The direct and indirect radiative effects of biogenic secondary organic aerosol

C E Scott,K S Carslaw,P Tunved,H Lihavainen,N Kivekäs,P M Forster,D V Spracklen,M Kulmala,A Rap,K J Pringle,G W Mann

doi:10.5194/acp-14-447-2014

Abstract

Abstract. We use a global aerosol microphysics model in combination with an offline radiative transfer model to quantify the radiative effect of biogenic secondary organic aerosol (SOA) in the present-day atmosphere. Through its role in particle growth and ageing, the presence of biogenic SOA increases the global annual mean concentration of cloud condensation nuclei (CCN; at 0.2% supersaturation) by 3.6–21.1%, depending upon the yield of SOA production from biogenic volatile organic compounds (BVOCs), and the nature and treatment of concurrent primary carbonaceous emissions. This increase in CCN causes a rise in global annual mean cloud droplet number concentration (CDNC) of 1.9–5.2%, and a global mean first aerosol indirect effect (AIE) of between +0.01 W m−2 and −0.12 W m−2. The radiative impact of biogenic SOA is far greater when biogenic oxidation products also contribute to the very early stages of new particle formation; using two organically mediated mechanisms for new particle formation, we simulate global annual mean first AIEs of −0.22 W m−2 and −0.77 W m−2. The inclusion of biogenic SOA substantially improves the simulated seasonal cycle in the concentration of CCN-sized particles observed at three forested sites. The best correlation is found when the organically mediated nucleation mechanisms are applied, suggesting that the first AIE of biogenic SOA could be as large as −0.77 W m−2. The radiative impact of SOA is sensitive to the presence of anthropogenic emissions. Lower background aerosol concentrations simulated with anthropogenic emissions from 1750 give rise to a greater fractional CCN increase and a more substantial first AIE from biogenic SOA. Consequently, the anthropogenic indirect radiative forcing between 1750 and the present day is sensitive to assumptions about the amount and role of biogenic SOA. We also calculate an annual global mean direct radiative effect of between −0.08 W m−2 and −0.78 W m−2 in the present day, with uncertainty in the amount of SOA produced from the oxidation of BVOCs accounting for most of this range.

Highlights

Vegetation emits biogenic volatile organic compounds (BVOCs), such as monoterpenes (C10H16) and isoprene (C5H8), into the atmosphere (Guenther et al, 1995)
secondary organic aerosol (SOA) increases the condensation sink, potentially suppressing new particle formation and growth. This results in the global annual mean total particle (N3; greater than 3 nm dry diameter) concentration being reduced by 7.9 % when monoterpene emissions are included with activation boundary layer nucleation and by 0.4 % when binary homogeneous nucleation (BHN) is the only new particle formation mechanism
We used a global aerosol microphysics model (GLOMAP) and an offline radiative transfer model to quantify the impact of biogenic SOA on cloud condensation nuclei (CCN) concentrations and cloud droplet number concentration (CDNC), and the subsequent radiative implications

Summary

Introduction

Vegetation emits biogenic volatile organic compounds (BVOCs), such as monoterpenes (C10H16) and isoprene (C5H8), into the atmosphere (Guenther et al, 1995). BVOCs oxidise to yield a range of lower volatility oxidation products that can partition into the aerosol phase, forming secondary organic aerosol (SOA) (Kavouras et al, 1998; O’Dowd et al, 2002; Kanakidou et al, 2005; Jimenez et al, 2009). The oxidation products of monoterpenes and isoprene have been observed in both ambient aerosol Scott et al.: The direct and indirect radiative effects of biogenic SOA been found to contain an aerosol mass that is proportional to the length of time the air has spent over forested land (Tunved et al, 2006, 2008), suggesting an important contribution to aerosol from BVOC emissions

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