Field comparison of two novel open-path instruments that measure dry deposition and emission of ammonia using flux-gradient and eddy covariance methods
Authors: Swart Daan , Zhang Jun , van der Graaf Shelley , Rutledge-Jonker Susanna , Hensen Arjan , Berkhout Stijn , Wintjen Pascal , van der Hoff René , Haaima Marty , Frumau Arnoud , van den Bulk Pim , Schulte Ruben , van Zanten Margreet , van Goethem Thomas 
Published: January 2023

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; however, 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 as the gold
standard for these measurements, which complicates the testing of new
techniques. Here, we present the results of an intercomparison of two novel
measurement set-ups aimed at measuring dry deposition of NH3 at
half hourly resolution. Over a 5-week period, we operated two novel optical
open-path techniques side by side at the Ruisdael station in Cabauw, the
Netherlands: the RIVM-miniDOAS 2.2D using the aerodynamic gradient
technique, and the commercial Healthy Photon HT8700E using the eddy
covariance technique. These instruments are widely different in their
measurement principle and approach to derive deposition values from measured
concentrations; however, both techniques showed very similar results (r=0.87)
and small differences in cumulative fluxes (∼ 10 %) as long
as the upwind terrain was homogeneous and free of nearby obstacles. The
observed fluxes varied from ∼ −80 to ∼ +140 ng NH3 m−2 s−1. Both the absolute flux values and the temporal
patterns were highly similar, which substantiates that both instruments were
able to measure NH3 fluxes at high temporal resolution. However, for
wind directions with obstacles nearby, the correlations between the two
techniques were weaker. The uptime of the miniDOAS system reached 100 %
once operational, but regular intercalibration of the system 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 NH3 concentrations measured by the HT8700E proved sensitive to air
temperature, causing substantial differences (range: −15 to +6 µg m−3) between the two systems. To conclude, the miniDOAS system appears
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 relatively dry and low-dust conditions,
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 an improved understanding of these
processes and better parameterisation in chemical transport models.