Closure between measured and modeled cloud condensation nuclei (CCN) using size-resolved aerosol compositions in downtown Toronto

Abstract

Abstract. Measurements of cloud condensation nuclei (CCN) were made in downtown Toronto during August and September, 2003. CCN measurements were performed at 0.58% supersaturation using a thermal-gradient diffusion chamber, whereas the aerosol size distribution and composition were simultaneously measured with a TSI SMPS and APS system and an Aerodyne Aerosol Mass Spectrometer (AMS), respectively. Aerosol composition data shows that the particles were predominately organic in nature, in particular for those with a vacuum aerodynamic diameter of <0.25 µm. In this study, the largest contribution to CCN concentrations came from this size range, suggesting that the CCN are also organic-rich. Using the size and composition information, detailed CCN closure analyses were performed. In the first analysis, the particles were assumed to be internally mixed, the organic fraction was assumed to be insoluble, and the inorganic fraction was assumed to be ammonium sulfate. The AMS time-of-flight data were used for Köhler theory predictions for each particle size and composition to obtain the dry diameter required for activation. By so doing, this closure analysis yielded an average value of CCN predicted /CCN observed =1.12±0.05. However, several sample days showed distinct bimodal distributions, and a closure analysis was performed after decoupling the two particle modes. This analysis yielded an average value of CCN predicted /CCN observed =1.03±0.05. A sensitivity analysis was also performed to determine the aerosol/CCN closure if the organic solubility, droplet surface tension, or chamber supersaturation were varied.

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

“…This contrasts with urban closure studies indicating insoluble organics (Lance et al, 2009), especially at small sizes (e.g. Broekhuizen et al, 2006;Cubison et al, 2008;Quinn et al, 2008), although this can also be true for remote locations . Here we conduct a study of the hygroscopicity of the organic component at supersaturated conditions, focusing on the oxygenated component under the assumption that the unoxygenated component is non-hygroscopic.…”

“…Similar results have been published for subsaturated conditions (McFiggans et al, 2005;Raatikainen et al, 2010) and more recently for laboratory and ambient measurements ), although laboratory experiments have shown that organic aerosol hygroscopicity under subsaturated conditions can be lower than under supersaturated conditions (Petters et al, 2009b;Prenni et al, 2007;Wex et al, 2009). This study builds on the results from previous closure studies conducted in our group at urban and rural sites (Broekhuizen et al, 2006;Chang et al, 2007). (Rupakheti et al, 2005).…”

“…An extensive review of early aerosol-CCN closure studies that had limited information about the organic component of the aerosol can be found in Broekhuizen et al (2006). 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.…”

“…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 .…”

Cold and transition season cloud condensation nuclei measurements in western Colorado

“…Broekhuizen et al (2006) and Cubison et al (2008) both show evidence that CCN closure can be improved with knowledge of size-dependent composition. Cubison et al (2008) also found that assumptions about the aerosol mixing state (internal or external) can adversely impact CCN predictions.…”