Investigations On High Time-Resolution Optical Sensor For Monitoring Atmospheric Nitrogen Oxides

Nitrogen oxides(NOx),mainly nitrogen dioxide(NO2)and nitric oxide(NO),are very common pollutants in the atmosphere.They have a serious harm to the ecological environment and human’s health.With the improvement of industry and transit mobility,growing amount of NOx have been produced and emitted into the atmosphere by fossil fuels combustion.Hence,high-time resolution monitoring atmospheric NOx is of great importance for improving and controlling air quality.In this study,based on the Griess-Saltzman(G-S)reaction,a portable and fast NOx optical sensor was developed for monitoring NOx in the urban air.NO2 optical sensor was composed of injection system and detection system.Five solenoid valves and a piece of PTFE tube formed gas injection system.The porous polypropylene membrane tube was filled with G-S reagents and covered with a coaxial jacket tube as gas permeation and detection system.Its two ends were respectively fixed with a green light emitting diode for light source and a photoelectric converter for optic signal reception.The operating conditions for monitoring atmospheric NO2 were systemically studied,the effect of the air flow rate and the concentration of G-S reagents for detection of NO2 were analyzed.Under the optimized experimental conditions,a limit of detection of 5.09 ppbv,an intra-day precision of 4.1%,an inter-day precision of 5.7%,and a linearity range extended to 700 ppbv were achieved for NO2 determination of the device with a sample injection of 200 mL.The developed device presented comparable precision to the standard G-S method of China when analyzing outdoor air.Its stability and application potential had also been evaluated by continuously monitoring of NO2 in urban atmosphere.Beyond that,in-suit and real-time monitoring of NO2 in urban air was successfully realized with this device.Additionally,an oxidation device of acid potassium permanganate was fixed in front of the NO2 injection system to form a NO optical detection platform.The influence of the oxidative efficiency of acid potassium permanganate for detection of NO was investigated.Under the optimized experimental conditions,a limit of detection of 1.62 ppbv,a precision of 2.7%and a linearity range extended to 1000 ppbv were achieved for NO determination of the device with a sample injection of 200 mL.The device was used for detection of NO in car exhaust and urban atmosphere and verified its accuracy for detection of NO.The developed NOx optical sensor in this study integrates sample collection and detection,requires less volume of air sample and has a high time resolution characteristic with 15 times per hour.These results indicate that the device is practical and feasible for routine NOx analysis.