Widening the gap between measurement and modelling of secondary organic aerosol properties?

Abstract

The link between measured sub-saturated hygroscopicity
and cloud activation potential of secondary organic
aerosol particles produced by the chamber photo-oxidation
of α-pinene in the presence or absence of ammonium sulphate
seed aerosol was investigated using two models of
varying complexity. A simple single hygroscopicity parameter
model and a more complex model (incorporating surface
effects) were used to assess the detail required to predict
the cloud condensation nucleus (CCN) activity from the subsaturated water uptake. Sub-saturated water uptake measured by three hygroscopicity tandem differential mobility analyser (HTDMA) instruments was used to determine the water activity for use in the models. The predicted CCN activity was compared to the measured CCN activation potential using a continuous flow CCN counter.
Reconciliation using the more complex model formulation
with measured cloud activation could be achieved widely
different assumed surface tension behavior of the growing
droplet; this was entirely determined by the instrument used
as the source of water activity data. This unreliable derivation of the water activity as a function of solute concentration from sub-saturated hygroscopicity data indicates a limitation in the use of such data in predicting cloud condensation nucleus behavior of particles with a significant organic fraction. Similarly, the ability of the simpler single parameter model to predict cloud activation behaviour was dependent on the instrument
used to measure sub-saturated hygroscopicity and
the relative humidity used to provide the model input. However, agreement was observed for inorganic salt solution particles, which were measured by all instruments in agreement with theory. The difference in HTDMA data from validated and extensively used instruments means that it cannot be stated with certainty the detail required to predict the CCN activity from sub-saturated hygroscopicity. In order to narrow the gap between measurements of hygroscopic growth and CCN activity the processes involved must be understood and the instrumentation extensively quality assured. It is impossible to say from the results presented here due to the differences in HTDMA data whether: i) Surface tension suppression occurs ii)
Bulk to surface partitioning is important iii) The water activity coefficient changes significantly as a function of the solute concentration.