Abstract
Abstract. During the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE), a plume of organic aerosol was produced by a smoke generator and emitted into the marine atmosphere from aboard the R/V Point Sur. In this study, the hygroscopic properties and the chemical composition of the plume were studied at plume ages between 0 and 4 h in different meteorological conditions. In sunny conditions, the plume particles had very low hygroscopic growth factors (GFs): between 1.05 and 1.09 for 30 nm and between 1.02 and 1.1 for 150 nm dry size at a relative humidity (RH) of 92%, contrasted by an average marine background GF of 1.6. New particles were produced in large quantities (several 10 000 cm−3), which lead to substantially increased cloud condensation nuclei (CCN) concentrations at supersaturations between 0.07 and 0.88%. Ratios of oxygen to carbon (O : C) and water-soluble organic mass (WSOM) increased with plume age: from < 0.001 to 0.2, and from 2.42 to 4.96 μg m−3, respectively, while organic mass fractions decreased slightly (~ 0.97 to ~ 0.94). High-resolution aerosol mass spectrometer (AMS) spectra show that the organic fragment m/z 43 was dominated by C2H3O+ in the small, new particle mode and by C3H7+ in the large particle mode. In the marine background aerosol, GFs for 150 nm particles at 40% RH were found to be enhanced at higher organic mass fractions: an average GF of 1.06 was observed for aerosols with an organic mass fraction of 0.53, and a GF of 1.04 for an organic mass fraction of 0.35.
Highlights
The interaction of atmospheric aerosol particles with water is a crucial factor affecting their evolution in the atmosphere
aerosol mass spectrometer (AMS) measurements show that the nonrefractory, submicrometer fraction of the background aerosol is dominated by sulfate and organics (Fig. 3)
Black carbon (BC) concentrations during Regime 1 were lower (1.03 ± 1.06 ng m−3) than during Regime 2 (1.8 ± 1.01 ng m−3, the difference is statistically significant at the 99 % confidence level), but short spikes in BC concentrations were observed in both regimes, confirming that the aerosol encountered during Regime 1 was still far from pristine
Summary
The interaction of atmospheric aerosol particles with water is a crucial factor affecting their evolution in the atmosphere. Cloud droplets and water in deliquesced aerosol particles provide an aqueous medium for chemical reactions, which can lead to a change in the chemical composition of the particles (Hegg, 1985; Blando and Turpin, 2000; El Haddad et al, 2009; Bateman et al, 2011; Ervens et al, 2011). Organic compounds can have a profound impact on the water-uptake properties of particles. An increase in the organic mass fraction of aerosol particles can reduce water uptake at relative humidities (RH) above the deliquescence RH (DRH) of salts, while simultaneously enabling hygroscopic
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