The Δ <sup>17</sup> O and <i>δ</i> <sup>18</sup> O values of atmospheric nitrates simultaneously collected downwind of anthropogenic sources – implications for polluted air masses
Bibliographic record
Abstract
There are clear motivations for better understanding the atmospheric processes that transform nitrogen (N) oxides (NO x ) emitted from anthropogenic sources into nitrates (NO3-), two of them being that NO3- contributes to acidification and eutrophication of terrestrial and aquatic ecosystems, and particulate nitrate may play a role in climate dynamics. For these reasons, oxygen isotope delta values ( δ 18 O, Δ 17 O) are frequently applied to infer the chemical pathways leading to the observed mass-independent isotopic anomalies from interaction with 17 O-rich ozone (O 3 ). Recent laboratory experiments suggest that the isotopic equilibrium between NO 2 (the main precursor of NO3-) and O 3 may take long enough under certain field conditions that nitrates may be formed near emission sources with lower isotopic values than those formed further downwind. Indeed, previously published field measurements of oxygen isotopes in NO3- in precipitation (wNO3-) and in particulate (pNO3-) samples suggest that abnormally low isotopic values might characterize polluted air masses. However, none of the air studies have deployed systems allowing collection of samples specific to anthropogenic sources in order to avoid shifts in isotopic signature due to changing wind directions, or separately characterized gaseous HNO 3 with Δ 17 O values. Here we have used a wind-sector-based, multi-stage filter sampling system and precipitation collector to simultaneously sample HNO 3 and pNO3-, and co-collect wNO3-. The nitrates are from various distances (<1 to >125 km) downwind of different anthropogenic emitters, and consequently from varying time lapses after emission. The separate collection of nitrates shows that the HNO 3 δ 18 O ranges are distinct from those of w - and pNO3-. Interestingly, the Δ 17 O differences between pNO3- and HNO 3 shift from positive during cold sampling periods to negative during warm periods. The low pNO3-Δ17O values observed during warm periods may partly derive from the involvement of 17 O-depleted peroxy radicals (RO 2 ) oxidizing NO during that season. Another possibility is that nitrates derive from NO x that has not yet reached isotopic equilibrium with O 3 . However, these mechanisms, individually or together, cannot explain the observed p NO 3 minus HNO 3 isotopic changes. We propose differences in dry depositional rates, faster for HNO 3 , as a mechanism for the observed shifts. Larger proportions of pNO3- formed via the N 2 O 5 pathway would explain the opposite fall–winter patterns. Our results show that the separate HNO 3 , wNO3- and pNO3- isotopic signals can be used to further our understanding of NO x oxidation and deposition. Future research should investigate all tropospheric nitrate species as well as NO x to refine our understanding of nitrate distribution worldwide and to develop effective emission reduction strategies.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".