Abstract
In this study we describe the hygroscopic properties of accumulation-and coarse-mode aerosol particles sampled at the Zotino Tall Tower Observatory (ZOTTO) in central Siberia (61 degrees N, 89 degrees E) from 16 to 21 June 2013. The hygroscopic growth measurements were supplemented with chemical analyses of the samples, including inorganic ions and organic/elemental carbon. In addition, the microstructure and chemical compositions of aerosol particles were analyzed by x-ray micro-spectroscopy (STXM-NEXAFS) and transmission electron microscopy (TEM). A mass closure analysis indicates that organic carbon accounted for 61 and 38% of particulate matter (PM) in the accumulation mode and coarse mode, respectively. The water-soluble fraction of organic matter was estimated to be 52 and 8% of PM in these modes. Sulfate, predominantly in the form of ammoniated sulfate, was the dominant inorganic component in both size modes: similar to 34% in the accumulation mode vs. similar to 47% in the coarse mode.
The hygroscopic growth measurements were conducted with a filter-based differential hygroscopicity analyzer (FDHA) over the range of 5-99.4% RH in the hydration and dehydration operation modes. The FDHA study indicates that both accumulation and coarse modes exhibit pronounced water uptake approximately at the same relative humidity (RH), starting at similar to 70 %, while efflorescence occurred at different humidities, i.e., at similar to 35% RH for submicron particles vs. similar to 50% RH for supermicron particles. This similar to 15% RH difference was attributed to higher content of organic material in the submicron particles, which suppresses water release in the dehydration experiments.
The kappa mass interaction model (KIM) was applied to characterize and parameterize non-ideal solution behavior and concentration-dependent water uptake by atmospheric aerosol samples in the 5-99.4% RH range. Based on KIM, the volume-based hygroscopicity parameter, kappa(v), was calculated. The kappa(v,ws) value related to the water-soluble (ws) fraction was estimated to be similar to 0.15 for the accumulation mode and similar to 0.36 for the coarse mode, respectively. The obtained kappa(v,ws) for the accumulation mode is in good agreement with earlier data reported for remote sites in the Amazon rain forest (kappa(v) approximate to 0.15) and a Colorado mountain forest (kappa(v) approximate to 0.16).
We used the Zdanovskii-Stokes-Robinson (ZSR) mixing rule to predict the chemical composition dependent hygroscopicity, kappa(v,p). The obtained kappa(v,p) values overestimate the experimental FDHA-KIM-derived kappa(v,ws) by factors of 1.8 and 1.5 for the accumulation and coarse modes, respectively. This divergence can be explained by incomplete dissolution of the hygroscopic inorganic compounds resulting from kinetic limitations due to a sparingly soluble organic coating. The TEM and STXM-NEXAFS results indicate that aged submicron (> 300 nm) and supermicron aerosol particles possess core-shell structures with an inorganic core, and are enriched in organic carbon at the mixed particle surface. The direct FDHA kinetic studies provide a bulk diffusion coefficient of water of similar to 10(-12) cm(2) s(-1) indicating a semi-solid state of the organic-rich phase leading to kinetic limitations of water uptake and release during hydration and dehydration cycles.
Overall, the present ZOTTO data set, obtained in the growing season, has revealed a strong influence of organic carbon on the hygroscopic properties of the ambient aerosols. The sparingly soluble organic coating controls hygroscopic growth, phase transitions, and microstructural rearrangement processes. The observed kinetic limitations can strongly influence the outcome of experiments performed on multi-second timescales, such as the commonly applied HTDMA (Hygroscopicity Tandem Differential Mobility Analyzer) and CCNC (Cloud Condensation Nuclei Counter) measurements.