AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
In-cloud Observations of Aerosol Hygroscopicity and Cloud Droplet Activation
OLLI VÄISÄNEN, Pasi Miettinen, Arttu Ylisirniö, Sami Romakkaniemi, Kari Lehtinen, Annele Virtanen, University of Eastern Finland
Abstract Number: 440 Working Group: Aerosols, Clouds, and Climate
Abstract Simultaneous measurements of aerosol hygroscopicity and cloud droplet activation were carried out in Kuopio, Finland during the autumn 2014. The measurement station was located at the top floor of the Puijo observation tower approximately 240 m above the surrounding lake level, providing a possibility to study the aerosol-cloud interactions in real atmospheric conditions. By using the special inlet setup with a switching valve system, we were able to separate the non-activated (interstitial) particles from the total aerosol.
The hygroscopic tandem differential mobility analyzer (HTDMA) was attached to the changing inlet and it was operated to measure the hygroscopic growth factors (GF) at 90 % relative humidity for three different dry sizes (80, 120 and 150 nm). Overall, the measured GF distributions (GF-PDF) appeared bimodal with main peaks around 1.05 and 1.5. However, the fraction of more hygroscopic particles increased substantially with size, causing an increasing trend for overall hygroscopicity. Also, the interstitial particles turned out considerably less hygroscopic (e.g. GF$_(Int,150) = 1.28) compared to residual particles (GF$_(Res,150) = 1.41).
By using the non-normalized GF-PDFs, we managed to estimate the activation efficiencies of low (0.8 ≤ GF ≤ 1.2) and high hygroscopicity (1.2 ≤ GF ≤ 1.8) particles separately. Since the 80 nm particles didn’t usually activate into cloud droplets, the average activation efficiencies were around 20 % regardless of hygroscopicity. However, the 120 and 150 nm particles already showed distinctive differences between different hygroscopicity populations. On average, the activation efficiencies of more hygroscopic particles were approximately 65 % (120 nm) and 78 % (150 nm) while the corresponding values for less hygroscopic particles remained well below 20 %.