AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Characterization of the Photochemistry in a Potential Aerosol Mass (PAM) Chamber by Modeling and Measurements: Radical Formation, OH Exposure Quantification and Calibration Equation Derivation
RUI LI, Joost de Gouw, William Brune, Brett Palm, Amber Ortega, Weiwei Hu, Douglas Day, Jose-Luis Jimenez, NOAA & CIRES & University of Colorado-Boulder
Abstract Number: 297 Working Group: Instrumentation and Methods
Abstract The Potential Aerosol Mass (PAM) chamber is a highly oxidizing reactor for studying the formation and oxidation of secondary organic aerosols (SOA). The PAM system was designed for a wide range of oxidant exposure times and short experiment duration with reduced wall loss effects, and has been used in many field and laboratory studies on SOA formation. However, the detailed balance of radical formation and destruction under various environmental and input conditions is not completely understood. The uncertainties in radical quantification can significantly affect the interpretation of SOA formation in the reactor. To better characterize the photochemistry and OH radical formation in the PAM chamber, a model has been developed to simulate the formation and evolution of oxidants, which was compared against PAM measurements under a variety of experimental conditions. The PAM experiments were conducted with CO and SO2 as reagents for quantifying OH exposure. The model outputs of OH exposure and ozone concentration were evaluated using measurements and agreed within a factor of 2. A sensitivity study was performed to characterize the dependence of the OH exposure, ozone concentration and HO2/OH radical ratio on photon flux, humidity, temperature, pressure, residence time and external OH reactivity in the PAM chamber. Model results show that OH exposure is strongly suppressed depending on the reactivity of the sampled gases and on the photon flux, and depends more weakly on humidity. An OH exposure calibration equation was derived as a function of ozone, humidity, and external OH reactivity. The OH exposure calculated from the derived calibration equation shows good agreement with those calibrated from several field campaigns, which supports the validity of the model. The quantification of OH exposure and improved understanding of the PAM technique resulting from this work further establishes the viability of this tool for aerosol formation and aging research.