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Model-measurement comparison of ammonia bi-directional air-surface exchange fluxes over agricultural fields
Citation:
He, Z., L. P. Wright, AND L. Zhang. Model-measurement comparison of ammonia bi-directional air-surface exchange fluxes over agricultural fields. Atmospheric and Climate Sciences. Scientific Research Publishing, Inc., Irvine, CA, 3(4):465-474, (2013).
Impact/Purpose:
This manuscript described the evaluation and improvement of an ammonia air-surface exchange model to be used by Environment Canada within a regional air quality model.
Description:
Modeling of the bi-directional fluxes (BDFs) of ammonia (NH3) over fertilized soybean and corn canopies was evaluated for three intensive sampling periods: the first, during the summer of 2002 in Warsaw, North Carolina (NC), USA; and the second and third during the summer of 2007 in Lillington, NC. The data, grouped for dry and non-dry conditions, are summarized. Comparisons with measurements were focused on daytime (07:00-19:00) and nighttime (19:00-07:00) conditions and grouped according to measured meteorological variables such as relative humidity, leaf wetness conditions, and rain. Compared with the original dry deposition model (non-BDF), the new BDF model shows significant improvement in capturing the daytime flux profile. For example, the average modeled daytime flux increased from -48.4 ng m-2 s-1 to 2.15 ng m-2 s-1, which more closely resembles the measured flux of -2.1 ng m-2 s-1 at Warsaw; and from -7.6 ng m-2 s-1 to 67.67 ng m-2 s-1, corresponding to the measured flux of 67.5 ng m-2 s-1 from the second sampling period at Lillington. Sensitivity testing based on agricultural growth status and the soil ground emission potential was also conducted for the first sampling period in Lillington to determine the factors that impact the modeling underestimation. The ground emission potential was found to have the most significant effect on the predicted flux. An appropriate g could produce modeling results comparable to measurements. For the first sampling period, the overall mean daytime flux from the sensitivity tests using the BDF model increased from 62.5 ng m-2 s-1 to 741.4 ng m-2 s-1, which is significantly closer to the measured value of 789.6 ng m-2 s-1.
