Measuring dry deposition of ammonia using flux-gradient and eddy covariance methods with two novel open-path instruments

Abstract

Abstract. Dry deposition of ammonia (NH3) is the largest contributor to the nitrogen deposition from the atmosphere to soil and vegetation in the Netherlands, causing eutrophication and loss of biodiversity. Yet, data sets of NH3 fluxes are sparse and in general have monthly resolution at best. An important reason for this is that measurement of the NH3 flux under dry conditions is notoriously difficult. There is no technique that can be considered the golden standard for these measurements, which complicates testing of new techniques and judging their quality. Here, we present the results of an intercomparison of two novel measurement setups aimed at measuring dry deposition of NH3 at half-hourly resolution. In a five-week comparison period, we operated two optical open-path techniques side by side at the Ruisdael station in Cabauw, the Netherlands: the novel RIVM-miniDOAS 2.2D using the aerodynamic gradient technique, and the novel commercial Healthy Photon HT8700E using the eddy covariance technique. Both are open-path optical instruments, leaving NH3 in the air during measurement. Otherwise, they are widely different in their measurement principle and approach to derive deposition values from measured concentrations. The two different techniques showed very similar results when the upwind terrain was both homogeneous and free of nearby obstacles (r = 0.87). The observed fluxes varied from a deposition of ~80 ng NH3 m-2 s-1 to an emission of ~140 ng NH3 m-2 s-1. We obtained similar results from two widely different techniques, both in absolute flux values as in their temporal pattern, which substantiated that both instruments were able to measure NH3 fluxes at high temporal resolution for a consecutive period of at least several weeks. However, for wind directions with nearby obstacles, the correlations between the two techniques were weaker. Moreover, the technical performance (e.g., uptime, precision) and practical limitations of both systems were discussed. The uptime of the miniDOAS system reached 100 % once operational, but regular intercalibration of the two instruments was applied in this campaign (35 % of the 7-week uptime). Conversely, the HT8700E did not measure during, and shortly after, rain, and the coating of its mirrors tended to degrade (21 % data loss during the 5-week uptime). In addition, the HT8700E measured NH3 concentrations proved sensitive to air temperature, causing substantial differences (range: -15 to + 6 µg m-3) between the two systems. To conclude, the miniDOAS system appeared ready for long-term hands-off monitoring. The current HT8700E system, on the other hand, had a limited stand-alone operational time under the prevailing weather conditions. However, under the right circumstances, the system can provide sound results, opening good prospects for future versions, also for monitoring applications. The new high temporal resolution data from these instruments can facilitate the study of processes behind NH3 dry deposition, allowing improved understanding of these processes and better parametrization in chemical transport models.