Effects of aerosol organics on cloud condensation nucleus (CCN) concentration and first indirect aerosol effect

Abstract

Abstract. Aerosol microphysics, chemical composition, and CCN properties were measured on the Department of Energy Gulfstream-1 aircraft during the Marine Stratus/Stratocumulus Experiment (MASE) conducted over the coastal waters between Point Reyes National Seashore and Monterey Bay, California, in July 2005. Aerosols measured during MASE included free tropospheric aerosols, marine boundary layer aerosols, and aerosols with high organic concentration within a thin layer above the cloud. Closure analysis was carried out for all three types of aerosols by comparing the measured CCN concentrations at ∼0.2% supersaturation to those predicted based on size distribution and chemical composition using Köhler theory. The effect of aerosol organic species on predicted CCN concentration was examined using a single hygroscopicity parameterization. For aerosols with organics volume fraction up to 70%, such as the marine boundary layer and free troposphere aerosols, CCN concentration and the corresponding first indirect aerosol effect are insensitive to the properties of organics, and can be accurately predicted with a constant hygroscopicity for all organic species. This simplification can facilitate the prediction of indirect aerosol effects using physically-based parameterizations in large scale models. However, for the aerosols within the thin layers above clouds, organics contributed up to 90% of the total aerosol volume, and an accurate knowledge of the overall organic hygroscopicity is required to accurately predict CCN concentrations. Derivations of organic properties in future closure studies, when aerosols are dominated by organic species, would help constrain the descriptions of organics and aerosol-cloud parameterizations in large scale models.

The hygroscopicity parameter (κ) of ambient organic aerosol at a field site subject to biogenic and anthropogenic influences: relationship to degree of aerosol oxidation

“…In both cases, the resulting κ for the entire aerosol is consistently lower than the 0.22 found for the OOA components in this study, where the m/z 44 to total organic fraction was 0.10 to 0.19. Other studies that have attempted to quantify κ org in the field found that using a value of 0.1 can adequately describe the CCN-activity of the aerosol at cloud-level (Wang et al, 2008) and in the Amazon rainforest (Gunthe et al, 2009). This is comparable to the campaign-wide average for this study of 0.15 for κ org , calculated using both Eqs.…”

“…2). While the slope from the linear regression is comparable to other field studies (Broekhuizen et al, 2006;Chang et al, 2007;Medina et al, 2007;Wang et al, 2008) which have found similar values, it is calculated in a different way and puts more emphasis on the data points at high concentrations. In an attempt to lessen this effect, a linear regression of the logarithm of the concentrations was also attempted, although this resulted in a similar slope of 1.02.…”

“…However, with the recent widespread use of the AMS and following the initial study of Broekhuizen et al (2006), a few closure studies have since been conducted in which the aerosol chemical composition is highly time resolved and the organic fraction quantified. These studies from the field achieved closure by determining the hygroscopicity of either the entire aerosol by assuming an average chemical composition in continental China (Rose et al, 2010), or the entire organic component by assuming that the hygroscopicity in the organic component was constant at rural locations (Chang et al, 2007;Medina et al, 2007;Stroud et al, 2007), in the Amazon (Gunthe et al, 2009;Roberts et al, 2002), at cloud level (Wang et al, 2008), as well as sites that are removed from major source areas . A general conclusion is that the organic fraction is hygroscopic.…”

The direct and indirect radiative effects of biogenic secondary organic aerosol

“…We assign the following individual hygroscopicity parameters: sulphate (0.61, assuming ammonium sulphate), sea salt (1.28), black carbon (0.0), and particulate organic matter (0.1). Whilst there is uncertainty associated with the hygroscopicity parameter for organic material in the atmosphere due to the wide range of solubilities observed, κ values close to 0.1 have been reported for secondary organic components (Engelhart et al, 2008(Engelhart et al, , 2011Gunthe et al, 2009;Dusek et al, 2010;King et al, 2010), and the entire organic fraction (Wang et al, 2008;Chang et al, 2010). We calculate CCN concentrations at a supersaturation of 0.2 %, which is equivalent to an activation dry diameter of approximately 80 nm (assuming a composition of pure ammonium sulphate).…”

CCN activity and organic hygroscopicity of aerosols downwind of an urban region in central Amazonia: seasonal and diel variations and impact of anthropogenic emissions

“…However, atmospheric aerosols consist of a large number of organic compounds, which often dominate the total fine aerosol mass, especially in forested areas (e.g., de Sá et al, 2017a;Zhang et al, 2007). The hygroscopicity of aerosol organics (κ org ) have been examined in both laboratory (e.g., Asa-Awuku et al, 2009;Duplissy et al, 2011;King et al, 2009;Lambe et al, 2011;Massoli et al, 2010;Raymond and Pandis, 2003) and field studies (e.g., Cerully et al, 2015;Chang et al, 2010;Dusek et al, 2010;Gunthe et al, 2009;Lathem et al, 2013;Mei et al, 2013a, b;Moore et al, 2011Moore et al, , 2012Pöhlker et al, 2016;Rose et al, 2010;Shantz et al, 2008;Wang et al, 2008). Overall, these studies show that aerosol organics exhibit a wide range of κ values from 0 to ∼ 0.3, and κ org often increases substantially during aerosol aging in the atmosphere (e.g., Duplissy et al, 2011;Lambe et al, 2011;Massoli et al, 2010;Mei et al, 2013a, b).…”