Standoff sub-ppb level measurement of atmospheric ammonia with calibration-free wavelength modulation spectroscopy

Ammonia (NH₃) plays a significant role in the formation of atmospheric particulate matter, and influences on environmental and public health as well as climate change. Thus, it is important to sensitive measurement of atmospheric NH₃. In the present work, a sub-ppb level standoff open-path NH₃ sensor was developed for on line, sensitive measurement of atmospheric NH₃. A 9.06 μm distributed feedback quantum cascade laser was employed to probe the ammonia absorption lines located on fundamental rotational-vibrational absorption band and calibration-free wavelength modulation spectroscopy technique was employed to retrieve NH₃ concentration directly. The standoff open-path NH₃ sensor performance was investigated in laboratory corridor with 80 m open path length (Hefei, China) and a minimum detection limit of 0.46 ppb (3σ) was obtained. Finally, field campaign measurement was carried out in a winter wheat farmland (Changshu, China). Field measurement shown that the concentration of NH₃ varies from 7 ppb to 30 ppb with an average of 14 ppb. The developed standoff sensor has high potential to be a robust tool for monitoring atmospheric NH₃ or study of regional ammonia emissions in farmland or feedlot scale.