Brown carbon: a significant atmospheric absorber of solar radiation?

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.

Observations of fine particulate nitrated phenols in four sites in northern China: concentrations, source apportionment, and secondary formation

“…Due to their strong absorption in the near-ultraviolet and visible regions, nitrated phenols are classified as poorly characterized brown carbon (BrC) (Desyaterik et al, 2013;Teich et al, 2017). Though the absorption of BrC is weak compared to that of black carbon (BC), it can enhance the absorption of solar radiation and may have an indirect effect on regional climate (Feng et al, 2013;Mohr et al, 2013;Laskin et al, 2015;Lu et al, 2015;Zhao et al, 2015). Recent studies have shown that several semivolatile nitrated phenols, including nitrophenol, nitrocatechol, and methyl-nitrocatechol (NC), are important photochemical oxidation products of gas-phase precursors, which may play a part in the formation of secondary organic…”

Aerosol optical properties and trace gas emissions by PAX and OP-FTIR for laboratory-simulated western US wildfires during FIREX

“…Despite the large BB emissions of greenhouse gases and BC, it has been assumed that BB OA contributes to negative radiative forcing by BB overall. However, the overall BB forcing could be positive if the emitted weakly-absorbing OA known as brown carbon (BrC) is sufficiently absorbing and long lived 20 (Feng et al, 2013;Jacobsen, 2014;Saleh et al, 2014;Forrister et al, 2015). This could generate a positive feedback with the expected increase in BB due to a warming climate (Feng et al, 2013;Doerr and Santin, 2016;Bowman et al, 2017).…”

“…However, the overall BB forcing could be positive if the emitted weakly-absorbing OA known as brown carbon (BrC) is sufficiently absorbing and long lived 20 (Feng et al, 2013;Jacobsen, 2014;Saleh et al, 2014;Forrister et al, 2015). This could generate a positive feedback with the expected increase in BB due to a warming climate (Feng et al, 2013;Doerr and Santin, 2016;Bowman et al, 2017). Thus, comprehensive understanding of wildfire contributions to air quality and climate requires further evaluation.…”

“…Critically though, we do report the OA absorption due to BrC at 401 nm, a poorly characterized term that needs to be improved in climate models to better estimate the radiative forcing of BB aerosol (Feng et al, 2013). The mass ratio of BC to the simultaneous co-located CO 2 , measured by the FTIR, was multiplied by EF for CO 2 to determine mass EFs for BC (g kg -1 ).…”

Evidence for pyrazine-based chromophores in cloudwater mimics containing methylglyoxal and ammonium sulfate

“…Light absorbing organic carbon in atmospheric aerosol (brown carbon or BrC) has been shown to impact radiative forcing with an estimated contribution of 19% to total aerosol absorption globally (Feng et al, 2013). AERONET measurements over California have shown that in the brown carbon region (440 nm), brown carbon absorption is 40% of that attributed to elemental carbon (Bahadur et al, 2012).…”