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

Abstract

Abstract. 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 µm). 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 µm 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 (χ). 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.

A simplified parameterization of isoprene-epoxydiol-derived secondary organic aerosol (IEPOX-SOA) for global chemistry and climate models

“…Sa is the wet aerosol surface area on which IEPOX can be taken up (cm 2 cm -3 ), ra is the wet aerosol radius (cm), Dg is gas-phase diffusion coefficient of IEPOX (cm 2 s -1 ), and vmms is the mean molecular speed (cm s -1 ) of gas-phase IEPOX. 145 Second, we calculate the submicron aerosol pH without sea salt based on the results from previous studies (Noble and Prather, 1996;Middlebrook et al, 2003;Hatch et al, 2011;Allen et al, 2015;Guo et al, 2016;Bondy et al, 2018;Murphy et al, 2018), which showed that sea salt aerosols were dominantly externally mixed with sulfate-nitrate-ammonium rather than internally mixed. Therefore, sea salt is not expected to impact submicron aerosol pH significantly in the real atmosphere.…”

Aerosol Mixing State: Measurements, Modeling, and Impacts

“…The relative abundance of particles from different primary sources that are aged by different secondary processes also changes, both temporally and with respect to particle size. The variability within the atmospheric aerosol of composition at the single‐particle level has been documented by field measurements globally for decades (Ault et al, ; Bondy et al, ; Casuccio et al, ; Dall'Osto et al, ; Gard et al, ; Middlebrook et al, ; Murphy et al, ; Pratt & Prather, ; Reinard et al, ; Sullivan et al, ) but effectively accounting for that complexity within global models remains challenging (Bauer et al, ; Fierce et al, ).…”

“…According to his definition “internal mixing” is present when the population of particles within an aerosol consist of the same mixture of chemical species, while “external mixing” is present when all particles in an aerosol consist of pure chemical species and have distinct compositions. In reality purely internal or external aerosol mixing states are rare in the atmosphere (Bondy et al, ; Healy et al, ; O'Brien et al, ). Winkler () went on to point out that the “same net composition of an aerosol can be caused by an infinite variety of different internal distributions of the various compounds.” Note that the term “mixing state,” as originally defined, refers to a property of the overall particle population within an aerosol, not to the property of an individual particle.…”

Structural changes of CAST soot during a thermal–optical measurement protocol

“…Depending on its origin carbonaceous material ranges from agglomerations of primary spherical particles with a graphitic-like internal structure to non-ordered organic material. While primary carbonaceous particles produced under well-defined combustion conditions are rather homogeneous, real-atmosphere carbonaceous particles are internal mixtures of different carbonaceous and non-carbonaceous materials of various origins (e.g., Okada and Hitzenberger, 2001;Zhang et al, 2014;Ye et al, 2018;IPCC, 2013;Deboudt et al, 2010;Pratt and Prather, 2010;Bondy et al, 2018;Adachi et al, 2010;Adachi and Buseck, 2013;China et al, 2013;Cappa et al, 2012). As carbonaceous aerosol material plays such an important role and is so diverse, the correct determination of carbonaceous fractions in the aerosol is essential.…”