Summer variability of the atmospheric NO<sub>2</sub> :  NO ratio at Dome C on the East Antarctic Plateau
<p>Previous Antarctic summer campaigns have shown unexpectedly high levels of oxidants in the lower atmosphere of the continental plateau and at coastal regions, with atmospheric hydroxyl radical (OH) concentrations up to 4 <span class="inline-formula">×</span> 10<span...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2022-09-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://acp.copernicus.org/articles/22/12025/2022/acp-22-12025-2022.pdf |
| Summary: | <p>Previous Antarctic summer campaigns have shown unexpectedly high levels of oxidants in the lower atmosphere of the continental plateau and at coastal regions, with atmospheric hydroxyl radical (OH) concentrations up to 4 <span class="inline-formula">×</span> 10<span class="inline-formula"><sup>6</sup></span> cm<span class="inline-formula"><sup>−3</sup></span>. Such high reactivity in the summer Antarctic boundary layer results in part from the emissions of nitrogen oxides (NO<span class="inline-formula"><sub><i>x</i></sub></span> <span class="inline-formula">≡</span> NO <span class="inline-formula">+</span> NO<span class="inline-formula"><sub>2</sub></span>) produced during photo-denitrification of the snowpack, but its underlying mechanisms are not yet fully understood, as some of the chemical species involved (NO<span class="inline-formula"><sub>2</sub></span>, in particular) have not yet been measured directly and accurately. To overcome this crucial lack of information, newly developed optical instruments based on absorption spectroscopy (incoherent broadband cavity-enhanced absorption spectroscopy, IBBCEAS) were deployed for the first time at Dome C (<span class="inline-formula">−75.10</span> lat., 123.33 long., 3233 m a.s.l.) during the 2019–2020 summer campaign to investigate snow–air–radiation interaction. These instruments directly measure NO<span class="inline-formula"><sub>2</sub></span> with a detection limit of 30 pptv (parts per trillion by volume or 10<span class="inline-formula"><sup>−12</sup></span> mol mol<span class="inline-formula"><sup>−1</sup></span>) (3<span class="inline-formula"><i>σ</i></span>). We performed two sets of measurements in December 2019 (4 to 9) and January 2020 (16 to 25) to capture the early and late photolytic season, respectively. Late in the season, the daily averaged <span class="inline-formula">NO<sub>2</sub>:NO</span> ratio of 0.4 <span class="inline-formula">±</span> 0.4 matches that expected for photochemical equilibrium through Leighton's extended relationship involving RO<span class="inline-formula"><sub><i>x</i></sub></span> (0.6 <span class="inline-formula">±</span> 0.3). In December, however, we observed a daily averaged <span class="inline-formula">NO<sub>2</sub>:NO</span> ratio of 1.3 <span class="inline-formula">±</span> 1.1, which is approximately twice the daily ratio of 0.7 <span class="inline-formula">±</span> 0.4 calculated for the Leighton equilibrium. This suggests that more NO<span class="inline-formula"><sub>2</sub></span> is produced from the snowpack early in the photolytic season (4 to 9 December), possibly due to stronger UV irradiance caused by a smaller solar zenith angle near the solstice. Such a high sensitivity of the <span class="inline-formula">NO<sub>2</sub>:NO</span> ratio to the sun's position is of importance for consideration in atmospheric chemistry models.</p> |
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| ISSN: | 1680-7316 1680-7324 |