Uncertainties in global aerosols and climate effects due to biofuel emissions

Abstract

Abstract. Aerosol emissions from biofuel combustion impact both health and climate; however, while reducing emissions through improvements to combustion technologies will improve health, the net effect on climate is largely unconstrained. In this study, we examine sensitivities in global aerosol concentration, direct radiative climate effect, and cloud-albedo aerosol indirect climate effect to uncertainties in biofuel emission factors, optical mixing state, and model nucleation and background secondary organic aerosol (SOA). We use the Goddard Earth Observing System global chemical-transport model (GEOS-Chem) with TwO Moment Aerosol Sectional (TOMAS) microphysics. The emission factors include amount, composition, size, and hygroscopicity, as well as optical mixing-state properties. We also evaluate emissions from domestic coal use, which is not biofuel but is also frequently emitted from homes. We estimate the direct radiative effect assuming different mixing states (homogeneous, core-shell, and external) with and without absorptive organic aerosol (brown carbon). We find the global-mean direct radiative effect of biofuel emissions ranges from −0.02 to +0.06 W m−2 across all simulation/mixing-state combinations with regional effects in source regions ranging from −0.2 to +0.8 W m−2. The global-mean cloud-albedo aerosol indirect effect (AIE) ranges from +0.01 to −0.02 W m−2 with regional effects in source regions ranging from −1.0 to −0.05 W m−2. The direct radiative effect is strongly dependent on uncertainties in emissions mass, composition, emissions aerosol size distributions, and assumed optical mixing state, while the indirect effect is dependent on the emissions mass, emissions aerosol size distribution, and the choice of model nucleation and secondary organic aerosol schemes. The sign and magnitude of these effects have a strong regional dependence. We conclude that the climate effects of biofuel aerosols are largely unconstrained, and the overall sign of the aerosol effects is unclear due to uncertainties in model inputs. This uncertainty limits our ability to introduce mitigation strategies aimed at reducing biofuel black carbon emissions in order to counter warming effects from greenhouse gases. To better understand the climate impact of particle emissions from biofuel combustion, we recommend field/laboratory measurements to narrow constraints on (1) emissions mass, (2) emission size distribution, (3) mixing state, and (4) ratio of black carbon to organic aerosol.

The impact of residential combustion emissions on atmospheric aerosol, human health, and climate

“…Downwind of source regions, N 3 concentrations are reduced by up to ∼ 400 cm −3 . This reduction is caused by primary particles acting as a coagulation sink for nucleated particles and a condensation sink for nucleating and condensing vapours, suppressing new particle formation (Spracklen et al, 2006), which is broadly consistent with the findings of Kodros et al (2015) for particle number concentrations due to the effect of biofuel emissions.…”

“…The review of Bond et al (2013) identified a net negative RE (DRE and AIE) for biofuel with large uncertainty but a slight net positive RE (with low certainty) from residential coal (Bond et al, 2013). However, a recent detailed global modelling study found that the climate effects of residential biofuel combustion aerosol are largely unconstrained because of uncertainties in emission mass flux, emitted size distribution, optical mixing state, and ratio of BC to POM (Kodros et al, 2015) In addition to impacting climate, aerosol from residential fuel combustion degrades air quality with adverse implications for human health. Epidemiologic research has confirmed a strong link between exposure to particulate matter (PM) and adverse health effects, including premature mortality (Pope III and Dockery, 2006;Brook et al, 2010).…”

“…al., 2011a; Kodros et al, 2015). Aerosols can further alter cloud properties through the second aerosol indirect effect and through semi-direct effects (Koch and Del Genio, 2010).…”

“…Coefficients for absorption and scattering, and asymmetry parameters, are then obtained from look-up tables containing all realistic combinations of refractive index and Mie parameter (particle radius normalised to the wavelength of radiation), as described by Bellouin et al (2013). The assumption that BC is internally or homogeneously mixed with scattering species is unrealistic, providing an upper bound for DRE (Jacobson, 2001;Kodros et al, 2015).…”

“…Modelling studies have shown the importance of carbonaceous combustion aerosols to global CCN concentrations (Pierce et al, 2007;Spracklen et al, 2011a) and modification of cloud properties Jacobson, 2010). However, there is considerable variability in the size of particles emitted by combustion sources including those from residential sources (Venkataraman and Rao, 2001;Shen et al, 2010;Pagels et al, 2013;Bond et al, 2006) that will impact simulated CCN concentrations (Pierce et al, 2007; Reddington et al, 2011;Spracklen et al, 2011a;Kodros et al, 2015) and AIE …”