Development of an Atmospheric Pressure Laser Induced Fluorimeter (AP-LIF) for NO₂ and Application of AP-LIF for Study of Heterogeneous NO₂ Chemistry

摘要

Nitrogen dioxide (NO₂) is a pollutant of interest for study both because of its controlling role in the oxidant capacity of the atmosphere and the health risks it poses. Concerns about the health effects of NO₂ and its role in forming deleterious atmospheric species have made it desirable to have low-cost, sensitive ambient measurements of NO₂. A continuous-wave laser-diode laser-induced fluorescence (LIF) system for NO₂ was developed here which operates at ambient pressure, thereby eliminating the need for an expensive pumping system. The current prototype system has achieved sensitivity several orders of magnitude beyond previous efforts at ambient pressure (limit of detection of 2 ppb, 60 s averaging time). Ambient measurements of NO₂ were made in Portland, Oregon using both the standard NO₂ chemiluminescence method and the LIF instrument and showed good agreement (r² = 0.92). In addition, investigations into surface mediated chemistry involving oxides of nitrogen (namely, NO₂) have stimulated new inquiry into potential heterogeneous sources of NO₂ as well as challenged the stability of permanent sinks for NO₂. The possibility that surface mediated chemistry plays a significant role in NOy chemistry in urban air has for the past few decades received considerable attention. The AP-LIF NO₂ instrument is uniquely suited to measure surface chemistry under near ambient conditions. The so called u27renoxificationu27 reaction of gaseous NO with surface bound HNO₃ yielding NO₂ (2HNO₃(surface) + NO--u3e 3NO₂ +H₂O(surface)) was suggested as a potentially important source of NO₂ which also degraded the stability of nitric acid as a sink of active oxides of nitrogen. Yet, there is disagreement in the literature as to the importance of this reaction. The disagreement stems from differing measurements of the rate for the renoxification reaction. Because there are differences in experimental setups no one research group has studied the renoxification reaction under ambient conditions, i.e., at moderate concentrations of NOy and in a static cell held at 1 atm. In this work, the production of NO₂ was measured using a novel AP-LIF. This setup made it possible to measure the rate of production of NO₂ due to the heterogeneous reaction of NO with HNO₃ under ambient conditions. Under these conditions it was found that renoxification due to gas-phase NO on surface HNO₃ is not a significant source of NO₂. However, this study did show the importance of water vapor in the renoxification of surface HNO₃.