Impacts of Nanoparticle Events on Cloud Condensation Nuclei and Cloud Microphysical

This dissertation examines aerosol properties above a forested site in the Piedmont of central Virginia and evaluates the effects of those aerosols on convective cloud properties using a cloud resolving model. During the measurement period between October and December, 2006, aerosol size distributions and hygroscopicity varied among air mass source regions. In stagnant air masses, aerosol concentrations and surface area were relatively high and hygroscopicity was enhanced in particles with dry diameter less than 0.100 µm (average growth factor increase = 0.05). Aerosol nucleation was a frequent source of new particles in the atmosphere, as inferred from nanoparticle growth events, which were observed on 350f all days. These events occurred preferentially (500f all observed events) in relatively cool, dry air masses (median values: Tair = 6 o C; relative humidity (RH) = 49%) from the northwest, in the climatologically dominant flow regime. Preexisting aerosol surface area (a competing sink for nucleating vapors) were lowest in this relatively clean flow (mean total surface area =142 µm 2 cm -3 ). Temporal trends in aerosol size distributions, concentration, growth rates, and the relationship between solubility and concentration, indirectly suggest that biogenic hydrocarbons had a decreasing influence on aerosol properties in time as canopy defoliation progressed. The observed diurnal cycle in aerosol hygroscopicity had the largest amplitude of any previously reported values. Calculated aerosol soluble mass fraction increased by up to 800etween 0800 local time (LT) and 1700 LT. We observed an association of activation diameter and hygroscopicity of a growing population of recently formed particles. These changes in aerosol solubility were independently confirmed (also for the first time) to affect the aerosol activation diameter 4 (Dp 50 ) into cloud condensation nuclei (CCN). During a case study the CCN activation diameter varied from 40 nm to 57 nm at 0.6upersaturation over the course of one day. A sensitivity analysis using representative aerosol size distributions showed that neglecting this diurnal variation in activation diameter could result in errors in calculated CCN concentrations of up to 3000 particles cm -3 , or an average relative error of 250ver the course of a three-day case study. These unique results show for the first time, to our knowledge, that variations in aerosol composition (as inferred from hygroscopicity) can affect CCN concentrations locally on short time scales, not just over large spatial scales, as has been previously reported. Four scenarios were evaluated in a cloud resolving model to determine the effects of these characteristic changes in aerosol size and solubility on convective cloud development. Novel model simulations were produced by representing aerosol activation into CCN based on field measurements. This was achieved by implementing the appropriate version of Köhler theory and assumed aerosol composition to accurately represent measured aerosol activation diameters. Precipitation was found to increase with increasing CCN concentration. In the low CCN cases, warm rain processes were inhibited when cloud water mass became concentrated at higher levels in the cloud. This reduced cloud vigor because precipitation evaporated as it fell through dry lower levels, which increased static stability. The high CCN cases developed vigorous mixed phase clouds which produced precipitation efficiently through both warm and cold rain processes. The relatively small changes in CCN concentration associated with the increase in aerosol solubility resulted in an increase in accumulated modeled precipitation of 12% after 180 minutes of simulation time. The increase due to the increase in aerosol size associated with a growth event was 93%.

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