Abstract
Abstract. Carbonaceous aerosols are responsible for large uncertainties in climate models, degraded visibility, and adverse health effects. The Carbonaceous Aerosols and Radiative Effects Study (CARES) was designed to study carbonaceous aerosols in the natural environment of the Central Valley, California, and learn more about their atmospheric formation and aging. This paper presents results from spectro-microscopic measurements of carbonaceous particles collected during CARES at the time of a pollution accumulation event (27–29 June 2010), when in situ measurements indicated an increase in the organic carbon content of aerosols as the Sacramento urban plume aged. Computer-controlled scanning electron microscopy coupled with an energy dispersive X-ray detector (CCSEM/EDX) and scanning transmission X-ray microscopy coupled with near-edge X-ray absorption spectroscopy (STXM/NEXAFS) were used to probe the chemical composition and morphology of individual particles. It was found that the mass of organic carbon on individual particles increased through condensation of secondary organic aerosol. STXM/NEXAFS indicated that the number fraction of homogenous organic particles lacking inorganic inclusions (greater than ~50 nm equivalent circular diameter) increased with plume age, as did the organic mass per particle. Comparison of the CARES spectro-microscopic dataset with a similar dataset obtained in Mexico City during the MILAGRO campaign showed that fresh particles in Mexico City contained three times as much carbon as those sampled during CARES. The number fraction of soot particles at the Mexico City urban site (ranging from 16.6 to 47.3%) was larger than at the CARES urban site (13.4–15.7%), and the most aged samples from CARES contained fewer carbon–carbon double bonds. Differences between carbonaceous particles in Mexico City and California result from different sources, photochemical conditions, gas phase reactants, and secondary organic aerosol precursors. The detailed results provided by these spectro-microscopic measurements will allow for a comprehensive evaluation of aerosol process models used in climate research.
Highlights
Uncertainties in predicting future climate change are, in large part, due to an incomplete knowledge of atmospheric aerosols (IPCC, 2007)
Carbon monoxide tracer forecasts indicate that the urban plume originating at T0 was effectively transported to T1 during this time
These plume characteristics allowed for spectro-microscopic measurements of carbonaceous aerosol aging at the T0 urban site and the T1 rural site
Summary
Uncertainties in predicting future climate change are, in large part, due to an incomplete knowledge of atmospheric aerosols (IPCC, 2007). Aerosols affect climate by scattering and absorbing solar radiation (direct effect) and by modification of cloud properties (indirect effects). The optical properties of strongly absorbing soot particles may change as a result of condensation of non-absorbing material that behaves as a “lens” to enhance the solar radiation. Moffet et al.: SpecFtriog-umricersoscopic measurements of carbonaceous aerosol incident on the strongly absorbing soot inclusion (Moffet and Prather, 2009; Ackerman and Toon, 1981; Jacobson, 2001; Cappa et al, 2012). Developing accurate global climate models requires a processlevel understanding of the aerosol life cycle from initial for-2 mation to loss through their incorporation into precipitating clouds or dry deposition (Ghan and process-level understanding can only bSechowbtaaritnze,d2t0h0ro7u).gShuccoh-3 ordinated measurements of different time and length scales from the microscopic to the synoptic
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