Observations of aerosol–cloud interactions at the Puijo semi-urban measurement station

In 2006, a new measurement station for aerosol-cloud interactions was established in Kuopio, Finland (Leskinen et al., submitted to Boreal Env. Res.). The station is located on top of the Puijo observation tower (306 m a.s.l, 224 m above the surrounding lake level), offering good conditions for particle and cloud measurements. We are using a twin-DMPS system for particle size distribution measurement with a size range of 10-500 nm until March 2007 and 7-800 nm thereafter. We have a heating in the sample inlet for drying the cloud droplets, so we observe the total (interstitial+residual) size distribution. Cloud droplet size distribution (3-50 μm) is measured with a cloud droplet probe (Droplet Measurement Technologies). We calculated hourly averages of weather parameters, particle and cloud droplet size distribution data. We classified all hourly averages with visibility below 200 m and rain intensity below 0.2 mm/h as cloud event hours. We use the limit for rain intensity, since rain drops remove unactivated aerosol particles and would disturb analysis. We classified this data into four groups based on the particle number concentration in order to distinguish the correlations between concentration and parameters involved in cloud-particle interaction. We found three indications of the aerosol indirect effects (Table 1): 1) Cloud droplet concentration Nd increased with the increasing particle concentration Np. 2) Nd and average droplet diameter dd were inversely proportional, as previous studies have shown (e.g. Komppula et al., 2005; Twohy et al. 2005). 3) Ratio of Nd to accumulation mode particle concentration Nacc was smaller when Np is higher. We did trajectory analysis to classify the air masses into marine and continental. It should be noted that the marine air masses can not be considered purely marine since they spent on average 40 hours over the continent before arriving to Puijo. Marine air masses were characterized by the higher Np, higher Nait/Nacc ratio and smaller Nd compared to continental (Table 2). The higher Nait/Nacc ratio in the marine air masses is consistent with the pronounced Aitken mode of usual marine size distributions (e.g. Anttila et al., 2008). The lower Np in continental air masses compared to marine was not what one would expect. It seems that during the cloud events, the continental air masses were relatively clean, the average Np being below the annual average. Table 1. Total particle concentration Np, cloud droplet concentration Nd, droplet diameter dd (μm), accumulation mode particle concentration Nacc, Nd/Nacc and ratio of Aitken mode particle concentration to accumulation mode particle concentration Nait/Nacc during the observed cloud events. Last line shows average values during all cloud events. All concentrations are in cm.