NO<sub>2</sub> measurement using fiber coupled broadband LED source combining a Herriott multi-pass cell

Abstract

An NO<sub>2</sub> sensor based on a fiber coupled broadband LED source with a Herriott multi-pass cell is developed and demonstrated for calibrating aerosol absorption photoacoustic (PA) spectrometer. At first, a Herriott multi-pass cell with an effective optical path of 26.1 m is designed based on the theory of light transmission, for increasing the light absorption of NO<sub>2</sub> in the cell. It is difficult to obtain a high-quality beam of LED by using the conventional collimating method that enables the collimated output beam to transmit back and forth in the multi-pass cell, due to the large emitting surface and divergence angle of the LED. So, in the present work, the emission of the LED is coupled into a single model fiber, and then collimated by using a lens. The LED spectrum does not change before and after fiber coupling. The collimated beam with a central wavelength of 438.5 nm is coupled into the multi-pass cell. The output beam passing through the multi-pass cell is detected by using a spectrometer for retrieving the NO<sub>2</sub> concentration. Finally, an expected concentration detection limit of 1 μmol/mol (3<i>σ</i>) is achieved within 1 s acquisition time and the signal-to-noise ratio (SNR) is 40. By analyzing the result measured with 46 μmol/mol NO<sub>2</sub>, a measurement precision of 0.1% is achieved. In order to calibrate the aerosol absorption PA spectrometer, the NO<sub>2</sub> sensor and the aerosol absorption PA spectrometer based on 450 nm are used to measure different concentrations of NO<sub>2</sub>, simultaneously. The results show that the measured NO<sub>2</sub> concentration has a good linear relationship with the PA spectrum signal, and the linearity is better than 99.9%. This good linear relationship further shows the feasibility and reliability of the NO<sub>2</sub> sensor. The slope of calibration curve after normalizing the power is 0.95 nV/(mW·Mm<sup>-1</sup>). Using this calibration result, the PA signal measured with aerosol absorption PA spectrometer is transformed into the absorption coefficient. The developed NO<sub>2</sub> measuring system based on a broadband LED light source and Herriott multi-pass cell has the advantages of low cost, simple structure and easy use. It can be used to calibrate the aerosol absorption PA spectrometer, and also to measure NO<sub>2</sub> in industry.