Abstract
Abstract. We present two Differential Optical Absorption Spectroscopy (DOAS) instruments built at RIVM: the RIVM DOAS and the miniDOAS. Both instruments provide virtually interference-free measurements of NH3 concentrations in the atmosphere, since they measure over an open path, without suffering from inlet problems or interference problems by ammonium aerosols dissociating on tubes or filters. They measure concentrations up to at least 200 μg m−3, have a fast response, low maintenance demands, and a high up-time. The RIVM DOAS has a high accuracy of typically 0.15 μg m−3 for ammonia for 5-min averages and over a total light path of 100 m. The miniDOAS has been developed for application in measurement networks such as the Dutch National Air Quality Monitoring Network (LML). Compared to the RIVM DOAS it has a similar accuracy, but is significantly reduced in size, costs, and handling complexity. The RIVM DOAS and miniDOAS results showed excellent agreement (R2 = 0.996) during a field measurement campaign in Vredepeel, the Netherlands. This measurement site is located in an agricultural area and is characterized by highly variable, but on average high ammonia concentrations in the air. The RIVM-DOAS and miniDOAS results were compared to the results of the AMOR instrument, a continuous-flow wet denuder system, which is currently used in the LML. Averaged over longer time spans of typically a day, the (mini)DOAS and AMOR results agree reasonably well, although an offset of the AMOR values compared to the (mini)DOAS results exists. On short time scales, the (mini)DOAS shows a faster response and does not show the memory effects due to 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 (or more) systems in a gradient set-up and applying the aerodynamic gradient technique.
Highlights
Excessive deposition of ammonia leads to acidification and eutrophication, which form important threats to biodiversity of nature areas
These measurements are performed for each Differential Optical Absorption Spectroscopy (DOAS) residual is mainly due to photon noise and detector noise
In this paper we described two Differential Optical Absorp-In Fig. 20 we plotted the minute values of the AMOR tion Spectroscopy (DOAS) instruments built at RIVM, the versus those of the miniDOAS and the minute RIVM DOAS and the miniDOAS
Summary
Excessive deposition of ammonia leads to acidification and eutrophication, which form important threats to biodiversity of nature areas. Wyers et al, 1993; Van Putten et al, 1994) Such annular denuder systems are used in the Dutch National Air Quality Monitoring Network (LML) to monitor ammonia concentrations throughout the Netherlands. They are extremely costly in maintenance, i.e. 15 000 C per instrument per annum. Most techniques are sampling techniques and involve direct contact with the highly adhesive ammonia resulting in inlet problems, e.g. causing slow response times and problems with interference by ammonium aerosols dissociating on tubes or filters (e.g. von Bobrutzki et al, 2010) This is a severe disadvantage for process studies.
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