, Volume 65, Issue 2-3, pp 111-125
Date: 19 Feb 2011

Evaluation of ultraviolet light-emitting diodes for detection of atmospheric NO2 by photolysis - chemiluminescence

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Commercially available ultraviolet light-emitting diodes (UV-LEDs) are evaluated in the laboratory as a light source for photolysis of atmospheric nitrogen dioxide (NO2) followed by chemiluminescence detection (P-CL) of the resulting nitric oxide (NO). Sensitivity, selectivity, and engineering simplicity of three UV-LED sources are compared. The most powerful source uses two 9-W Nichia LED modules and provides an NO2 photolysis frequency (j) of 8.4 s−1 corresponding to a nine-fold improvement over a 100-W Hg arc lamp; this source provides the most sensitivity, but requires water cooling. A pair of Hamamatsu 0.25-W LED modules provides intermediate sensitivity with photolysis efficiency comparable to a 100-W Hg arc lamp. The Hamamatsu LEDs require no additional cooling and are a simple and inexpensive package, providing sensitivity and stability appropriate for long-term, unattended measurements at remote surface sites. A Droplet Measurement Technologies Blue-Light Converter (BLC) represents the most complete off-the-shelf system available for atmospheric measurements of NO2. The BLC provides the least sensitivity of the LED-based converters, but is less subject to interference from HONO. Implementing the Nichia LEDs into the NOAA WP-3D aircraft P-CL instrument, in conjunction with improved sample gas handling, improves instrument time response by a factor of eight and permits 1-Hz retrieval of ambient NO and NO2 using a single detector simply by modulating the LEDs on and off. The performance and stability of the Nichia LEDs are further assessed during the CalNex (California Research at the Nexus of Air Quality and Climate Change) field study in May and June of 2010.