Insight into particle growth rates and controls from two years of SGP SMPS and HTDMA data

Poster PDF

Description

Data from the SGP scanning mobility particle sizer (SMPS) and humidified tandem differential mobility analyzer (HTDMA) have been used to study the controls on growth of recently formed particles. New particle growth events during 2009 and 2011 that spanned at least several hours were identified and both the particle size distributions and size-resolved hygroscopicity distributions from each episode were fitted with lognormals. Time series profiles of the resulting growing mode median particle diameters and hygroscopic growth factors were then fitted using appropriate mathematical forms and their time-dependent slopes used to quantify the rates of change of particle size and hygroscopicity, reflecting the overall addition of mass and the net properties of that mass, respectively. These changes were also translated into those of the hygroscopicity parameter, kappa, to better link with other datasets and to more easily partition the contributions of generic aerosol components to the observed growth. After subtracting the change attributable to coagulation, the remaining growth was assumed to result from condensation of a more hygroscopic inorganic component and a less hygroscopic organic component having a kappa estimated from available aerosol chemical speciation monitor (ACSM) data describing the PM1 aerosol organics. The rates of addition of those two condensable components were related to requisite gas phase concentrations to better separate the hourly, daily, and seasonal variations in responsible atmospheric chemistry of interest from interference from the quantifiable size-dependence of the growth rate. Both the frequency of the observed growth events and the particle growth rates during them have summer maxima and winter minima. The growth profiles of the tracked aerosol modes typically exhibit a decreasing growth rate with increasing size (e.g., 8.5 nm/hr for 20 nm and smaller, 5.6 nm/hr for 20-30 nm, 4.1 nm/hr for 30-40 nm during summer, 2009), which differs from reported observations at many other locations. Partitioning of the more and less hygroscopic condensing species often varies during an event, with the more hygroscopic component contributing relatively more during the daytime. Both representative case studies and averaged profiles will be presented.