10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Cloud Effects on the Aitken and Accumulation Modes
JAMES HUDSON, Stephen Noble, Desert Research Institute
Abstract Number: 937 Working Group: Clouds and Climate
Abstract At SGP correlation coefficients, R, between remotely-sensed cloud fraction, cf, and time-lagged surface aerosol mean diameter (md) increased from zero (simultaneity) to maxima for aerosol ~five hours after cf. Delay was due to transit from cloud altitudes to surface. Peak Rs were greater for instruments with greater sky coverage (pyranometer gridded network [PGN]; total sky imager [TSI]) than ceilometer. But TSI and PGN operated only during daylight. Ceilometer cf-md Rs were also positive during daylight but negative at night. All three instrument peak Rs were also positive for cf-Aitken (< 100 nm; R = 0.54, 0.60 and 0.65; two-tailed probabilities, P2 < 10-8 for ceilometer [daylight], TSI and PGN, respectively) and accumulation mode md (> 100 nm; R = 0.27, 0.42 and 0.38; P2 4x10-5, < 10-8, < 10-8, respectively). Negative peak Rs for cf-Aitken concentration were -0.30, -0.36, -0.35; P2 = 2x10-6, < 10-8, < 10-8, respectively. Positive peak Rs for cf-accumulation concentrations were 0.23, 0.34, 0.34; P2 4x10-4, 3x10-8, 3x10-8, for ceilometer, TSI and PGN, respectively.
Lagged md responses to cf > median cf steadily increased for ~five hours whereas lagged md responses to cf < median cf steadily decreased by similar md for ~five hours. When this was analyzed for consecutive hours of cloudiness or clearness, peak md increased/decreased by greater amounts.
More accumulation mode particles with larger sizes for greater cloudiness is consistent with aqueous oxidation, coalescence and Brownian scavenging by cloud droplets. These cloud processes thus account for the positive 0.47 R (P2 < 10-8) between accumulation mode concentrations and sizes.
Negative Rs for cf-Aitken concentrations indicate more photochemical production for less cloudiness. Positive cf-Aitken size Rs indicate that initial photochemical particles are small. Brownian capture of cloud-interstitial particles also reduces Aitken concentrations, especially smaller Aitken particles because of their greater Brownian motion. This increases sizes of remaining Aitkens. Many Aitkens are converted to accumulation particles. The sizes of some larger Aitken particles are also increased by aqueous chemistry on those that nucleated cloud droplets. All of these processes make the negative R for Aitken concentration versus size (R = -0.42, P2 < 10-8).
Separating Aitken concentrations by each hour of the day and according to above or below mean cf, all three instruments (six plots) show similar concentrations (~2000 cm-3) for the first daylight hours. But after 10 AM concentrations increase by a factor of three for all plots except PGN cf > median cf, which remains below 2000 cm-3 all day. For cases when all three instruments simultaneously report cf > median cf, Aitken concentrations remained below 2000 cm-3 all day. The smaller numbers of cases when ceilometer or TSI cf > median cf but PGN cf < median cf cause the high midday ceilometer and TSI Aitken concentrations for cf > median cf. The consistently low PGN cf > median cf Aitken concentrations indicate that PGN greater sky coverage better represents the cloud conditions that inhibit photochemical Aitken particle production. The more local TSI and ceilometer sky coverages, therefore, misrepresent conditions most favorable for photochemical Aitken particle production.
Six similar hourly plots of accumulation mode concentrations also divided according to median cf are nearly identical for the three instruments; i.e., low accumulation mode concentrations for low cf throughout the day and factor of two higher concentrations for high cf with early afternoon peaks. Thus, the accumulation mode responds more to local cloudiness whereas the Aitken mode responds to more widespread clear skies.