Reduced anthropogenic aerosol radiative forcing caused by biogenic new particle formation

Abstract

The magnitude of aerosol radiative forcing caused by anthropogenic emissions depends on the baseline state of the atmosphere under pristine preindustrial conditions. Measurements show that particle formation in atmospheric conditions can occur solely from biogenic vapors. Here, we evaluate the potential effect of this source of particles on preindustrial cloud condensation nuclei (CCN) concentrations and aerosol-cloud radiative forcing over the industrial period. Model simulations show that the pure biogenic particle formation mechanism has a much larger relative effect on CCN concentrations in the preindustrial atmosphere than in the present atmosphere because of the lower aerosol concentrations. Consequently, preindustrial cloud albedo is increased more than under present day conditions, and therefore the cooling forcing of anthropogenic aerosols is reduced. The mechanism increases CCN concentrations by 20-100% over a large fraction of the preindustrial lower atmosphere, and the magnitude of annual global mean radiative forcing caused by changes of cloud albedo since 1750 is reduced by [Formula: see text] (27%) to [Formula: see text] Model uncertainties, relatively slow formation rates, and limited available ambient measurements make it difficult to establish the significance of a mechanism that has its dominant effect under preindustrial conditions. Our simulations predict more particle formation in the Amazon than is observed. However, the first observation of pure organic nucleation has now been reported for the free troposphere. Given the potentially significant effect on anthropogenic forcing, effort should be made to better understand such naturally driven aerosol processes.

Keywords: aerosol; biogenic; climate; forcing.

Fig. 1.

Nucleation rates at $3\text{-nm}$ diameter (${\text{J}}_3{,\hspace{0.17em}\text{centimeters}}^{-3}\hspace{0.17em}{\text{second}}^{-1}$) within approximately $500\hspace{0.17em}\text{m}$ of the surface averaged over June without pure biogenic nucleation in (A) preindustrial and (B) present day conditions and with pure biogenic nucleation in (C) preindustrial and (D) present day conditions.

Fig. 2.

Percentage of particles produced via pure biogenic (PB) nucleation within approximately $500\hspace{0.17em}\text{m}$ of the surface averaged over June in (A) preindustrial and (B) present day conditions. We note that our model predicts large changes to particle formation at the surface and very little change above the boundary layer.

Fig. 3.

Concentrations of CCN calculated at 0.2% supersaturation in ${\text{centimeters}}^{-3}$ annually averaged at cloud base level in (A) preindustrial and (B) present day conditions, including pure biogenic nucleation and (C and D) percentage changes to these concentrations when pure biogenic nucleation is introduced. In this figure, we assume HOM formation and pure biogenic nucleation proceed at the rates measured at the CLOUD chamber.

Fig. 4.

(A) Distribution of cloud albedo radiative forcing, including pure biogenic nucleation and (B) change to this distribution when pure biogenic nucleation is included in the model. EQ, Equator; GM, Greenwich Meridian; RF, radiative forcing.