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Bias in modeled bi-directional NH3 fluxes associated with temporal averaging of atmospheric NH3 concentrations
Citation:
Walker, Johnt. Bias in modeled bi-directional NH3 fluxes associated with temporal averaging of atmospheric NH3 concentrations. Spring meeting of the National Atmospheric Deposition Program, Madison, WI, April 22 - 25, 2013.
Impact/Purpose:
This presentation examines the fesibility of using time integrated air concentrations, such as measurements from passive monitoring networks, to estimate NH3 fluxes using bi-directional air-surface exchange models run with hourly meteorology.
Description:
Direct flux measurements of NH3 are expensive, time consuming, and require detailed supporting measurements of soil, vegetation, and atmospheric chemistry for interpretation and model parameterization. It is therefore often necessary to infer fluxes by combining measurements of atmospheric concentrations, meteorology, and surface chemistry with a suitable air-surface exchange model. Two-week integrated NH3 air concentrations are currently measured using passive diffusion samplers at ≈ 65 sites across the U.S. as part of the Ammonia Monitoring Network (AMON). Ideally, these measurements could be used to estimate NH3 fluxes in a manner similar to CASTNet, in which deposition is estimated by combining hourly meteorological measurements with weekly integrated air concentrations within the Multi-Layer Model (MLM). The use of time-integrated (temporally averaged) air concentrations may produce error in the modeled NH3 flux due to: covariance between the atmospheric concentration and the flux transfer velocity and non-linearities in the relationships between air concentration and surface conditions that produce bi-directionality in the actual flux on a time-scale of up to several hours. This presentation investigates temporal averaging as a source of error in modeled fluxes using a series of simulations in which temporally aggregated fluxes (seasonal and annual) derived from hourly versus time-integrated air concentrations are compared for a range of atmospheric and surface conditions. Analysis of the ability of the air-surface exchange model to accurately reproduce hourly fluxes by comparison to field data is forthcoming.