Worldwide biogenic soil NO<sub>x</sub> emissions inferred from OMI NO<sub>2</sub> observations
Biogenic NO<sub>x</sub> emissions from soils are a large natural source with substantial uncertainties in global bottom-up estimates (ranging from 4 to 15 Tg N yr<sup>−1</sup>). We reduce this range in emission estimates, and present a top-down soil NO<sub>x</s...
Main Authors: | , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2014-09-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | http://www.atmos-chem-phys.net/14/10363/2014/acp-14-10363-2014.pdf |
Summary: | Biogenic NO<sub>x</sub> emissions from soils are a large natural source
with substantial uncertainties in global bottom-up estimates
(ranging from 4 to 15 Tg N yr<sup>−1</sup>). We reduce this range
in emission estimates, and present a top-down soil NO<sub>x</sub>
emission inventory for 2005 based on retrieved tropospheric
NO<sub>2</sub> columns from the Ozone Monitoring Instrument (OMI). We
use a state-of-science soil NO<sub>x</sub> emission inventory
(Hudman et al., 2012) as a priori in the GEOS-Chem chemistry transport
model to identify 11 regions where tropospheric NO<sub>2</sub> columns
are dominated by soil NO<sub>x</sub> emissions. Strong correlations
between soil NO<sub>x</sub> emissions and simulated NO<sub>2</sub> columns
indicate that spatial patterns in simulated NO<sub>2</sub> columns in
these regions indeed reflect the underlying soil NO<sub>x</sub>
emissions. Subsequently, we use a mass-balance approach to
constrain emissions for these 11 regions on all major continents
using OMI observed and GEOS-Chem simulated tropospheric NO<sub>2</sub>
columns. We find that responses of simulated NO<sub>2</sub> columns to
changing NO<sub>x</sub> emissions are suppressed over low NO<sub>x</sub>
regions, and account for these non-linearities in our inversion
approach. In general, our approach suggests that emissions need to
be increased in most regions. Our OMI top-down soil NO<sub>x</sub>
inventory amounts to 10.0 Tg N for 2005 when only
constraining the 11 regions, and 12.9 Tg N when
extrapolating the constraints globally. Substantial regional
differences exist (ranging from −40% to +90%), and
globally our top-down inventory is 4–35% higher than the
GEOS-Chem a priori (9.6 Tg N yr<sup>−1</sup>). We evaluate
NO<sub>2</sub> concentrations simulated with our new OMI top-down
inventory against surface NO<sub>2</sub> measurements from monitoring
stations in Africa, the USA and Europe. Although this comparison
is complicated by several factors, we find an encouraging improved
agreement when using the OMI top-down inventory compared to using
the a priori inventory. To our knowledge, this study provides, for
the first time, specific constraints on soil NO<sub>x</sub> emissions
on all major continents using OMI NO<sub>2</sub> columns. Our results
rule out the low end of reported soil NO<sub>x</sub> emission
estimates, and suggest that global emissions are most likely around
12.9 ± 3.9 Tg N yr<sup>−1</sup>. |
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ISSN: | 1680-7316 1680-7324 |