Ammonia monitoring at ppb level using photoacoustic spectroscopy for environmental application

摘要

A novel photoacoustic (PA) trace-ammonia analyzer for measuring ambient ammonia concentration in the lower ppb concentration range has been developed. It is based on a wavelength-modulated, telecommunication type, room temperature-operated diode laser light source, an acoustically optimized, longitudinal resonator type small volume photoacoustic cell and a compact multi-functional electronics unit, which ensures the fully automatic long-term operation of the system. It has a response time below 2 min, which is achieved by using a photoacoustic cell made of polyvinylidene fluoride (PVDF) and gas-handling tubes made of polyamide 11 (PA-11). A dual wavelength measurement method with optimized measurement wavelengths and laser modulation parameters was introduced, which proved to efficiently suppress cross-sensitivity to other atmospheric components, most importantly to water vapor, while improving the sensitivity of the system. The developed PA system was tested with reference to a continuous-flow denuder system (AMANDA) under both laboratory and simulated field conditions, and it featured highly reliable, fully automatic operation with a detection limit of about 50 ppb of ammonia.