 |
| Titel |
Phase state is a limiting factor in hygroscopic growth of secondary organic aerosol |
| VerfasserIn |
Aki Pajunoja, Annele Virtanen |
| Konferenz |
EGU General Assembly 2014
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250097768
|
| Publikation (Nr.) |
 EGU/EGU2014-13377.pdf |
|
|
|
|
|
| Zusammenfassung |
| Secondary organic aerosol (SOA) particles formed from oxidation products of volatile
organic compounds (VOC) form a significant fraction of the total atmospheric particulate
matter affecting climate both directly and indirectly. The dependence of hygroscopicity on
particle composition is often represented with the single parameter κ, commonly used in
global models to describe the hygroscopic properties of atmospheric aerosol particles. The
physical phase state of SOA particles affects the partitioning of organic vapors and also may
affect the uptake of water vapor and particle activation into cloud droplets. Thus, hygroscopic
behaviour of SOA particles is affected by composition (i.e. oxidation state and molecular
size) but also by phase of particles.
In this study the following three distinct studies were performed: (1) particle bounced
fraction (BF) measurements, which are qualitatively related to particle phase, as a function of
relative humidity using an Aerosol Bounce Instrument (ABI). We assume that the particles
with BF > 0 are solid or semisolid, and that particles with BF = 0 behave mechanically as
liquids (2) water uptake measured in the sub-saturated region using hygroscopicity tandem
differential mobility analyzer (HTDMA) by measuring the ratio of wet to dry particle
diameter following exposure to water vapor at a controlled RH (3) cloud droplet formation in
the supersaturated region using a cloud condensation nuclei counter (CCNc). Particle
composition and oxidation state was measured with a compact time of flight aerosol mass
spectrometer (c-ToF-AMS).
In this study we show that at sub-saturation conditions water uptake by SOA particles is
restricted due to the kinetic limitations. Diffusion and solubility limitations inhibit water
uptake until the humidity is high enough for dissolution to occur. Our studies show that this
”threshold” humidity is dependent on particle composition, oxidation state, and average
molecular size. Our laboratory results explain several observations both in laboratory and in
the atmosphere that have reported discrepancies between SOA particle hygroscopicity
measurements in subsaturated and in supersaturated conditions. The results reported here
provide new information about one of the most central aerosol processes in the atmosphere. |
|
|
|
|
|
|