Two instruments based on differential optical absorption spectroscopy (DOAS) to measure accurate ammonia concentrations in the atmosphere

摘要

We present two Differential Optical Absorption Spectroscopy (DOAS)instruments built at RIVM: the RIVM DOAS and the miniDOAS. Both instrumentsprovide virtually interference-free measurements of NH₃ concentrationsin the atmosphere, since they measure over an open path, without sufferingfrom inlet problems or interference problems by ammonium aerosolsdissociating on tubes or filters. They measure concentrations up to at least200 μg m?3, have a fast response, low maintenance demands,and a high up-time. The RIVM DOAS has a high accuracy of typically0.15 μg m?3 for ammonia for 5-min averages and over a totallight path of 100 m. The miniDOAS has been developed for application inmeasurement networks such as the Dutch National Air Quality MonitoringNetwork (LML). Compared to the RIVM DOAS it has a similar accuracy, but issignificantly reduced in size, costs, and handling complexity. The RIVM DOASand miniDOAS results showed excellent agreement (R2 = 0.996) during afield measurement campaign in Vredepeel, the Netherlands. This measurementsite is located in an agricultural area and is characterized by highlyvariable, but on average high ammonia concentrations in the air. TheRIVM-DOAS and miniDOAS results were compared to the results of the AMORinstrument, a continuous-flow wet denuder system, which is currently used inthe LML. Averaged over longer time spans of typically a day, the (mini)DOASand AMOR results agree reasonably well, although an offset of the AMOR valuescompared to the (mini)DOAS results exists. On short time scales, the(mini)DOAS shows a faster response and does not show the memory effects dueto inlet tubing and transport of absorption fluids encountered by the AMOR.Due to its high accuracy, high uptime, low maintenance and its open path, the(mini)DOAS shows a good potential for flux measurements by using two (ormore) systems in a gradient set-up and applying the aerodynamicgradient technique.