| Popis: |
Aerosols in the atmosphere are chemically complex with thousands or more chemical species distributed in different proportions across individual particles in an aerosol population. An internal mixing assumption, with species present in the same proportions across all aerosols, is used in many models and calculations of secondary organic aerosol (SOA) formation, cloud activation, and aerosol optical properties. However, many of these effects depend on the distribution of species within individual particles, and important information can be lost when internal mixtures are assumed. Herein, we show that during the Southern Oxidant and Aerosol Study (SOAS) in Centreville, Alabama, at a rural, forested location, that aerosols frequently are not purely internally mixed, even in the accumulation mode (0.2–1.0 µm). A range of aerosol sources and mixing states were obtained using computer controlled scanning electron microscopy with energy dispersive X-ray spectroscopy (CCSEM-EDX) and scanning transmission X-ray microscopy-near-edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS). Particles that were dominated by SOA and inorganic salts were the majority of particles by number fraction from 0.2–5 microns with an average of 78 % SOA in the accumulation mode. However, during certain periods contributions by sea spray aerosol (SSA) and mineral dust were significant to accumulation (22 % SSA and 26 % dust) and coarse mode number concentrations (38 % SSA and 63 % dust). The fraction of particles containing key elements (Na, Mg, K, Ca, and Fe) were determined as a function of size for specific classes of particles. Within internally mixed SOA/sulfate particles |
