Global deposition of total reactive nitrogen oxides from 1996 to 2014 constrained with satellite observations of NO<sub>2</sub> columns
Reactive nitrogen oxides (NO<sub><i>y</i></sub>) are a major constituent of the nitrogen deposited from the atmosphere, but observational constraints on their deposition are limited by poor or nonexistent measurement coverage in many parts of the world. Here we apply NO<...
Main Authors: | , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2017-08-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | https://www.atmos-chem-phys.net/17/10071/2017/acp-17-10071-2017.pdf |
Summary: | Reactive nitrogen oxides (NO<sub><i>y</i></sub>) are a major constituent of the
nitrogen deposited from the atmosphere, but observational constraints on
their deposition are limited by poor or nonexistent measurement coverage in
many parts of the world. Here we apply NO<sub>2</sub> observations from multiple
satellite instruments (GOME, SCIAMACHY, and GOME-2) to constrain the global
deposition of NO<sub><i>y</i></sub> over the last 2 decades. We accomplish this by
producing top-down estimates of NO<sub><i>x</i></sub> emissions from inverse modeling of
satellite NO<sub>2</sub> columns over 1996–2014, and including these emissions in
the GEOS-Chem chemical transport model to simulate chemistry, transport, and
deposition of NO<sub><i>y</i></sub>. Our estimates of long-term mean wet nitrate
(NO<sub>3</sub><sup>−</sup>) deposition are highly consistent with available measurements
in North America, Europe, and East Asia combined (<i>r</i> = 0.83, normalized mean
bias = −7 %, <i>N</i> = 136). Likewise, our calculated trends in wet
NO<sub>3</sub><sup>−</sup> deposition are largely consistent with the measurements, with
129 of the 136 gridded model–data pairs sharing overlapping 95 % confidence
intervals. We find that global mean NO<sub><i>y</i></sub> deposition over 1996–2014 is
56.0 Tg N yr<sup>−1</sup>, with a minimum in 2006 of 50.5 Tg N and a maximum in
2012 of 60.8 Tg N. Regional trends are large, with opposing signs in
different parts of the world. Over 1996 to 2014, NO<sub><i>y</i></sub> deposition
decreased by up to 60 % in eastern North America, doubled in regions of
East Asia, and declined by 20 % in parts of western Europe. About 40 % of
the global NO<sub><i>y</i></sub> deposition occurs over oceans, with deposition to the
North Atlantic Ocean declining and deposition to the northwestern Pacific
Ocean increasing. Using the residual between NO<sub><i>x</i></sub> emissions and NO<sub><i>y</i></sub>
deposition over specific land regions, we investigate how NO<sub><i>x</i></sub> export via
atmospheric transport has changed over the last 2 decades. Net export from
the continental United States decreased substantially, from 2.9 Tg N yr<sup>−1</sup> in 1996 to 1.5 Tg N yr<sup>−1</sup> in 2014. Export
from China more than tripled between 1996 and 2011 (from 1.0
to 3.5 Tg N yr<sup>−1</sup>), before a striking decline to 2.5 Tg N yr<sup>−1</sup> by
2014. We find that declines in NO<sub><i>x</i></sub> export from some western European
countries have counteracted increases in emissions from neighboring
countries to the east. A sensitivity study indicates that simulated NO<sub><i>y</i></sub>
deposition is robust to uncertainties in NH<sub>3</sub> emissions with a few
exceptions. Our novel long-term study provides timely context on the rapid
redistribution of atmospheric nitrogen transport and subsequent deposition to
ecosystems around the world. |
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ISSN: | 1680-7316 1680-7324 |