Forest–atmosphere exchange of reactive nitrogen in a remote region – Part I: Measuring temporal dynamics

[摘要] Long-term dry deposition flux measurements of reactive nitrogen based on theeddy covariance or the aerodynamic gradient method are scarce. Due to thelarge diversity of reactive nitrogen compounds and high technical requirementsfor the measuring devices, simultaneous measurements of individual reactivenitrogen compounds are not affordable. Hence, we examined the exchangepatterns of total reactive nitrogen ( ΣN r ) anddetermined annual dry deposition budgets based on measured data at a mixedforest exposed to low air pollution levels located in the Bavarian ForestNational Park (NPBW), Germany. Flux measurements of ΣN r were carried out with the Total ReactiveAtmospheric Nitrogen Converter (TRANC) coupled to a chemiluminescencedetector (CLD) for 2.5 years. The average ΣN r concentration was 3.1  µg N m −3 . Denuder measurements with DELTA samplers and chemiluminescencemeasurements of nitrogen oxides (NO x ) have shown that NO x has thehighest contribution to ΣN r ( ∼51.4 % ), followed by ammonia ( NH 3 ) ( ∼20.0 % ),ammonium ( NH 4 + ) ( ∼15.3 % ), nitrate NO 3 - ( ∼7.0 % ), and nitric acid ( HNO 3 ) ( ∼6.3 % ). Onlyslight seasonal changes were found in the ΣN r concentration level, whereas a seasonal pattern was observed for thecontribution of NH 3 and NO x . NH 3 showed highestcontributions to ΣN r in spring and summer,NO x in autumn and winter. We observed deposition fluxes at the measurement site with median fluxesranging from − 15 to − 5  ng N m - 2 s - 1 (negative fluxesindicate deposition). Median deposition velocities ranged from 0.2 to0.5  cm s −1 . In general, highest deposition velocities wererecorded during high solar radiation, in particular from May to September. Ourresults suggest that seasonal changes in composition of ΣN r , global radiation ( R g ), andother drivers correlated with R g were most likely influencingthe deposition velocity ( v d ). We found that from May toSeptember higher temperatures, lower relative humidity, and dry leaf surfacesincrease v d of ΣN r . At themeasurement site, ΣN r concentration did notemerge as a driver for the ΣN r v d . No significant influence of temperature, humidity, friction velocity, or windspeed on ΣN r fluxes when using themean-diurnal-variation (MDV) approach for filling gaps of up to 5 days wasfound. Remaining gaps were replaced by a monthly average of the specifichalf-hourly value. From June 2016 to May 2017 and June 2017 to May 2018, weestimated dry deposition sums of 3.8 and 4.0  kg N ha - 1 a - 1 ,respectively. Adding results from the wet deposition measurements, wedetermined 12.2 and 10.9  kg N ha - 1 a - 1 as total nitrogendeposition in the 2 years of observation. This work encompasses (one of) the first long-term flux measurements of ΣN r using novel measurements techniques forestimating annual nitrogen dry deposition to a remote forest ecosystem.

[学科分类] 大气科学