Hygroscopic properties of the ambient aerosol in southern Sweden – a two year study

Abstract

Abstract. The hygroscopic growth of the atmospheric aerosol is a critical parameter for quantifying the anthropogenic radiative forcing. Until now, there has been a lack of long term measurements due to limitations in instrumental techniques. In this work, for the first time the seasonal variation of the hygroscopic properties of a continental background aerosol has been described, based on more than two years of continuous measurements. In addition to this, the diurnal variation of the hygroscopic growth has been investigated, as well as the seasonal variation in CCN concentration. These physical properties of the aerosol have been measured with a Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA), a Differential Mobility Particle Sizer (DMPS), and a Cloud Condensation Nuclei Counter (CCNC). The results show that smaller particles are generally less hygroscopic than larger ones, and that there is a clear difference in the hygroscopic properties between the Aitken and the accumulation mode. A seasonal cycle was found for all particle sizes. In general, the average hygroscopic growth is lower during wintertime, due to an increase in the relative abundance of less hygroscopic or barely hygroscopic particles. Monthly averages showed that the hygroscopic growth factors of the two dominating hygroscopic modes (one barely hygroscopic and one more hygroscopic) were relatively stable. The hygroscopic growth additionally showed a diurnal cycle, with higher growth factors during day time. CCN predictions based on H-TDMA data underpredicted the activated CCN number concentration with 7 % for a 1 % water supersaturation ratio. The underprediction increases with decreasing s, most likely due to a combination of measurement and modeling uncertainties. It was found that although the aerosol is often externally mixed, recalcuCorrespondence to: E. O. Fors (erik.fors@nuclear.lu.se) lating to an internal mixture with respect to hygroscopicity did not change the CCN concentration as a function of supersaturation significantly.

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

“…The median number fraction of hydrophilic mode for 50 nm particles was 0.6, which was smaller than that of larger particles. Fors et al (2011) also reported that smaller particles had a higher fraction of less hygroscopic particles in southern Sweden. Larger particles (here, above 150 nm) constituting a larger fraction of the hydrophilic mode can be explained as such: in the urban area, traffic emissions are major sources for particles below 100 nm.…”

Size-resolved aerosol composition and its link to hygroscopicity at a forested site in Colorado

“…Laboratory and field studies of secondary organic aerosol (SOA) species have found κ values around ∼0.1 (Prenni et al, 2007;Engelhart et al, 2008;Engelhart et al, 2011;Wex et al, 2009;Pierce et al, 2012) although individual species, when separated by polarity, show a larger range (Suda et al, 2012). Studies have shown that κ of more oxidized secondary organic species can be as high as 0.22 (Chang et al, 2010), although Frosch et al (2011) found no relationship between κ and O / C ratio for α-pinene SOA aged in a smog chamber. Black carbon and dust are typically measured or assumed to have κ values of ∼0-0.05 (Koehler et al, 2009;Yamashita et al, 2011).…”

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

“…In addition to the DMPS data, CCN number concentrations at fixed supersaturations were measured in Hyytiälä and Vavihill using a cloud condensation nuclei counter (CCNC) over limited time periods. For detailed descriptions of the measurements systems at each location, we refer to Hari and Kulmala (2005), Kristensson et al (2008), Laakso et al (2008), , Fors et al (2011) and Sihto et al (2011).…”