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

Abstract

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 lowcost, 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, NOy) 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 ‘renoxification’ reaction of gaseous NO with surface bound HNO3 yielding NO2 (2HNO3(surface) + NO −→ 3NO2 + H2O(surface)) was