Evaluation of HNO3, SO2, and NH3 in the Surface Tiled Aerosol and Gaseous Exchange (STAGE) option in the Community Multiscale Air Quality Model version 5.3.2 against field-scale, in situ and satellite observations

The Surface Tiled Aerosol and Gaseous Exchange (STAGE) model was developed as a unified system for estimating dry deposition and bidirectional exchange for field-scale applications and for use within the CMAQ v5.3.2 regional scale model. The field-scale model was evaluated against micrometeorological flux measurements of NH 3 , HNO 3 , and SO 2 at a managed grassland site at Duke Forest Blackwood Division, NC (35.58° N, 79.05° W) and NH 3 in a cultivated corn field in Lillington, NC (35.38, 78.78° N). When using data collected at the field-scale for soil and vegetation NH 3 compensation points, modeled fluxes for all species agreed well with the observations, with mean bias within or near the reported measurement uncertainty. However, when using the default CMAQ v5.3.2 values for NH 3 emission potentials for soil (Γ soil =20) and vegetation (Γ apoplast =247) at the Duke Forest grassland site, the model estimated a mean net deposition (−1.3 ng m −2 h −1 ) while a mean NH 3 evasive flux (8.4 ng m −2 h −1 ) was observed. An annual 2016 model simulation of CMAQ v5.3.2 was evaluated against Cross-Track Infrared Sounder (CrIS) satellite NH 3 observations to assess if the box model biases at the field scale where site specific of indicative of more general model biases. The evaluation with CrIS observations shows a broad underestimation of NH 3 concentrations by approximately 1 to 2 ppb in the U.S. Great Plains. This is in general agreement with the results from the grassland field data indicating that there is likely an underestimation of the evasive NH 3 flux in grassland sites in CMAQ due to the model's default tabular values of the vegetation/litter NH4+ concentrations. The sensitivities of the STAGE model to the soil and vegetation emission potentials indicates that regional scale model results for NH 3 can be further improved with additional micrometeorological flux and vegetation and soil chemistry measurements over different land use types, soil types, and vegetation phenological stages.