An intercomparison of total column-averaged nitrous oxide between ground-based FTIR TCCON and NDACC measurements at seven sites and comparisons with the GEOS-Chem model

<p>Nitrous oxide (<span class="inline-formula">N₂O</span>) is an important greenhouse gas and it can also generate nitric oxide, which depletes ozone in the stratosphere. It is a common target species of ground-based Fourier transform infrared (FTIR) near-infrared (TCCON) and mid-infrared (NDACC) measurements. Both TCCON and NDACC networks provide a long-term global distribution of atmospheric <span class="inline-formula">N₂O</span> mole fraction. In this study, the dry-air column-averaged mole fractions of <span class="inline-formula">N₂O</span> (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">X</mi><mrow><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="7083feeaa337c360bc1dec6cdd9e436c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1393-2019-ie00001.svg" width="25pt" height="14pt" src="amt-12-1393-2019-ie00001.png"/></svg:svg></span></span>) from the TCCON and NDACC measurements are compared against each other at seven sites around the world (Ny-Ålesund, Sodankylä, Bremen, Izaña, Réunion, Wollongong, Lauder) in the time period of 2007–2017. The mean differences in <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">X</mi><mrow><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="5e6a681c49fd20b61f27782a4f0ae370"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1393-2019-ie00002.svg" width="25pt" height="14pt" src="amt-12-1393-2019-ie00002.png"/></svg:svg></span></span> between TCCON and NDACC (NDACC–TCCON) at these sites are between <span class="inline-formula">−3.32</span> and 1.37&thinsp;ppb (<span class="inline-formula">−1.1</span>&thinsp;%–0.5&thinsp;%) with standard deviations between 1.69 and 5.01&thinsp;ppb (0.5&thinsp;%–1.6&thinsp;%), which are within the uncertainties of the two datasets. The NDACC <span class="inline-formula">N₂O</span> retrieval has good sensitivity throughout the troposphere and stratosphere, while the TCCON retrieval underestimates a deviation from the a priori in the troposphere and overestimates it in the stratosphere. As a result, the TCCON <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">X</mi><mrow><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="50b5fa68b9780aad29d3bc59a335671d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1393-2019-ie00003.svg" width="25pt" height="14pt" src="amt-12-1393-2019-ie00003.png"/></svg:svg></span></span> measurement is strongly affected by its a priori profile.</p> <p><span id="page1394"/>Trends and seasonal cycles of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">X</mi><mrow><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="99677be2b065f598f9fe943d745811ab"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1393-2019-ie00004.svg" width="25pt" height="14pt" src="amt-12-1393-2019-ie00004.png"/></svg:svg></span></span> are derived from the TCCON and NDACC measurements and the nearby surface flask sample measurements and compared with the results from GEOS-Chem model a priori and a posteriori simulations. The trends and seasonal cycles from FTIR measurement at Ny-Ålesund and Sodankylä are strongly affected by the polar winter and the polar vortex. The a posteriori <span class="inline-formula">N₂O</span> fluxes in the model are optimized based on surface <span class="inline-formula">N₂O</span> measurements with a 4D-Var inversion method. The <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">X</mi><mrow><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="e10d4b76078a1e8806f098c6d853566d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1393-2019-ie00005.svg" width="25pt" height="14pt" src="amt-12-1393-2019-ie00005.png"/></svg:svg></span></span> trends from the GEOS-Chem a posteriori simulation (<span class="inline-formula">0.97±0.02</span> (<span class="inline-formula">1<i>σ</i></span>)&thinsp;ppb&thinsp;yr<span class="inline-formula">⁻¹</span>) are close to those from the NDACC (0<span class="inline-formula">.93±0.04</span>&thinsp;ppb&thinsp;yr<span class="inline-formula">⁻¹</span>) and the surface flask sample measurements (<span class="inline-formula">0.93±0.02</span>&thinsp;ppb&thinsp;yr<span class="inline-formula">⁻¹</span>). The <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">X</mi><mrow><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="466b2088eb3ba38a6fcc0d0b8ea69279"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1393-2019-ie00006.svg" width="25pt" height="14pt" src="amt-12-1393-2019-ie00006.png"/></svg:svg></span></span> trend from the TCCON measurements is slightly lower (<span class="inline-formula">0.81±0.04</span>&thinsp;ppb&thinsp;yr<span class="inline-formula">⁻¹</span>) due to the underestimation of the trend in TCCON a priori simulation. The <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M24" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">X</mi><mrow><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="90450f00fd870e5f84133e6e1a36cf6c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1393-2019-ie00007.svg" width="25pt" height="14pt" src="amt-12-1393-2019-ie00007.png"/></svg:svg></span></span> trends from the GEOS-Chem a priori simulation are about 1.25&thinsp;ppb&thinsp;yr<span class="inline-formula">⁻¹</span>, and our study confirms that the <span class="inline-formula">N₂O</span> fluxes from the a priori inventories are overestimated. The seasonal cycles of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M27" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">X</mi><mrow><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="bf7b54d1602258a6bd4e0a2baf736945"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1393-2019-ie00008.svg" width="25pt" height="14pt" src="amt-12-1393-2019-ie00008.png"/></svg:svg></span></span> from the FTIR measurements and the model simulations are close to each other in the Northern Hemisphere with a maximum in August–October and a minimum in February–April. However, in the Southern Hemisphere, the modeled <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M28" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">X</mi><mrow><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="da6994d65f4a61e38d189bbe5fbdd62a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1393-2019-ie00009.svg" width="25pt" height="14pt" src="amt-12-1393-2019-ie00009.png"/></svg:svg></span></span> values show a minimum in February–April while the FTIR <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M29" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">X</mi><mrow><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="8900c9f9c19d990507a65d899d2e82ec"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1393-2019-ie00010.svg" width="25pt" height="14pt" src="amt-12-1393-2019-ie00010.png"/></svg:svg></span></span> retrievals show different patterns. By comparing the partial column-averaged <span class="inline-formula">N₂O</span> from the model and NDACC for three vertical ranges (surface–8, 8–17, 17–50&thinsp;km), we find that the discrepancy in the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M31" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">X</mi><mrow><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub><mi mathvariant="normal">O</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="0d208196834a80a82d174963af43b993"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1393-2019-ie00011.svg" width="25pt" height="14pt" src="amt-12-1393-2019-ie00011.png"/></svg:svg></span></span> seasonal cycle between the model simulations and the FTIR measurements in the Southern Hemisphere is mainly due to their stratospheric differences.</p>