AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Quantification of the Catalytic Effect of Nitric Acid on Dehydration of Particulate Cyclic Hemiacetals
APRIL RANNEY, Paul Ziemann, UC Riverside
Abstract Number: 456 Working Group: Aerosol Chemistry
Abstract Dehydration of cyclic hemiacetals in particles formed by OH radical-initiated reactions of alkanes and alkenes has been shown to be catalyzed by nitric acid (HNO$^3). This reaction forms unsaturated dihydrofurans, which can evaporate from particles and react with OH radicals, O$^3, or NO$^3 radicals to form a variety of multifunctional products that can potentially form secondary organic aerosol (SOA). In order to quantify the effects of HNO$^3 on the rate of dehydration of cyclic hemiacetals, a series of environmental chamber experiments were conducted in which particulate cyclic hemiacetals were formed from the OH radical-initiated reaction of n-pentadecane in the absence of NOx and exposed to a range of concentrations of HNO$^3. n-Pentadecane, dioctyl sebacate (DOS) seed particles, various gas phase concentrations of HNO$^3, and O$^3 were added to the chamber and then the reaction was initiated by adding tetramethylethene (TME), which reacted with O$^3 in 2–3 minutes to form OH radicals with approximately unit yield. A particle beam mass spectrometer was used to monitor the rapid formation and much slower decay of cyclic hemiacetals in real-time, with wall loss accounted for using the decay of DOS signal. HNO$^3 was measured using ion chromatography to analyze water extracts of particles sampled onto filters and gas sampled through an impinger. Dehydration rate constants increased linearly with HNO$^3 concentration, as expected for an acid-catalyzed reaction, and the amount of HNO$^3 measured in SOA particles was proportional to the gas-phase concentration of HNO$^3 and the SOA mass concentration, which allowed for an estimate of the Henry’s law constant for HNO$^3 in SOA particles. These two quantities can be used to model the behavior of cyclic hemiacetals in dry SOA particles as well as concentrations of HNO$^3.