AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Constraints on the Parameters Dictating Organic Aerosol Volatility from Dual Thermodenuder Field Measurements in the Southeastern US
PROVAT SAHA, Andrey Khlystov, Andrew Grieshop, North Carolina State University
Abstract Number: 372 Working Group: Air Quality and Climate in the Southeast US: Insights from Recent Measurement Campaigns
Abstract The volatility of organic aerosols (OA) has emerged as a property of primary importance in understanding their source and atmospheric lifecycle. Parameters used to describe ambient OA volatility, such as the distribution of material among lumped components with varying effective saturation concentrations (C*; e.g. in the volatility basis set (VBS) approach) and the associated enthalpies of evaporation (ΔHvap), and evaporation coefficient (alpha) are highly uncertain. However, since OA evaporation is dictated by a large number of independent parameters (C*, ΔHvap, and alpha), it is difficult to constrain all of the volatility parameters with single-dimensional (e.g. thermodenuder temperature) perturbation to the initial equilibrium. Here, we discuss measurements of ambient OA volatility at two sites in the Southeastern US: (i) near Centreville, Alabama, at the main ground site of Southern Oxidant and Aerosol Study (SOAS), in June and July, 2013, and (ii) at the North Carolina State University (NCSU) main campus in October and November, 2013. In these studies, we simultaneously operated two thermodenuders (TDs): one stepping through various temperature settings (40-180 degC) at reasonably high residence time (RT; ~50 s) and another operating isothermally (at 60 or 90 degC), with varying residence time (~1 to 50 s). This dual TD approach provides more insight into OA evaporation kinetics and volatility parameters when coupled with an evaporation kinetics model. Approximately 50-65% OA mass evaporated at 100 degC with RT of 50 s, indicating that much of the OA is semi-volatile under atmospheric conditions; whereas about 10% of OA mass remained at 180 degC, indicating that extremely low volatile material consistently contributes to OA. An optimizing VBS-based mass-transfer model was used to invert observations and derive campaign-average volatility distributions and parameter values. Modeling results suggest that ΔHvap of OA was likely in the range of 100-120 KJ/mol and alpha in the range of 0.1 to 1. Results from these and future analyses enable the direct evaluation of emerging treatments of OA volatility in atmospheric models.