8.3 EPA-developed Non-agricultural Land NH3 Emissions

8.3.1 Methodology Overview

Since the 2020 NEI, EPA has developed an approach for estimating biogenic non-agricultural land NH3 emissions from soils and vegetation constrained with recent flux observations from EPA-ORD measurement programs [ref 11, 12]. These measurements indicated that NH3 emissions from biogenic sources were underestimated by the bidirectional exchange parameterization in CMAQ v5.3 and earlier. This was rectified in CMAQ v5.4 through the incorporation of revised NH3 emission potentials, which were based on soil and vegetation surveys at some Ammonia Monitoring Network (AMoN) sites and soil and vegetation NH4+ observations in the global TRY plant trait database [ref 13]. The ground and vegetation emission potentials for non-agricultural lands may be found in the “STAGE_LU_DATA” table of STAGE’s CMAQ control namelist (CMAQ_Control_STAGE.nml) and are listed in Table 8.3. Emission potentials represent the ratio of leaf or soil surface NH4+ and H+ and govern the emissions of NH3 from aqueous media following Henry’s Law [ref 14, 15]. The emission potentials and the approach to calculating NH3 emissions from biogenic sources in 2023 are consistent with the 2020 NEI but is tabulated separately with a unique SCC.

Table 8.3: Ground and vegetation NH3 emission potentials for non-agricultural lands.
Land use category Ground NH3 emission potential (mol mol-1) Vegetation NH3 emission potential (mol mol-1)
Water 0 0
Ice 438 797
Barren 438 797
Urban 0 0
Grasslands 438 797
Agriculture* 438 797
Shrub 52 81
Evergreen Needleleaf 13 18
Evergreen Broadleaf 46 71
Deciduous Needleleaf 67 107
Deciduous Broadleaf 198 340
  • The agricultural NH3 flux uses soil ammonium simulated with FEST-C.

The approach to estimate 2023 non-agricultural land emissions of NH3 is illustrated in Figure 8.4 and consists of these steps:

  • CMAQ v5.4 was modified to output non-agricultural land biogenic NH3 emissions separately from agricultural land emissions.
  • Run CMAQ v 5.4 model with the Surface Tiled Aerosol and Gaseous Exchange (STAGE) deposition option with bidirectional (“bidi”) NH3 exchange to generate gaseous biogenic ammonia NH3 emission estimates.
  • Calculate the month- and county-specific total non-agricultural land biogenic NH3 emissions (short tons)
  • Assign the non-agricultural land biogenic NH3 emissions from soil and vegetation to SCC: 2803001000
This figure shows the simplified CMAQ system flow of operations in estimating non-agricultural land biogenic NH3 emissions. Note that the role of deposition as a source of soil nitrogen is not currently represented due to the lack of a FEST-C-like biogeochemical model for non-agricultural land uses

Figure 8.4: Simplified CMAQ system flow of operations in estimating non-agricultural land biogenic NH3 emissions. Note that the role of deposition as a source of soil nitrogen is not currently represented due to the lack of a FEST-C-like biogeochemical model for non-agricultural land uses

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