Using CCN and Particle Size Distributions to Observe Modifications to CCN Hygroscopicity from Aerosol–Cloud Processing

Modification of natural or anthropogenic atmospheric aerosols occur in natural 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 the residual material as an aerosol altered from initial activation. The resulting new aerosol is larger and more easily activates at lower supersaturations. This activation improvement has impacts on cloud droplet number and size, and drizzle amount (Hudson et al., 2015; Hudson et al., 2017; Hudson and Noble, 2018, this conference), which have implications for the indirect aerosol effect and climate. This process creates a separation between the processed (accumulation mode) and unprocessed (Aitken mode) aerosol creating bimodal distributions. These bimodal distributions have been observed in particle size measurements (Hoppel et al., 1986) and cloud condensation nuclei (CCN) measurements (Hudson et al., 2015). The size and CCN distributions can be compared when hygroscopicity (κ) is used to convert particle size to critical supersaturation, which is used in CCN distribution measurements. Overlaying particle size and CCN distributions and tuning the κ value to ensure agreement gives κ for the distribution. Hygroscopicity is often tuned separately for the Atiken (unprocessed) and accumulation (processed) modes. Differences in κ between the two modes reveal information about cloud processing type. Results from two different 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); highlight these different processes and differences in κ. Particle size distributions were measured by differential mobility analyzers and CCN distributions were measured by the Desert Research Institute CCN spectrometer in both projects. In ICE-T, 53% of bimodal distributions had the same κ for both modes while in MASE only 41% had the same κ. This indicates dominance of collision and coalescence processing in ICE-T where similarly hygroscopic CCN are combined from droplet coalescence. However, in MASE where 59% of the bimodal distributions had different κ for the two modes, chemical processing via aqueous oxidation dominated. This is consistent with Hudson et al. (2015). Figure 1 also points to chemical processing in MASE and coalescence in ICE-T. In MASE, hygroscopicity is larger for processed κ between 0.35-0.75 (Fig. 1A). The processed mode had an influx of soluble material from chemical processing which increased κ versus the unprocessed κ. Above 0.75 processed κ is lower than unprocessed κ (Fig. 1A). When unprocessed κ is high and sulfate material is added, processed κ tends towards κ of the added material. Thus, processed κ is reduced by additional material that is less soluble than the original material. In ICE-T, with the exception of the high unprocessed κ, processed κ does not vary much from unprocessed κ (Fig. 1B). This is consistent with coalescence processing. 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. However, more investigation between cloud types is needed. Hygroscopicity determined through this new analysis method is continuous; i.e., no interruptions of ambient particle size or CCN measurements. Therefore, this κ measurement allows for investigation of aerosol evolution due to these natural cloud processes while providing information about particle hygroscopicity.

Hoppel, W.A., G.M. Frick, and R.E. Larson (1986), Geophys. Res. Lett., 13, 125-128.

Hudson, J.G., S. Noble, and S. Tabor (2015), J. Geophys. Res. Atmos., 120, 3436–3452.

Hudson, J.G., S.R. Noble, and S. Tabor (2017), Submitted, J. Geophys. Res. Atmos..

Hudson, J.G., and S.R. Noble (2018), AMS Annual Meeting.