Measurements of aerosol and cloud droplet residual composition in boundary layer clouds

Optical and physical properties of clouds are dependent on the number of cloud droplets forming the cloud. In addition to atmospheric dynamics, the cloud droplet number concentration is controlled by atmospheric aerosol particles. A subset of these particles may act as cloud condensation nuclei (CCN), so that the CCN activity of particles depends primarily on their size but also on their chemical composition. Usually elevated CCN concentrations increase the number concentration of droplets in a cloud leading to decrease in their mean size. This may lead to optically thicker clouds and to prolonged lifetime of clouds in some situations.

Here we show how the composition of particles is affecting their CCN activity in boundary layer clouds. We present results from the measurement campaigns carried out in the Puijo measurement station (62°54′34′′N, 27°39′19′′E) where the activation of aerosol particles to form cloud droplets can be studied in situ for boundary layer clouds. When the station is surrounded by clouds, the aerosol is sampled through two sampling lines, one measuring only the interstitial (not activated) particles and other one measuring both interstitial particles and cloud droplet residuals. Thus we will gain information on both the size and chemical composition of particles activated.

In the typical conditions prevailing at the station the minimum size of particles forming cloud droplets is between 80 to 200nmm and the composition of particles is dominated by organics, sulfates and nitrates. However, a distinct difference in the chemical compositions between the cloud interstitial particles and cloud droplet residuals can be seen. We have observed much higher mass fractions of organic nitrate and less oxidized organic compounds in the cloud interstitial particles than in the cloud droplet residuals. By contrast, the mass fractions of (NH4)2SO4 and NH4NO3 in the cloud droplet residuals were observed to be higher than in interstitial particles. This is caused both by the activation ability (and the size dependent composition) of aerosol particles into cloud droplets and the gas-particle partitioning of semivolatile chemical compounds inside the cloud after the cloud droplet formation. This partitioning might affect the chemistry in the cloud droplets and cloud processing of aerosol particles.