A CIMS Technique for Fast Time Response PAN Measurements

We have developed a chemical ionization mass spectrometric (CIMS) technique for detecting atmospheric peroxyacetyl nitrate (PAN) utilizing I^- chemistry. I^- is a very selective reagent ion because it is unreactive with most atmospheric species including ozone, nitric acid, and water. It is synthesized by dissociative electron attachment to CF₃I. Ambient air is sampled into the CIMS through heated 0.375" o.d. PFA Teflon tubing. PAN thermally dissociates in the Teflon tube to form CH₃C(O)O₂ (PA) and NO₂. The CH₃C(O)O₂ reacts with I^- in the CIMS flow tube at the gas kinetic rate [Villalta and Howard, 1996] to form CH₃C(O)O^- and IO, thus allowing for selective and sensitive detection of PAN. Preliminary data suggest that our sensitivity is at least one Hz/pptv of PAN. We routinely observed 1000-2000 Hz of CH₃C(O)O^- in room air this summer in Atlanta with a background of <50 Hz. The background was determined by either cooling the inlet or adding excess NO to the inlet to react away the peroxyacetyl radical before it entered the CIMS. These results indicate that a detection limit of less than 20 pptv for a one second integration period is feasible with this technique. A potential problem with this technique is destruction of the PA radicals via reaction with ambient NO. However, due to a very short effective reaction time in the inlet (5-10 ms), we estimate that reaction of PA with NO decreases the measured PAN level by less than 10% at a NO mixing ratio of 40 ppbv. Complications due to self reaction of the PA radical should also be insignificant for PAN levels less than 40 ppbv. Potential interferences from water, acetone, and other species will be addressed. Correlations of PAN with ozone and NO_x measured in Atlanta will also be presented.