Abstract
Abstract. We investigate the CCN activity of freshly emitted biomass burning particles and their hygroscopic growth at a relative humidity (RH) of 85%. The particles were produced in the Mainz combustion laboratory by controlled burning of various wood types. The water uptake at sub- and supersaturations is parameterized by the hygroscopicity parameter, κ (c.f. Petters and Kreidenweis, 2007). For the wood burns, κ is low, generally around 0.06. The main emphasis of this study is a comparison of κ derived from measurements at sub- and supersaturated conditions (κG and κCCN), in order to see whether the water uptake at 85% RH can predict the CCN properties of the biomass burning particles. Differences in κGand κCCN can arise through solution non-idealities, the presence of slightly soluble or surface active compounds, or non-spherical particle shape. We find that κG and κCCN agree within experimental uncertainties (of around 30%) for particle sizes of 100 and 150 nm; only for 50 nm particles is κCCN larger than κG by a factor of 2. The magnitude of this difference and its dependence on particle size is consistent with the presence of surface active organic compounds. These compounds mainly facilitate the CCN activation of small particles, which form the most concentrated solution droplets at the point of activation. The 50 nm particles, however, are only activated at supersaturations higher than 1% and are therefore of minor importance as CCN in ambient clouds. By comparison with the actual chemical composition of the biomass burning particles, we estimate that the hygroscopicity of the water-soluble organic carbon (WSOC) fraction can be represented by a κWSOC value of approximately 0.2. The effective hygroscopicity of a typical wood burning particle can therefore be represented by a linear mixture of an inorganic component with κ ≅ 0.6, a WSOC component with κ ≅ 0.2, and an insoluble component with κ = 0.
Highlights
Both, CCN properties and hygroscopic growth of aerosols are determined by the amount and nature of the soluble and insoluble materials contained in the particles
The κ values of freshly emitted biomass burning particles are small, ranging between 0.02 and 0.15. This means that the water uptake of the biomass particles is equivalent to particles consisting of 5–20 % ammonium sulfate and otherwise insoluble material
The fraction of inorganic ions in impactor samples of the biomass burning particles is much smaller than 5–20 %, indicating that the water uptake is mainly governed by organic material
Summary
CCN properties and hygroscopic growth of aerosols are determined by the amount and nature of the soluble and insoluble materials contained in the particles. It is reasonable to assume a relationship between the hygroscopic growth at subsaturated conditions and the critical supersaturation of droplet activation. There has been one study comparing hygroscopic growth and CCN activation of biomass burning aerosol (Petters et al, 2009a). These closure studies have had various degrees of success, but in principle it can be concluded that predicting CCN properties based on hygroscopic growth factors is successful as predicting CCN properties based on chemical information
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