Work Description

Title: Sensitivity of modeled soil NOx emissions to soil moisture: Data

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Creator/Author: Daniel Huber

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Contact Information: dehuber@umich.edu

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Abstract:

Nitrogen oxides (NOx = NO + NO2) are reactive nitrogen gases that have a direct negative impact on health and
air quality. While most NOx in the United States derives from fossil fuel combustion, a substantial fraction is
released into the atmosphere by microbial processes in the soil. One frequently used model parameterization estimates
soil NOx emissions from environmental factors including soil moisture, with a spatially constant relationship between
soil NOx and soil moisture despite most regions exhibiting spatially varying soil and ecosystem characteristics. We
update this model parameterization to produce variable soil moisture-NOx relationships for different locations based
on local soil moisture history, as informed by previous soil NOx emissions studies.

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Definitions and Acronyms:

- NOx: nitrogen oxides, which are reactive nitrogen gases representing the sum of nitric oxide (NO) and nitrogen
dioxide (NO2) concentrations.

- BDSNP: The Berkeley Dalhousie Soil NOx Parameterization (BDSNP) is a soil NOx emissions model developed by Hudman
et al., 2012 (https://doi.org/10.5194/acp-12-7779-2012). The model estimates soil NOx emissions for a spatial domain
of interest using soil moisture, soil temperature and soil nitrogen content input data.

- ERA5-Land: ERA5-Land is a land surface reanalysis model developed by the European Center for Medium-range Weather
Forecasts (ECMWF). The data are provided at 9 km spatial resolution for the entire globe. Data access:
https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-land?tab=form

- MERRA2: The Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) is a global
meteoroglogical reanalysis dataset provided by the National Aeronautics and Space Administrataion (NASA). The data
are provided globally at a spatial resolution of 0.5 degrees x 0.625 degrees.
Data Access: https://search.earthdata.nasa.gov/search?q=MERRA2

- NLDAS2-Mosaic: The North American Land Data Assimilation System (NLDAS-2) provides four separate land surface
models, one of which is the Mosaic model, which is named to reference the mosaic of grid cells comprising the
dataset. NLDAS2-Mosaic provides modeled land surface variables for the contiguous United States at a spatial
resolution of 1/8 degree x 1/8 degree. Data Access: https://search.earthdata.nasa.gov/search?q=NLDAS2%20MOSAIC

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Methodology:

We produce monthly averaged 2011-2020 soil NOx emissions for the contiguous United States by modifying the soil
moisture function within the existing BDSNP model. The standard BDSNP soil moisture function classifies each grid
cell as either arid (dry summers) or non-arid (wet summers), and applies one static relationship between soil
moisture and soil NOx emissions for each classification. This results in broad regions with highly variable soil and
ecosystem characteristics to respond identically to changes in soil moisture. This is an unrealistic assumption,
given that previous soil chamber emissions studies and satellite studies have shown soil NOx emissions to exhibit a
variety of responses to changes in soil moisture.

We update the BDSNP soil moisture function to better reflect the relationships documented in the literature. The
updated parameterization no longer consists of two static relationships between soil moisture and soil NOx
emissions. We assign a unique relationship to each grid cell based on the range of soil moisture data at each
location, with peak soil NOx emission occurring at the median soil moisture value at each grid cell. This results in
modelled soil NOx emissions that are larger in non-arid (wetter) regions than the standard parameterization, which
favored larger emissions in arid (drier) regions.

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File Descriptions:

Here we provide three versions of monthly-averaged model output from our update of the BDSNP model. Each file
contains soil NOx emissions for the contiguous United States for the years 2011-2020 on a 0.25 x 0.25 spatial grid.
The data have units of ng N m^-2 s^-1. We provide three versions of our model output, with each version produced
with a different soil moisture dataset as input for the model. The three versions are produced using input soil
moisture data from the ERA5-Land, MERRA2 and NLDAS2-Mosaic modeling systems (see acronyms above). The files are
provided as NetCDF (.nc) files. Data from NetCDF files can be accessed using most coding languages, including Python,
R and MATLAB. NetCDF files can also be opened using open source software, such as ncview
(http://meteora.ucsd.edu/~pierce/ncview_home_page.html) or panoply (https://www.giss.nasa.gov/tools/panoply/).

- “soilNOx_updated_MOSAIC.nc”: Soil NOx emissions produced using NLDAS2-Mosaic soil moisture as input for the model.

- “soilNOx_updated_MERRA2.nc”: Soil NOx emissions produced using MERRA2 soil moisture as input for the model.

- “soilNOx_updated_ERA5LAND.nc”: Soil NOx emissions produced using ERA5-Land soil moisture as input for the model.