Rachel E. O'Brien

Microspectroscopic imaging and characterization of individually identified ice nucleating particles from a case field study

The effect of anthropogenic and biogenic organic particles on atmospheric glaciation processes is poorly understood. We use an optical microscopy setup to identify the ice nuclei (IN) active in immersion freezing (IMF) and deposition ice nucleation within a large population of particles collected on a substrate from an ambient environment in central California dominated by urban and marine aerosols. Multimodal microspectroscopy methods are applied to characterize the physicochemical properties and mixing state of the individual IN and particle populations to identify particle-type classes. The temperature onsets of water uptake occurred between 235 and 257 K at subsaturated conditions, and the onsets of IMF proceeded at subsaturated and saturated conditions for 235–247 K, relevant for ice nucleation in mixed-phase clouds. Particles also took up water and nucleated ice between 226 and 235 K and acted as deposition IN with onset temperatures below 226 K, a temperature range relevant to cirrus cloud formation. The identified IN belong to the most common particle-type classes observed in the field samples: organic coated sea salt and Na-rich, secondary, and refractory carbonaceous particles. Based on these observations, we suggest that the IN are not always particles with unique chemical composition and exceptional ice nucleation propensity; rather, they are common particles in the ambient particle population. The results suggest that particle-type abundance and total particle surface area are also crucial factors, in addition to particle-type ice nucleation efficiency, in determining ice formation within the particle population.

Physical properties of ambient and laboratory‐generated secondary organic aerosol

The size and thickness of organic aerosol particles collected by impaction in five field campaigns were compared to those of laboratory-generated secondary organic aerosols (SOA). Scanning transmission X-ray microscopy was used to measure the total carbon absorbance (TCA) by individual particles as a function of their projection areas on the substrate. Particles with higher viscosity/surface tension can be identified by a steeper slope on a plot of TCA versus size because they flatten less upon impaction. The slopes of the ambient data are statistically similar indicating a small range of average viscosities/surface tensions across five field campaigns. Steeper slopes were observed for the plots corresponding to ambient particles, while smaller slopes were indicative of the laboratory-generated SOA. This comparison indicates that ambient organic particles have higher viscosities/surface tensions than those typically generated in laboratory SOA studies.

Chemical imaging of ambient aerosol particles: Observational constraints on mixing state parameterization

A new parameterization for quantifying the mixing state of aerosol populations has been applied for the first time to samples of ambient particles analyzed using spectro-microscopy techniques. Scanning transmission X-ray microscopy/near edge X-ray absorption fine structure (STXM/NEXAFS) and computer-controlled scanning electron microscopy/energy dispersive X-ray spectroscopy (CCSEM/EDX) were used to probe the composition of the organic and inorganic fraction of individual particles collected on 27 and 28 June during the 2010 Carbonaceous Aerosols and Radiative Effects study in the Central Valley, California. The first field site, T0, was located in downtown Sacramento, while T1 was located near the Sierra Nevada Mountains. Mass estimates of the aerosol particle components were used to calculate mixing state metrics, such as the particle-specific diversity, bulk population diversity, and mixing state index, for each sample. The STXM data showed evidence of changes in the mixing state associated with a buildup of organic matter confirmed by collocated measurements, and the largest impact on the mixing state was due to an increase in soot dominant particles during this buildup. The mixing state from STXM was similar between T0 and T1, indicating that the increased organic fraction at T1 had a small effect on the mixing state of the population. The CCSEM/EDX analysis showed the presence of two types of particle populations: the first was dominated by aged sea-salt particles and had a higher mixing state index (indicating a more homogeneous population); the second was dominated by carbonaceous particles and had a lower mixing state index.

Molecular characterization of S‐ and N‐containing organic constituents in ambient aerosols by negative ion mode high‐resolution Nanospray Desorption Electrospray Ionization Mass Spectrometry: CalNex 2010 field study

Samples of ambient aerosols from the 2010 California Research at the Nexus of Air Quality and Climate Change (CalNex) field study were analyzed using negative ion mode Nanospray Desorption Electrospray Ionization High-Resolution Mass Spectrometry (nano-DESI/MS). Four samples per day (6 h each) were collected in Bakersfield, CA on 20–24 June. Four characteristic groups were identified: molecules composed of carbon, hydrogen, and oxygen only (CHO), sulfur- (CHOS), nitrogen- (CHON), and both nitrogen- and sulfur-containing organics (CHONS). The chemical formula and elemental ratios were consistent with the presence of organonitrates, organosulfate, and nitroxy organosulfates in the negative ion mode mass spectra. The number of observed CHO compounds increased in the afternoon samples, suggesting photochemical processing as a source. The average number of CHOS compounds had the smallest changes during the day, consistent with a more broadly distributed source. Both of the nitrogen-containing groups (CHONS and CHON) had greater numbers of compounds in the early morning (midnight to 6 A.M.) and night (6 P.M. to midnight) samples, respectively, consistent with nitrate radical chemistry as a likely source for those compounds. Most of the compounds were found in submicron particles. The size distribution of the number of CHON compounds was bimodal, potentially indicating two types of sources. We conclude that the majority of the compounds observed were secondary in nature with both biogenic and anthropogenic sources. These data are complementary to previous results from positive ion mode nano-DESI/MS analysis of a subset of the same samples providing a more complete view of aerosol chemical composition at Bakersfield.