10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Hygroscopicity Measurements near Cloud Indicate Aerosol-Cloud Processing without Interrupting Ambient Measurements
STEPHEN NOBLE, James Hudson, Desert Research Institute
Abstract Number: 228 Working Group: Clouds and Climate
Abstract Transformations of atmospheric aerosols occur through processes within clouds. Clouds process aerosols in three ways; aqueous oxidation of trace gases within the droplets that add soluble material (chemical); collision and coalescence of droplets to combine soluble material from the aerosol (physical); and Brownian capture of interstitial particles to add soluble or less soluble material (physical). As most droplets evaporate, this leaves an altered particle from the one initially activated. This resulting particle is larger and more easily activates at lower supersaturations in subsequent cloud cycles. This activation improvement impacts cloud droplet number and size, and drizzle amount (Hudson et al., 2015; Hudson et al., 2018), which have implications for climate. Aerosol-cloud processing separates the processed (accumulation mode) and unprocessed (Aitken mode) aerosol. The resulting bimodal distributions have been observed in particle size measurements (Hoppel et al., 1986) and cloud condensation nuclei (CCN) measurements (Hudson et al., 2015). Hygroscopicity (κ) can be determined from particle size and CCN distributions, measured by critical supersaturation. Overlaying particle size and CCN distributions and tuning the κ value to produce agreement gives κ for the distribution. Separate tuning for Aitken (unprocessed) and accumulation (processed) modes provides κ for each mode. Differences in processing type are highlighted by κ differences between the two modes. Two aircraft field projects; a polluted stratus cloud study (MArine Stratus/stratocumulus Experiment, MASE), and a clean summertime cumulus cloud study (Ice in Cloud Experiment-Tropical, ICE-T); provide κ of the two modes. Differential mobility analyzers measured particle size distributions while the Desert Research Institute CCN spectrometer measured CCN distributions in both projects. In ICE-T, 60% of bimodal distributions had the same κ for both modes while in MASE only 26% had the same κ. This indicates collision and coalescence dominating in ICE-T where CCN with similar κ were combined through droplet coalescence. However, in MASE chemical processing via aqueous oxidation dominated because the two modes had different κ for 74% of bimodal distributions. This is consistent with Hudson et al. (2015). Generally, adding hygroscopic material through cloud processing makes processed κ greater than unprocessed κ. However, even when unprocessed κ is high, aqueous oxidation adds material that is less soluble (sulfates added to sea salts) and the resulting processed κ tends towards κ of the added material. This was evident in MASE. Cloud types in these projects appear to affect cloud processing type where larger vertical motions (W) in cumulus clouds create larger droplets that coalesce, while shallow stratus clouds with limited W promote chemical processing. More investigations among cloud types are needed. Here, the evidence suggests both types of processing existed to some extent in both projects. This new method to determine hygroscopicity is continuous; i.e., no interruptions of ambient particle size or CCN measurements; but may not be as exact as other methods. But, this κ measurement allows for investigation of aerosol evolution due to these natural cloud processes while providing information about relative particle hygroscopicity and can be performed after measurements have already been taken; i.e., used on archived data.