Evolution of boundary-layer aerosol particles due to in-cloud chemical reactions during the 2nd Lagrangian experiment of ACE-2

The second Aerosol Characterisation Experiment (ACE-2) was aimed at investi gating the physical, chemical and radiative properties of aerosol and their evolution in the North Atlantic region. In the 2nd "Lagrangian" experiment , an air mass was tracked over a 30-h period during conditions of extensive stratocumulus cover. Boundary-layer measurements of the aerosol size distr ibution obtained with a passive cavity aerosol spectrometer probe (PCASP) d uring the experiment show a gradual growth in size of particles in the 0.1- 0.2 mu m diameter mode. Simultaneously, SO2 concentrations were found to de crease sharply from 800 to 20 ppt. The fraction of sulphate in aerosol ioni c mass increased from 0.68 +/- 0.07 to 0.82 +/- 0.09 for small particles (d iameter below 1.7 mu m) and from 0.21 +/- 0.04 to 0.34 +/- 0.03 for large p articles (diameter above 1.7 mu m). The measurements were compared with a m ulticyclic parcel model of gas phase diffusion into cloud droplets and aque ous phase chemical reactions. The model was able to broadly reproduce the o bserved transformation in the aerosol spectra and the timescale for the tra nsformation of SO2 to sulphate aerosol. The modelled SO2 concentration in t he boundary layer fell to below half its initial value over a 6.5-h time pe riod due to a combination of the entrainment of cleaner tropospheric air an d cloud chemical reactions. NH3 and HCl gas were also found to play an impo rtant role in cloud processing in the model.