Abstract
Abstract. Measurements of the effect of water uptake on particulate light extinction or scattering made at two locations during the 2010 Carbonaceous Aerosols and Radiative Effects Study (CARES) study around Sacramento, CA are reported. The observed influence of water uptake, characterized through the dimensionless optical hygroscopicity parameter γ, is compared with calculations constrained by observed particle size distributions and size-dependent particle composition. A closure assessment has been carried out that allowed for determination of the average hygroscopic growth factors (GFs) at 85% relative humidity and the dimensionless hygroscopicity parameter κ for oxygenated organic aerosol (OA) and for supermicron particles (defined here as particles with aerodynamic diameters between 1 and 2.5 microns), yielding κ = 0.1–0.15 and 0.9–1.0, respectively. The derived range of oxygenated OA κ values are in line with previous observations. The relatively large values for supermicron particles is consistent with substantial contributions of sea-salt-containing particles in this size range. Analysis of time-dependent variations in the supermicron particle hygroscopicity suggest that atmospheric processing, specifically chloride displacement by nitrate and the accumulation of secondary organics on supermicron particles, can lead to substantial depression of the observed GF.
Highlights
It is well established that atmospheric particles can have a strong influence on climate through their direct effect: scattering and absorption of solar and terrestrial radiation
Time series for the dried (RH < 40 %) and humidified (RH ∼ 85 %) particle extinction (532 nm) or scattering (525 nm), the submicron particle composition as volume fractions, and the volume-weighted particle size distributions are shown in Fig. 1 for both T0 and T1
Measurements of light extinction and light scattering by ambient particles (PM2.5) were made at two sites under low and high relative humidity (RH) conditions during the 2010 Carbonaceous Aerosols and Radiative Effects Study (CARES) campaign in Sacramento, CA to assess the influence of water uptake on the optical properties of the particles
Summary
It is well established that atmospheric particles can have a strong influence on climate through their direct effect: scattering and absorption of solar and terrestrial radiation. Atkinson et al.: Aerosol hygroscopicity measurements during CARES 2010 more explicitly for particle dynamics, aging and mixing state (e.g., Riemer et al, 2010; Zaveri et al, 2010). In both simple and complex models, the extent of particulate water is determined by the local atmospheric relative humidity (RH) and the particle composition, the latter of which controls the particle hygroscopicity. The connections between particle composition, hygroscopicity, and optical properties ( scattering and extinction) are examined through optical closure based on observations made during the 2010 Carbonaceous Aerosols and Radiative Effects Study (CARES) field intensive (Zaveri et al, 2012). The recirculation period produced an extensively processed organic aerosol (Setyan et al, 2012)
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