Water uptake by biomass burning aerosol at sub- and supersaturated conditions: closure studies and implications for the role of organics

摘要

We investigate the CCN activity of freshly emitted biomass burning particlesand their hygroscopic growth at a relative humidity (RH) of 85%. Theparticles were produced in the Mainz combustion laboratory by controlledburning of various wood types. The water uptake at sub- and supersaturationsis parameterized by the hygroscopicity parameter, κ (c.f. Pettersand Kreidenweis, 2007). For the wood burns, κ is low,generally around 0.06. The main emphasis of this study is a comparison ofκ derived from measurements at sub- and supersaturated conditions(κ_G and κ_CCN), in order to see whether the wateruptake at 85% RH can predict the CCN properties of the biomass burningparticles. Differences in κ_Gand κ_CCN can arisethrough solution non-idealities, the presence of slightly soluble or surfaceactive compounds, or non-spherical particle shape. We find that κ_G and κ_CCN agree within experimental uncertainties (ofaround 30%) for particle sizes of 100 and 150 nm; only for 50 nmparticles is κ_CCN larger than κ_G by a factor of 2.The magnitude of this difference and its dependence on particle size isconsistent with the presence of surface active organic compounds. Thesecompounds mainly facilitate the CCN activation of small particles, whichform the most concentrated solution droplets at the point of activation. The50 nm particles, however, are only activated at supersaturations higher than1% and are therefore of minor importance as CCN in ambient clouds. Bycomparison with the actual chemical composition of the biomass burningparticles, we estimate that the hygroscopicity of the water-soluble organiccarbon (WSOC) fraction can be represented by a κ_WSOC value ofapproximately 0.2. The effective hygroscopicity of a typical wood burningparticle can therefore be represented by a linear mixture of an inorganiccomponent with κ ≅ 0.6, a WSOC component with κ ≅ 0.2, and an insoluble component with κ = 0.