Diode-laser based field deployable continuous-wave differential absorption lidar for atmospheric NO 2 monitoring

A field deployable NO 2 -differential absorption lidar (NO 2 -DIAL) based on the Scheimpflug imaging principle, employing a high -power multimode 450-nm laser diode as the wavelength -tunable light source, has been designed and implemented for remote sensing of NO 2 . The on -resonance and off -resonance wavelengths generated by the multimode laser diode have been optimized through simulation studies to improve the measurement stability and accuracy. An unequal exposure scheme has been designed for capturing the on- and offwavelength backscattering signals to compromise the potential nonlinear effect of the CCD image sensor. Field campaigns have been carried out under various atmospheric conditions on the near -horizontal path to investigate the performance of the NO 2 -DIAL system. The atmospheric parameters reported by two nearby national monitoring stations are used to validate the NO 2 concentration measured by the NO 2 -DIAL system. Experimental results revealed that in most cases, the measured range -resolved NO 2 concentrations were in good agreement with the results reported by ambient air pollution monitoring stations, with a correlation coefficient generally beyond 0.9 for short -period measurements under various atmospheric conditions. The mean bias concentration between the NO 2 concentrations measured by the DIAL system and the pollution monitoring station was generally below 5.5 mu g/m 3 in most cases and could be reduced to 0.9 mu g/m 3 after correcting the aerosol -induced error. Meanwhile, a correlation between the bias concentration and the PM10 concentration has been found out, namely 0.574, indicating that correction of aerosol -induced error is particularly important in polluted atmospheric conditions. The spatiotemporal distribution of the NO 2 concentration has also been obtained in a continuous measurement campaign, which also exhibited good consistency with the monitoring stations. The promising result has demonstrated a great possibility of employing the present NO 2 -DIAL system for fielddeployable atmospheric sensing of NO 2 .