Field comparison of two novel open-path instruments that measure dry deposition and emission of ammonia using flux-gradient and eddy covariance methods

[摘要] Dry deposition of ammonia (NH 3 ) is the largestcontributor to the nitrogen deposition from the atmosphere to soil andvegetation in the Netherlands, causing eutrophication and loss ofbiodiversity; however, data sets of NH 3 fluxes are sparse and in generalhave monthly resolution at best. An important reason for this is thatmeasurement of the NH 3 flux under dry conditions is notoriouslydifficult. There is no technique that can be considered as the goldstandard for these measurements, which complicates the testing of newtechniques. Here, we present the results of an intercomparison of two novelmeasurement set-ups aimed at measuring dry deposition of NH 3 athalf hourly resolution. Over a 5-week period, we operated two novel opticalopen-path techniques side by side at the Ruisdael station in Cabauw, theNetherlands: the RIVM-miniDOAS 2.2D using the aerodynamic gradienttechnique, and the commercial Healthy Photon HT8700E using the eddycovariance technique. These instruments are widely different in theirmeasurement principle and approach to derive deposition values from measuredconcentrations; however, both techniques showed very similar results ( r =0.87 )and small differences in cumulative fluxes ( ∼  10 %) as longas the upwind terrain was homogeneous and free of nearby obstacles. Theobserved fluxes varied from ∼   −80 to ∼   + 140 ng NH 3  m −2  s −1 . Both the absolute flux values and the temporalpatterns were highly similar, which substantiates that both instruments wereable to measure NH 3 fluxes at high temporal resolution. However, forwind directions with obstacles nearby, the correlations between the twotechniques were weaker. The uptime of the miniDOAS system reached 100 %once operational, but regular intercalibration of the system was applied inthis campaign (35 % of the 7-week uptime). Conversely, the HT8700E did notmeasure during and shortly after rain, and the coating of its mirrorstended to degrade (21 % data loss during the 5-week uptime). In addition,the NH 3 concentrations measured by the HT8700E proved sensitive to airtemperature, causing substantial differences (range: −15 to +6   µ g m −3 ) between the two systems. To conclude, the miniDOAS system appearsready for long-term hands-off monitoring. The current HT8700E system, on theother hand, had a limited stand-alone operational time under the prevailingweather conditions. However, under relatively dry and low-dust conditions,the system can provide sound results, opening good prospects for futureversions, also for monitoring applications. The new high temporal resolutiondata from these instruments can facilitate the study of processes behindNH 3 dry deposition, allowing an improved understanding of theseprocesses and better parameterisation in chemical transport models.

[学科分类] 内科医学