Analyzing Growth, Hygroscopicity and Climate Effects of Atmospheric Particles at the Arm SGP Site
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Particle size and size resolved hygroscopicity distributions measured at the ARM SGP site from 2009 to 2012 were used to study new particle formation (NPF) and subsequent growth, variation in hygroscopicity and mixing state with time of day and year, upscatter fraction (β) and other optical properties, and the factors influencing estimates of the concentration of cloud condensation nuclei (NCCN). New particles were formed frequently between March and October in 2009, but those that formed grew faster from June to August. Usually particles between 12 nm and 22 nm grew faster in the afternoon when the gas phase precursor concentrations and photochemical reaction rates are expected to be higher. A two-component condensable gas model I developed describes the observed pattern, attributing daytime growth to higher concentration of hygroscopic inorganic and organic species and nighttime growth primarily to condensation of organics. Among three evaluated time-spans, the hygroscopicity parameter, k, was highest during the daytime and was relatively higher at night for particles < 100 nm. It was also higher for particles < 100 nm during days and nights on which NPF events were observed. On average, particles at the small and large tails of the measured size range had higher k due to higher inorganic content. Among the 7 particle sizes the minimum hygroscopicity was observed at an intermediate size (~50nm), which is thought to have higher organic content. A similar pattern was observed in fitted and categorized GF distribution modes, with the additional observation of frequently nearly-hydrophobic particle modes throughout the size range. Aerosol mixing state was quantified as the standard deviation (SD) of each GF distribution and a size-dependent threshold SD selected to roughly separate internal and external mixtures. The results show an increased frequency of internal mixtures during the daytime and during the summer when photochemistry and precursor emissions are highest. The effect of aerosols on climate is connected to the fraction of incoming solar radiation scattered into the upward hemisphere and back into space, β, and to NCCN. Calculated boundary layer β was highest around sunset and during winter. Aerosol optical properties including β were parameterized using size and GF distributions, solar position, and RH. NCCN for a supersaturation (S) range of 0.25 to 0.85% was calculated using a base model that incorporates all of the information from the size and GF distributions. The results of those base model estimates were compared to directly measured NCCN, with average fractional errors of 0.21, 0.23, and 0.28 for 2010, 2011, and 2012, respectively. The results were then compared with estimates based on simplified treatment of the aerosol composition and mixing state. Among all simplifications considered, assuming the aerosol is an internal mixture with sizedependent soluble fraction was the best alternative for base model. The NCCN above 0.3% S was also parameterized reasonably well (r² = 0.87) using the integrated particle concentration between 50 and 500 nm and S.
Mahish, Manasi (2017). Analyzing Growth, Hygroscopicity and Climate Effects of Atmospheric Particles at the Arm SGP Site. Doctoral dissertation, Texas A & M University. Available electronically from