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 0.22 W m −2 (27%) to −0.60 W m −2 . 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. M easurements in the European Organization for NuclearResearch (CERN) Cosmics Leaving Outdoor Droplets (CLOUD) chamber under atmospheric conditions show that new particles can form purely from the oxidation products of α-pinene, a compound emitted by the biosphere (1). Nucleation of new aerosol particles via gas to particle conversion has been studied for 50 years (2) and is responsible for around one-half of global cloud condensation nuclei (CCN) (3), which affect Earth's radiation balance via aerosol-cloud interactions. The involvement of Significance A mechanism for the formation of atmospheric aerosols via the gas to particle conversion of highly oxidized organic molecules is found to be the dominant aerosol formation process in the preindustrial boundary layer over land. The inclusion of this process in a global aerosol model raises baseline preindustrial aerosol concentrations and could lead to a reduction of 27% in estimates of anthropogenic aerosol radiative forcing.

Aerosols in the Pre-industrial Atmosphere

“…4 Map of regions analysed in Table 1 and Fig. 7 not reflected in the model simulations above, is pure biogenic nucleation [100], which could increase PI CCN concentrations by 4-19% [101]. These changes are comparable to the standard deviations in CCN concentrations caused by 26 other processes and emissions so would constitute a significant change to the model.…”

“…The second major factor is the lower sink of nucleating vapours in the PI. Gordon et al [101] showed that this can allow nucleation of biogenic vapours [100] to increase substantially in the PI, providing a "nucleation buffering" mechanism that raises PI aerosol concentrations above the level that might be expected based on the generally lower emissions. A third factor is that the volatility of biogenic nucleating vapours depends on [38] which species were involved in the oxidation steps (ozone, OH, HO 2 , and NO 3 ) as well as the concentrations of NO x (ref [102]), which have been strongly affected by anthropogenic activities [103,104].…”

“…Developments in all of these model components are needed. As shown by Gordon et al [101], progress is being made in exploiting welldesigned chamber measurements to understand the mechanism of particle nucleation under PI conditions. However, a much deeper knowledge of PI gas phase and aerosol chemistry is needed to refine our understanding of these and other aerosol processes.…”

New particle formation in the sulfuric acid–dimethylamine–water system: reevaluation of CLOUD chamber measurements and comparison to an aerosol nucleation and growth model

“…Even though oxidized organics seem to be globally important for NPF (Jokinen et al, 2015;Gordon et al, 2016;, the formation of new particles by sulfuric acid and amines should still be considered because sulfuric acid-amine nucleation rates exceed those from oxidized organics as soon as the concentrations of the precursor gases (sulfuric acid and amines) are high enough . Therefore, at least locally or regionally, i.e., close to sources, amines should be relevant.…”

Seasonal Characteristics of Black Carbon Aerosol and its Potential Source Regions in Baoji, China

“…Atmospheric aerosols are a widespread concern due to their physicochemical role in atmospheric processes, which can cause them to disturb the solar radiative balance of the earth (Bellouin et al, 2005;Gordon et al, 2016). The uncertainty regarding the radiative forcing induced by atmospheric particulate matter is a major obstacle to the accurate prediction of climate change (Kahn, 2012;Lee et al, 2016).…”