Size-resolved cloud condensation nuclei activity of aerosol particles sampled above the mixing layer

Cloud condensation nuclei (CCN) is a key factor in the formation and evolution of clouds and thereby the climate. An investigation on particle number size distributions (PNSDs) and CCN properties above the mixing layer (ML) was carried out, using a specifically designed sampler attached to a tethered balloon system. Aerosol microphysical and CCN properties above the ML differed distinctly from that near the ground. During the campaign, the average total particle number concentration (N-p) near the ground (6200 +/- 1700 cm(-3)) was found roughly twice that above the ML (2800 +/- 1200 cm(-3)). The hygroscopicity parameter (kappa), derived from size-resolved CCN measurements, ranged from 0.22 to 0.37 above the ML with an average of 0.29 +/- 0.04. While near the ground, where particle properties were more directly and readily affected by various emission sources, aging and deposition than those above the ML, a larger variability in kappa (from 0.17 to 0.47) was observed, with an average of 0.25 +/- 0.08. The analysis regarding one haze process and one fog process revealed that kappa above the ML stayed relatively stable, in spite of the simultaneous large variability in the surface kappa. This observational study implies that the accurate and detailed chemical composition of aerosol particles, though quite important to investigate aerosol hygroscopicity within the ML, might not be highly demanded for characterizing the variability of aerosol hygroscopicity above the ML. [GRAPHICS]