A Chamber Investigation of Nitric Acid-Soot Aerosol Chemistry at 298 K |
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| Authors: | R S Disselkamp M A Carpenter J P Cowin |
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| Institution: | (1) Environmental Molecular Sciences Laboratory and Atmospheric Sciences Technical Group, Pacific Northwest National Laboratory, Richland, WA, 99352, U.S.A. |
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| Abstract: | Long-pathlength infrared absorption spectroscopy wasused to investigate nitric acid-soot aerosol chemistryat 298 K and 0.5% relative humidity. Experimentswere performed by introducing nitric acid vapor(PHNO
3  3 Pa, Ptotal 40 kPa) intoateflon-coated chamber and initiating acquisition ofinfrared spectra at 3 minute time intervals. After 36minutes of data collection, soot powder was rapidlyexpanded into nitric acid contained in the chamber togenerate a soot-HNO3 aerosol. Infrared spectracollected before, and after, soot introduction to thechamber were used to characterize chamber wallreaction processes and soot aerosol chemistry,respectively. Three soot types were investigated(Degussa FW2, Cabot Monarch 1000, and crystallinegraphite), each yielding similar chemistry. Upon sootintroduction to the chamber both HNO3 uptake andNO2 production occurred, with the molar ratio ofHNO3 uptake to NO2 production varying from1.2 to 2.9 for the three soot types studied. Unreacted HNO3 was present at the conclusion ofeach of the aerosol experiments, indicating incompleteconversion of HNO3 into NO2. Thisobservation suggested that `active' sites at the sootsurface responsible for the reduction of HNO3 arenot regenerated (i.e., formed) in the reactionprocess. In essence, a titration occurred betweenthese active sites and HNO3. The NO2concentrations produced, the soot mass concentrationsused, and the BET measured specific surface area ofthe powders allowed computation of the surface densityof active sites of 4.0 × 10-18 m2/active site(describing all three powders studied). This is thefirst reported measurement of surface density ofactive sites for nitric acid chemistry on soot. Sinceatmospheric heterogeneous reactions that exhibitsurface deactivation may, in principle, affect tracegas concentration, we perform an assessment in thisregard. |
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| Keywords: | soot aerosol nitric acid |
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