Abstract
Abstract. Several recent observational studies have shown organic carbon aerosols to be a significant source of absorption of solar radiation. The absorbing part of organic aerosols is referred to as "brown" carbon (BrC). Using a global chemical transport model and a radiative transfer model, we estimate for the first time the enhanced absorption of solar radiation due to BrC in a global model. The simulated wavelength dependence of aerosol absorption, as measured by the absorption Ångström exponent (AAE), increases from 0.9 for non-absorbing organic carbon to 1.2 (1.0) for strongly (moderately) absorbing BrC. The calculated AAE for the strongly absorbing BrC agrees with AERONET spectral observations at 440–870 nm over most regions but overpredicts for the biomass burning-dominated South America and southern Africa, in which the inclusion of moderately absorbing BrC has better agreement. The resulting aerosol absorption optical depth increases by 18% (3%) at 550 nm and 56% (38%) at 380 nm for strongly (moderately) absorbing BrC. The global simulations suggest that the strongly absorbing BrC contributes up to +0.25 W m−2 or 19% of the absorption by anthropogenic aerosols, while 72% is attributed to black carbon, and 9% is due to sulfate and non-absorbing organic aerosols coated on black carbon. Like black carbon, the absorption of BrC (moderately to strongly) inserts a warming effect at the top of the atmosphere (TOA) (0.04 to 0.11 W m−2), while the effect at the surface is a reduction (−0.06 to −0.14 W m−2). Inclusion of the strongly absorption of BrC in our model causes the direct radiative forcing (global mean) of organic carbon aerosols at the TOA to change from cooling (−0.08 W m−2) to warming (+0.025 W m−2). Over source regions and above clouds, the absorption of BrC is higher and thus can play an important role in photochemistry and the hydrologic cycle.
Highlights
Methods and length dependence of aerosol absorption, as measured by the absorption Ångström exponent (AAE), increases from 0.9Absorption of solar radiatioDnabtyaaeSroysosltsehmas sa significant imfor non-absorbing organic carbon to 1.2 (1.0) for strongly pact on the atmospheric energy distribution and hydrologic absorbing brown carbon (BrC)
The global simulations suggest that the strongly cfaocneve(RctaivmeancaotuhpalninagndbeCtawEremaeincrhtthaheelS,a2ty0m0so8ts)ep. hTmehree and the surmagnitude of absorbing BrC contributes up to +0.25 W m−2 or 19 % of the atmospheric forcing induceSd cbyieanntchreopsogenic absorbthe absorption by anthropogenic aerosols, while 72 % is at- ing aerosols, mainly black carbon (BC) emitted from comtributed to black carbon, and 9 % is due to sulfate and non- bustion processes, has been suggested
Using a global chemical transfer model and a Monte Carlo radiative transport model, we examine the enhancement in atmospheric absorption due to BrC for year 2000 aerosol emissions and present the first global estimate of its direct perturbation on aerosol radiative forcing
Summary
Methods and length dependence of aerosol absorption, as measured by the absorption Ångström exponent (AAE), increases from 0.9. Inclusion of the strongly the atmospheric forcing due to GHGs, their dimming effect absorption of BrC in our model causes the direct radia- may compensate for or enhance the GHG warming at the tive forcing (global mean) of organic carbon aerosols at the surface, depending on vertical placement in the atmosphere. Model underestimation can reach factors of 2 to 5, over regions dominated by biomass burning, South and East Asia, and remote regions (Koch et al, 2009; Menon et al, 2010; Ganguly et al, 2012) Such large gaps between simulated and observed BC and aerosol absorption lead to disconcerting discrepancies in the estimated aerosol radiative forcing.
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