92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Tuesday, 24 January 2012: 2:30 PM
An Annual Cycle of Size-Resolved Aerosol Hygroscopicity At a Forested Site in Colorado
Room 244 (New Orleans Convention Center )
Ezra J.T. Levin, Colorado State University, Fort Collins, CO; and A. J. Prenni, M. D. Petters, S. M. Kreidenweis, R. C. Sullivan, S. A. Atwood, J. Ortega, P. J. DeMott, and J. N. Smith

The Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics & Nitrogen (BEACHON) program seeks to understand the interactions and feedbacks among the atmosphere, biosphere and hydrosphere through the carbon and water cycles. Biogenic secondary organic aerosol (BSOA) particles are key components in these processes as they can affect cloud and precipitation formation and characteristics by acting as liquid and ice cloud nuclei. To better understand the cloud condensation nuclei (CCN) properties of biogenic aerosol we conducted one year of size-resolved CCN measurements in the Manitou Experimental Forest in the Colorado Front Range mountains, as part of the larger BEACHON program. Based on prior studies, the aerosol at this site is expected to have a large BSOA component. From our measurements we were able to determine the aerosol hygroscopicity parameter, κ, at five water supersaturations (s) between ~0.14% and ~0.97%. The average κ value over the entire study and all s was κ = 0.16 ± 0.08, similar to values reported for aerosols observed in organic dominated boreal and tropical forest regions. We also observed some change in hygroscopicity as a function of s. Study average κ values decreasing from κ = 0.22 at s = 0.14 to κ = 0.13 at s = 0.97, suggesting a change in aerosol composition with decreasing dry diameter. We also observed seasonal changes in aerosol properties. During the summer months there was a large increase in CCN concentrations and activated particle number fraction, but a small decrease in hygroscopicity at the lower s measurements. Small particle events, indicative of new particle formation at or upwind of our site, were observed throughout the study period, especially in the summer, and were associated with large increases in CCN concentration. Due to the low hygroscopicity observed during these events, it is likely that these particles are predominantly composed of organic compounds.

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