Anthropogenic Perchlorate Increases since 1980 in the Canadian High Arctic
Bibliographic record
Abstract
An ice core of 15.5 m retrieved from Agassiz Ice Cap (Nunavut, Canada) in April 2009 was analyzed for perchlorate to obtain a temporal trend in the recent decades and to better understand the factors affecting High Arctic deposition. The continuous record dated from 1936 to 2007, covers the periods prior to and during the major atmospheric releases of organic chlorine species that affected the stratospheric ozone levels. Concentrations and yearly fluxes of perchlorate and chloride showed a significant correlation for the 1940–1959 period, suggesting a predominant tropospheric formation by lightning. While concentration of chloride remained unchanged from 1940s until 2009, elevated levels of perchlorate were observed after 1979. A lack of significant increases in either sulfate or chloride between 1980 and 2001 suggests that the effect of volcanic activities on the perchlorate at the study site during this period could be insignificant. Therefore, the elevated perchlorate in the ice could most likely be attributed to anthropogenic activities that influenced perchlorate sources and formation mechanisms after 1979. Our results show that anthropogenic contribution could be responsible for 66% of perchlorate found in the ice. Although with some differences in trends and amounts, deposition rate found in this study is similar to those observed at Devon Island (Nunavut, Canada), Eclipse Icefield (Yukon, Canada) and Summit Station (Greenland). Methyl chloroform, a chlorinated solvent largely used after 1976, peaked in the atmosphere in 1990 and has a much shorter atmospheric life than chlorofluorocarbons (CFCs). This study proposes methyl chloroform (CH 3 CCl 3 ) as the significant anthropogenic source of perchlorate in the Canadian High Arctic between 1980 and 2000, with HCFC-141b (Cl 2 FC–CH 3 ), a relatively short-lived CFC probably responsible for a slower decrease in perchlorate deposition after the late 1990s. The presence of aerosols in the stratosphere appears to suppress perchlorate production after 1974. As both methyl chloroform and HCFC-141b had no new significant emissions after 2003, deposition of perchlorate in High Arctic is expected to remain at pre-1980 levels.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.003 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 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".