The diverse chemical mixing state of aerosol particles in the southeastern United States

Aerosols in the atmosphere are chemically complex with thousands of 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 - as found during the Southern Oxidant and Aerosol Study (SOAS) in Centreville, Alabama, at a rural, forested location - aerosols frequently are not purely internally mixed, even in the accumulation mode (0.2-1.0 mu m). A range of aerosol sources and the mixing state were determined 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 (e.g., ammonium sulfate) were the majority of particles by number fraction from 0.2 to 5 mu m 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 <5% contained Na, Mg, K, Ca, or Fe, though these nonvolatile cations were present in particles from the other sources (e.g., SSA and dust). Mass estimates of the aerosol elemental components were used to determine the extent of internal versus external mixing by calculating the mixing state index (chi). The aerosol population was more externally mixed than internally mixed during all time periods analyzed. Accumulation mode aerosol ranged from more internally mixed during SOA periods to mostly externally mixed during dust periods. Supermicron aerosols were most externally mixed during SOA time periods, when more SOA particles added a distinct supermicron class, and more internally mixed when dominated by a single particle type (e.g., SSA or dust). These results emphasize that neither external nor internal mixtures fully represent the mixing state of atmospheric aerosols, even in a rural, forested environment, which has important implications for air quality and climate modeling.