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

Journal Article · Thu Oct 13 00:00:00 EDT 2016 · Atmospheric Chemistry and Physics

DOI:https://doi.org/10.5194/acp-16-12733-2016· OSTI ID:1329806

Wang, X. ^[1]; Heald, C. L. ^[1]; Sedlacek, A. ^[2]; de Sa, S. S. ^[3]; Martin, S. T. ^[3]; Alexander, M. L. ^[4]; Watson, T. B. ^[2]; Aiken, A. C. ^[5]; Springston, S. R. ^[2]; Artaxo, P. ^[6]

---

1. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
2. Brookhaven National Lab. (BNL), Upton, NY (United States)
3. Harvard Univ., Cambridge, MA (United States)
4. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
5. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
6. Univ. of Sao Paulo (Brazil)

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 AAE_{388/440 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.

• Research Organization: Brookhaven National Laboratory (BNL), Upton, NY (United States); Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Biological and Environmental Research (BER); USDOE Office of Science (SC). Biological and Environmental Research (BER) (SC-23)
• Grant/Contract Number: SC00112704; AC52-06NA25396
• OSTI ID: 1329806
• Alternate ID(s): OSTI ID: 1369212
• Report Number(s): BNL-112751-2016-JA; LA-UR-17-24402; R&D Project: 2016-BNL-EE630EECA-Budg; KP1701000
• Journal Information: Atmospheric Chemistry and Physics, Vol. 16, Issue 19; ISSN 1680-7316
• Publisher: European Geosciences Union
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
Earth Sciences
aerosol
black carbon
brown carbon
angstrom exponent
ambient aerosol
mass absorption coefficient
absorbing aerosol
GoAmazon
Atmospheric Radiation Measurement Facility