Coatings and clusters of carboxylic acids in carbon‐containing atmospheric particles from spectromicroscopy and their implications for cloud‐nucleating and optical properties

Abstract

Particle shape and distribution of chemical compounds within individual particles are implied in the parameterizations used in air quality and climate models for radiative transfer, volatility, and mass transfer. In this study we employ Scanning Transmission X‐Ray Microscopy with Near‐Edge X‐Ray Absorption Fine Structure Spectroscopy with image analysis and pattern recognition techniques to characterize the chemical structure of 636 particles collected on six field campaigns in the western hemisphere between 2004 and 2008. Many of the particles were chemically heterogeneous. A few observed types include black carbon particles covered by aqueous‐phase components (n = 90), dust particles with organic clumps (106), organic particles enriched in carboxylic acid at the surface (54), and inorganic cores encapsulated by organic shells (10). The 90 particles in the first category collectively contained 95 regions showing a strong black‐carbon spectral signature associated with the aqueous‐phase components, of which 78 were between 0.1 and 1 μm. Organic mass fraction of the organic dust particles varied significantly (mean and standard deviation of 0.3 ± 0.2), and over half of these dust particles were found to be nearly spherical. Thickness of acid‐enriched coatings and carbon on inorganic cores were less than 0.6 μm in most cases, but accounted for <0.01 to 0.98 of the particle volume fraction. More than half of the identified organic particles (359) were found to be chemically heterogeneous, and 32 particles were observed as agglomerations or inclusions but did not meet one or more of the criteria of the categories described here. The acidic coatings on black carbon are calculated to have a significant impact on the critical supersaturation of these particles. The measured distribution of aspect ratios of dust and other particles in our samples ranged nonuniformly between 1.0 and 4.6 with a mean of 1.4, which can affect assessment of extinction‐to‐backscatter ratios over the case where spherical geometry is assumed.