Modification of cloud properties by atmospheric aerosol particles is perhaps the most uncertain forcing component associated with human induced climate change. The basic issue in looking at aerosol-cloud interactions is to find a relation between the activating aerosol particle –and the resulting cloud droplet population. This is far from being well understood.
The Third PAllas Cloud Experiment, PACE III, took place from 9th of September to 9th of October. With a multi-disciplinary approach, studies of size-dependent activation efficiencies, of the role of chemistry and environmental factors on cloud droplet activation and cloud properties were made.
The Pallas – Sodankylä Global Atmosphere Watch station is located in northern Finland (Hatakka et al., 2003). The main measuring site is at the top of the Sammaltunturi fjeld 560 m asl. The station is occasionally inside a cloud, at least part of a day in about 10 % of all days. These are typically orographic clouds. There are no local pollution sources. The closest major sources of pollutants are the smelters in Kola Peninsula, about 300 km east from Pallas. Due to its location, Pallas is occasionally under very clean air masses coming straight from the Arctic Ocean and occasionally under polluted air masses from Kola Peninsula which originate from the Arctic Ocean. This makes it an ideal place to study anthropogenic vs. natural effects on cloud properties.
The campaign instrumentation included:
- DMPS's in PM2.5 –and a Total Inlet for both interstitial and total aerosol size distribution
- HTDMA for hygroscopic growth of the particles
- CCN counter
- AMS for single particle chemistry
- MAAP and nephelometer in PM2.5 and a Total Inlet for optical properties of both total and interstitial aerosol
- Fog Monitor for cloud droplet concentration
- FSSP for cloud droplet size distribution
- Fog collector for cloud water chemistry
- 3D Anemometers for estimation of the updraft velocity
- Aerosol sampler for analysis of aerosol physical chemistry (surface tension)
- Ceilometer for cloud base height
During the campaign period, station was inside the cloud 10 % of the time, with 9 distinct cloud events (fig 1). The average temperature during the campaign was about 3°C, varying between -7°C and +14°C. The average RH was 81 %, and the average wind speed was 7 m/s.
Fig. 1. Data coverage of the instruments and times when the station was in cloud.
Acknowledgements
This work was funded by the EUSAAR and the Maj & Tor Nessling Foundation.