Determination of external and internal mixing of organic and inorganic aerosol components from equilibrium water uptake by sub-micrometer particles.

The ability of a particle to gain and lose water with changes in relative humidity is fundamental to particle's effectiveness as a cloud condensation nucleus, chemical reactivity, atmospheric residence time and influence on global radiation balance. We describe a method developed to measure particle hygroscopicity over a range of relative humidities (RH) from 50% to 85%. Ambient aerosol particles were dried, monodisperse particles with diameters of usually 50 and 114 nm were selected, and their size distribution following humidification was measured. We measured particle hygroscopicity at Golden Ears Provincial Park and Eagle Ridge Mountain as part of the Pacific 2001 field study in the lower Fraser Valley in August of 2001. The humidified size distributions were sometimes monomodal and sometimes bimodal distribution with less and more hygroscopic peaks. The hygroscopicity of the monomodal particles varied between that of the less and more hygroscopic particles. The less hygroscopic particles were probably almost entirely organic in composition; they had consistent growth curves with wet/dry diameter ratios that increased from 1.04 at 50% relative humidity to 1.09 at 80% RH. These less hygroscopic particles constituted almost all the sampled aerosol at the forested site in Golden Ears Park and during the rainy periods at Eagle Ridge. At other times there were more hygroscopic particles, either as a single mixed mode or as a distinct mode in addition to the less hygroscopic particles. These showed little growth below 70% RH and pronounced growth above 70%. The increased water sorption above 70% RH is likely due to the particles containing (NH₄)₃H(SO₄)₂ or (NH₄)₂(SO₄), as these salts deliquesce at 70% and 80% RH, respectively. Since the growth of these particles was less than expected for the pure salts, we conclude that these particles consisted of a mixture of the organic and inorganic components. An estimate of the relative organic fraction was made using the observed growth factors for mixed and organic particles and calculated growth factors for pure inorganic particles. The results indicate that the sampled particles contained a significant organic fraction. 50% of the more hygroscopic particles had organic fractions higher than 0.63, this value was 0.84 for monomodal particles. Bimodal distributions of the humidified particles clearly demonstrated external mixing of the aerosol. In addition, the width of the humidified distribution (spread factor) was also used as an indicator of varying composition. The most common result following humidification of particles was a single mode which frequently exhibited significant spread when humidified above 70% RH. The least amount of spread was observed for particles measured at Golden Ears Forest. At Eagle Ridge, lower spread factors were observed for particles with growth factors at the high or low end of the values observed. The highest spread factors were observed for particles with mid-range growth factors. These results indicate that while particles at Golden Ears Forest were largely single component particles, at Eagle Ridge particle had two sources whose hygroscopicity was approximated by the upper and lower growth factor values observed for the site.