An annual cycle of size‐resolved aerosol hygroscopicity at a forested site in Colorado

Abstract

[1] The ability of particles composed wholly or partially of biogenic secondary organic compounds to serve as cloud condensation nuclei (CCN) is a key characteristic that helps to define their roles in linking biogeochemical and water cycles. In this paper, we describe size-resolved (14-350 nm) CCN measurements from the Manitou Experimental Forest in Colorado, where particle compositions were expected to have a large biogenic component. These measurements were conducted for 1 year as part of the Bio-hydro-atmosphere Interactions of Energy, Aerosols, Carbon, H 2 O, Organics, and Nitrogen program and determined the aerosol hygroscopicity parameter, k, at five water supersaturations between $0.14% and $0.97%. The average k value over the entire study and all supersaturations was k avg = 0.16 AE 0.08. Kappa values decreased slightly with increasing supersaturation, suggesting a change in aerosol composition with dry diameter. Furthermore, some seasonal variability was observed with increased CCN concentrations and activated particle number fraction, but slightly decreased hygroscopicity, during the summer. Small particle events, which may indicate new particle formation, were observed throughout the study period, especially in the summer, leading to increases in CCN concentration, followed by a gradual increase in the aerosol mode size. The condensing material appeared to be predominantly composed of organic compounds and led to a small decrease in k at the larger activation diameters during and immediately after those events.

Contribution from biogenic organic compounds to particle growth during the 2010 BEACHON-ROCS campaign in a Colorado temperate needleleaf forest

“…The simulated growth of particles above 15 nm compares well with the DMPS measurements. These simulations are also consistent with results from Levin et al (2012Levin et al ( , 2014 for hygroscopicity measurements at the BEACHON-ROCS site; these authors showed that growth of new particles was likely driven by biogenic secondary organic species, and the NPF events ultimately impacted aerosol chemical and physical properties for particles up to cloud condensation nuclei (CCN) sizes. Since lump sums of different oxidation products were used as the condensing vapors in this modeling study, it was not possible to assign exact physical properties to the vapors.…”

Particle hygroscopicity and its link to chemical composition in the urban atmosphere of Beijing, China, during summertime

“…In contrast, the newly formed particles consisted of a minor fraction of water-soluble fraction (16 %) in Hyytiälä, Finland (Ehn et al, 2007). Other observations in the clean atmospheric environments (relative to Beijing), such as a forested site in Colorado (Levin et al, 2012), Mace Head, Ireland (Väkevä et al, 2002), and Melpitz, Germany , also showed that low water-soluble compounds, most likely secondary organic species, mainly contributed to new particle growth. Differently, in the urban area of Atlanta (Sakurai et al, 2005), the water-soluble fraction dominated in the newly formed particles, which was similar to our observation in this study.…”

“…A similar size dependency of particle hygroscopicity was observed in various environments. For examples, Levin et al (2012Levin et al ( , 2014 and Paramonov et al (2013) reported that particle hygroscopicity increased with particle size at a forested site in Colorado and a boreal environment of southern Finland at the SMEAR station, respectively. Jurányi et al (2013) observed that particle hygroscopic growth increased with increasing dry diameter in the urban areas of Paris.…”

“…More recently, some studies have been performed to investigate the relationship between particle hygroscopicity and chemical composition in both field measurements and laboratory experiments (Massoli et al, 2010;Wong et al, 2011;Lambe et al, 2011;Rickards et al, 2013;Moore et al, 2012a, b;Suda et al, 2014;Paramonov et al, 2013;Levin et al, 2012). These works specially focused on parametrizing the empirical correlations between the atomic oxygen : carbon (O : C) ratio and organic hygroscopicity parameter (κ) derived from either hygroscopic growth factor (e.g., Wu et al, 2013;Rickards et al, 2013) or cloud condensation nuclei (CCN) activity (e.g., Mei et al, 2013;Wong et al, 2011;Lambe et al, 2011;Chang et al, 2010).…”

Cloud condensation nuclei production associated with atmospheric nucleation: a synthesis based on existing literature and new results

“…By combining this information with the observed values of κ between about 0.1 and 0.45 for sub-100 nm particles originating from nucleation (e.g. Dusek et al, 2010;Cerully et al, 2011;Sihto et al, 2011;Levin et al, 2012), we end up with threshold diameters larger than about 70-80 nm in boundary-layer clouds (Fig. 1).…”