC ∕ OA emission ratio, we estimate a range of 0.05–1.5 m2 g−1 for OA-MAC at 440 nm. Using the combination of AERONET and OMI UV absorption observations we estimate that the AAE388∕440 nm for BrC is generally  ∼ 4 worldwide, with a smaller value in Europe (< 2). Our analyses of observations at two surface sites (Cape Cod, to the southeast of Boston, and the GoAmazon2014/5 T3 site, to the west of Manaus, Brazil) reveal no significant relationship between BrC absorptivity and photochemical aging in urban-influenced conditions. However, the absorption of BrC measured during the biomass burning season near Manaus is found to decrease with photochemical aging with a lifetime of  ∼ 1 day. This lifetime is comparable to previous observations within a biomass burning plume but much slower than estimated from laboratory studies. Given the large uncertainties associated with AERONET retrievals of AAOD, the most challenging aspect of our analysis is that an accurate, globally distributed, multiple-wavelength aerosol absorption measurement dataset is unavailable at present. Thus, achieving a better understanding of the properties, evolution, and impacts of global BrC will rely on the future deployment of accurate multiple-wavelength absorption measurements to which AAE methods, such as the approach developed here, can be applied.

Short summary
We describe a new approach to estimate the absorption of brown carbon (BrC) from multiple-wavelength absorption measurements. By applying this method to column and surface observations globally, we find that BrC contributes up to 40 % of the absorption measured at 440 nm. The analysis of two surface sites also suggests that BrC absorptivity decreases with photochemical aging in biomass burning plumes, but not in typical urban conditions.
Final-revised paper