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

Abstract

Abstract. 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 -20øC 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 -5øC. 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.

Exploratory analysis of the distribution of condensation nuclei in the northern hemisphere upper troposphere and lower stratosphere during the late 1970s

“…In many cases these clouds could be in a region of recent upward transport, which in addition to bringing moisture upward, also bring CN up from the surface boundary layer to the upper troposphere. It also is possible that evaporating cloud particles acted to humidify the sample air stream and affect the response of the CN counters or that cloud particles shattering on the inlet walls artificially enhanced CN counts [Weber et aL, 1998]. For these reasons, only cloud-free observations are included in statistics on which our CN analyses are based, with the goal that our results be representative of clear air volumes not recently influenced by relatively rapid vertical transport or high humidity.…”

Out of Africa: High aerosol concentrations in the upper troposphere over Africa

“…There are many reports of artifact signals created at aerosol inlets on aircraft when operating in clouds (e.g., Clarke, et al, 1997;Hudson, 1993;Hudson and Frisbie, 1991;Weber, et al, 1998). Cloud drops shattering and splashing at the edges of the inlets are supposed to create large numbers of fragments that are aspirated, dried, and recorded as small residues in aerosol sensors.…”

Impacts of aerosol particles on the microphysical and radiative properties of stratocumulus clouds over the southeast Pacific Ocean

“…The mean time between in-cloud and below cloud samples was 18.5 min (about 125 km). Drizzle drops can break up within inlets and produce particulate artifacts downstream (Clarke et al, 1997;Weber et al, 1998;Hudson and Frisbie, 1991). For the CVI inlet, mod- eling (Kulkarni and Twohy, 2011) and measurements (Twohy et al, 2003) indicate that drops larger than ∼ 100 µm are prone to break up.…”