Investigating the CCN Activation Kinetics of Aerosol in Arctic Haze and Canadian Boreal Forest Fires
Abstract
It has long been hypothesized that the existence of (organic) hydrophobic films and slowly-dissolving compounds may impact the droplet activation kinetics of ambient Cloud Condensation Nuclei (CCN). This “compositional” effect on droplet formation may have an important impact on droplet number and size distribution, but is largely unconstrained to date. Measurements with a continuous flow CCN instrument that sizes the activated droplets can be used to identify whether composition affects droplet activation kinetics. The method used, termed “Threshold Droplet Growth Analysis (TDGA)” looks for evidence of delayed growth kinetics by comparing the droplet sizes of activated ambient particles against a “standard” that represents CCN with rapid activation kinetics. The standard we use is ammonium sulfate particles activated at a critical supersaturation equal to that of the instrument (for the same flow/temperature conditions as in the ambient measurements). This standard provides the smallest droplet size possible that corresponds to rapid activation kinetics. Particles with lower critical supersaturation will activate earlier in the instrument and grow to larger sizes than the standard, while particles with higher critical supersaturation will not activate and remain undetected. If ambient particles form droplets smaller than the calibration aerosol over the measured range of supersaturation, the ambient aerosol is said to exhibit kinetic limitations (delayed droplet growth). We apply TDGA on CCN activation data collected for Arctic haze and Canadian boreal forest fires sampled during the 2008 NASA ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) campaign. Within biomass burning plumes, we find evidence of significant kinetic limitations coincident with high organic mass fractions. In the Arctic, where organic mass fractions are lower and the air masses are more aged, we find little to no kinetic limitations. To further explore the influence of organics, we infer an optimum water vapor uptake coefficient by explicitly simulating the observed activated droplet distribution using the measured aerosol size distributions and chemical composition. This water vapor uptake coefficient is then correlated with aerosol properties to gain insight on the mechanisms responsible for creating and controlling CCN activation kinetics.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.A43A0162L
- Keywords:
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- 0305 ATMOSPHERIC COMPOSITION AND STRUCTURE / Aerosols and particles;
- 0320 ATMOSPHERIC COMPOSITION AND STRUCTURE / Cloud physics and chemistry;
- 3311 ATMOSPHERIC PROCESSES / Clouds and aerosols;
- 9315 GEOGRAPHIC LOCATION / Arctic region