Sensitivity of simulated ammonia fluxes in Rocky Mountain National Park to measurement time resolution and meteorological inputs

Gaseous ammonia ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mtext>NH</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:math> ) is an important precursor for secondary aerosol formation and contributes to reactive nitrogen deposition. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mtext>NH</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:math> dry deposition is poorly quantified due to the complex bidirectional nature of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mtext>NH</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:math> atmosphere-surface exchange and lack of high time-resolution in situ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mtext>NH</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:math> concentration and meteorological measurements. To better quantify <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mtext>NH</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:math> dry deposition, measurements of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mtext>NH</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:math> were made above a subalpine forest canopy in Rocky Mountain National Park (RMNP) and used with in situ micrometeorology to simulate bidirectional fluxes. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mtext>NH</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:math> dry deposition was largest during the summer, with 47 % of annual net <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mtext>NH</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:math> dry deposition occurring in June, July, and August. Because in situ, high time-resolution concentration and meteorological data are often unavailable, the impacts on estimated deposition from utilizing more commonly available biweekly <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mtext>NH</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:math> measurements and ERA5 meteorology were evaluated. Fluxes simulated with biweekly <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mtext>NH</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:math> concentrations, commonly available from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mtext>NH</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:math> monitoring networks, underestimated <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mtext>NH</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:math> dry deposition by 45 %. These fluxes were strongly correlated with 30 min fluxes integrated to a biweekly basis ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msup><mml:mi>R</mml:mi> <mml:mn>2</mml:mn></mml:msup> <mml:mo>=</mml:mo> <mml:mn>0.88</mml:mn></mml:mrow> </mml:math> ), indicating that a correction factor could be applied to mitigate the observed bias. Application of an average <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mtext>NH</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:math> diel concentration pattern to the biweekly <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mtext>NH</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:math> concentration data removed the observed low bias. Annual <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msub><mml:mtext>NH</mml:mtext> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:math> dry deposition from fluxes simulated with reanalysis meteorological inputs exceeded simulations using in situ meteorology measurements by a factor of 2.