Seasonal variation of CCN concentrations and aerosol activation properties in boreal forest

Abstract

Abstract. As a part of EUCAARI activities, the annual cycle of cloud condensation nuclei (CCN) concentrations and critical diameter for cloud droplet activation as a function of supersaturation were measured using a CCN counter and a HTDMA (hygroscopicity tandem differential mobility analyzer) at SMEAR II station, Hyytiälä, Finland. The critical diameters for CCN activation were estimated from (i) the measured CCN concentration and particle size distribution data, and (ii) the hygroscopic growth factors by applying κ-Köhler theory, in both cases assuming an internally mixed aerosol. The critical diameters derived by these two methods were in good agreement with each other. The effect of new particle formation on the diurnal variation of CCN concentration and critical diameters was studied. New particle formation was observed to increase the CCN concentrations by 70-110 %, depending on the supersaturation level. The average value for the κ-parameter determined from hygroscopicity measurements was κ = 0.18 and it predicted well the CCN activation in boreal forest conditions in Hyytiälä. The derived critical diameters and κ-parameter confirm earlier findings with other methods, that aerosol particles at CCN sizes in Hyytiälä are mostly organic, but contain also more hygrosopic, probably inorganic salts like ammonium sulphate, making the particles more CCN active than pure secondary organic aerosol.

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

“…In their study, the particle hygroscopicity showed no obvious size dependency and was higher than our observation in Beijing. In contrast, κs measured were relatively low at a forested site in Colorado (κ = 0.16 ± 0.08 detected by CCN counter (CCNc)), a boreal forest in Finland (κ = 0.18 at RH = 90 %) (Sihto et al, 2011), and a tropical forest site in the Amazon (κ = 0.16 ± 0.06 detected by CCNc) (Gunthe et al, 2009). At these forested locations, organic species were predominant in particles.…”

A synthesis of cloud condensation nuclei counter (CCNC) measurements within the EUCAARI network

“…A peak in aerosol hygroscopicity is observed around midday when D c reaches its minimum. Several previous studies have reported such behaviour in Hyytiälä and have attributed it to the vegetation activity, photochemistry and the ageing of organics during sunlight hours (Sihto et al, 2011;Cerully et al, 2011;Paramonov et al, 2013). While no diurnal pattern of aerosol hygroscopicity is visible for Jungfraujoch for winter and spring, a very clear pattern does exist in the summer and autumn.…”

Aerosol mixing state, hygroscopic growth and cloud activation efficiency during MIRAGE 2006

“…κ CCNc for all particle sizes reaches a maximum within 1-2 h of local noon, suggesting that photochemical processes are dominant at that time, which is consistent with the peak in oxygenated organic aerosol mass (Aiken et al, 2009) and water soluble organic carbon mass (Hennigan et al, 2008) observed during the daytime at the T0 and T1 sites, respectively. New particle formation (NPF) events occurred frequently during MIRAGE 2006(Smith et al, 2008, as illustrated by the particle size distributions (Fig. 2f).…”

“…The colder end of the CCN column was maintained at T c = 24.0 ±1.38 • C, while the warmer end was maintained at T h to obtain T ranging from from 1.87±0.032 • C to 13.7 ± 0.038 • C, corresponding to a range of supersaturations ∼ 0.07-1.15 %. For reference, ambient cloud supersaturations are generally <1 % (Seinfeld and Pandis, 1998). Supersaturation in the CCNc was calibrated in the field using atomized and classified NaCl particles, which were also used to simultaneously calibrate the HTDMA system.…”