Spurious aerosol measurements when sampling from aircraft in the vicinity of clouds

Extensive airborne measurements of aerosol particles in a pristine marine region were made during the first Aerosol Characterization Experiment (ACE 1) from November 15 to December 14, 1995. During this study, high concentrations of condensation nuclei (CN) were frequently observed both near and within clouds. Near clouds, in the absence of liquid water, Clarke et al. [1998] have reported that high CN levels were from new particle formation by homogeneous nucleation. Here we show, however, that within clouds, elevated CN concentrations were not authentic, but instead a sampling artifact, likely related to fragmentation of cloud drops impacting the aerosol inlet. By themselves, these fragments were often indistinguishable from ambient particles. Spurious CN from fragmenting droplets were observed at temperatures down to roughly -20°C and spanned a broad size range, with diameters down to 3 nm. Comparison of two different sized isokinetic aerosol inlets showed that inlets with smaller openings produce higher droplet fragment concentrations. The mechanism for producing these particles is not completely understood. Although fragmentation appeared to be the primary mechanism, for one instrument, an additional spurious source, correlated with liquid water, was observed when ambient temperatures were below -5°C. These findings show that care must be taken when interpreting airborne aerosol measurements in regions of liquid water. This is particularly pertinent to studies of new particle formation by homogeneous nucleation in the vicinity of clouds.