Simultaneous open-path determination of road side mono-nitrogen oxides employing mid-IR laser spectroscopy

Abstract

Within this work we present the application of a new, portable open-path measurement system for the simultaneous determination of NO and NO2 in the atmosphere. The system is based on two pulsed distributed feedback mid-IR quantum cascade laser and a fast thermo-electrically cooled mercury–cadmium-telluride detector. Limits of detection (LoD) below 1 μgm−3 are achieved for both analytes during one minute measurement time and using an optical path-length of up to 428 m. An accuracy below 10 ngm−3 and a precision below 0.76 μgm−3 could be calculated based on minute mean values for 100 m path-length. Reducing the measurement time to one second LoDs of approximately 7 μgm−3 are obtained. During the 300 ns laser pulses micro-spectra for NO and NO2, each of typically 1.2 cm−1 width and a spectral resolution of 0.02 cm−1, are recorded and evaluated. The chosen rotation-vibrational doublets are located at approximately 1900 cm−1 for NO and 1630 cm−1 for NO2. The obtained results show good correlation to the reference method based on chemiluminescence. A particular advantage of the new method is that it provides real time information on the existing NO/NO2 ratio in the measured air. This in turn allows distinguishing between the different emission sources and is demonstrated here by data obtained from different vehicles passing close to the measurement path. These events temporally increased the NO and NO2 concentrations at characteristic ratios from the background values.