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The long‐term trends and seasonality of many tropospheric pollutants are not well characterized in the high Arctic due to a dearth of trace‐gas measurements in this remote region. In this study, the inter‐ and intra‐annual variabilities of carbon monoxide (CO), acetylene (C2H2), ethane (C2H6), methanol (CH3OH), formaldehyde (H2CO), formic acid (HCOOH), and peroxyacetyl nitrate (PAN) in the high Arctic region were derived from the total column time‐series of ground‐based Fourier transform infrared (FTIR) measurements at Eureka, Nunavut (80.05°N, 86.42°W, 2006–2020) and Thule, Greenland (76.53°N, 68.74°W, 1999–2022). Consistent seasonal cycles were observed in the FTIR measurements at both sites for all species. Negative trends were observed for CO, C2H2, and CH3OH at both sites, and for HCOOH at Eureka. Positive trends were detected for C2H6 and H2CO at both sites, and for PAN at Eureka. Additionally, a 19‐year simulation was performed using the novel GEOS‐Chem High Performance model v14.1.1 for the period of 2003–2021. The model was able to reproduce the observed seasonality of all gases, but all species showed negative biases relative to observations, and CH3OH was found to have a particularly large bias of approximately −70% relative to the FTIR measurements. The GEOS‐Chem modeled trends broadly agreed with observations for all species except C2H6, H2CO, and PAN, which were found to have opposite trends in the model. For some species, the measurement‐model differences are suspected to be the result of errors or underestimations in the emissions inventories used in the simulation. Plain Language Summary: In this study, we investigate the seasonality and long‐term trends of seven atmospheric pollutants measured from ground‐based spectrometers at two locations in the North American high Arctic; Eureka, Nunavut (80.05°N, 86.42°W, 2006–2020), and Thule, Greenland (76.53°N, 68.74°W, 1999–2022). We observe consistent seasonal cycles at both sites for all gases. Carbon monoxide and acetylene were found to be decreasing over the years at both locations, while others gases, like ethane and formaldehyde were steadily increasing. At Eureka, formic acid concentrations were also found to be steadily decreasing. To better understand these changes, we used a chemical transport model to simulate the behavior of these gases in the high Arctic over a 19‐year period (2003–2021). The model broadly captured the shape of the seasonal cycles, however, the simulation displayed some discrepancies relative to the ground‐based measurements. For some gases, such as methanol and peroxyacetyl nitrate, the model showed greater discrepancies with the measurements than for others. The model reproduced the observed trends for all species except ethane, formaldehyde, and peroxyacetyl nitrate, which were found to have opposite trends in the model. Some of these discrepancies are believed to be attributed to errors in the model's emissions inventories. Key Points: Consistent seasonal cycles were observed for seven tropospheric pollutants from the measurements of two high Arctic ground‐based FTIRsCO, C2H2, and CH3OH showed clear negative trends in the Arctic, while C2H6 and PAN displayed positive trendsGEOS‐Chem High Performance reproduced the observed trends of CO, C2H2, and CH3OH, but showed differing trends for C2H6, H2CO, and PAN [ABSTRACT FROM AUTHOR]

Extreme enhancements in the total columns of carbon monoxide (CO), peroxyacetyl nitrate (PAN), ethylene (C2H4), methanol (CH3OH), and formic acid (HCOOH) were observed over the Canadian high Arctic during the period of 17–22 August 2017 by a ground‐based Fourier transform infrared (FTIR) spectrometer at Eureka, Nunavut (80.05°N, 86.42°W), and by the Infrared Atmospheric Sounding Interferometer (IASI) satellite instruments. These enhancements have been attributed to wildfires in British Columbia (BC) and the Northwest Territories (NWT) of Canada, and represent the largest short‐term perturbations of PAN, C2H4, and HCOOH above ambient concentrations over the 14‐year (2006–2020) Eureka time‐series. Enhancement ratios, emission ratios, and emission factors relative to CO were calculated for all species for both FTIR and IASI observations. The C2H4 and HCOOH emission factors are significantly larger than previous studies, suggesting unusually high emissions from these fires. The wildfire plumes were also simulated using the GEOS‐Chem model. Initial GEOS‐Chem simulations displayed a severe under‐estimation relative to observations for these fire plumes resulting from the injection height scheme of the model. Sensitivity tests highlighted that injection heights of 12.5 km for BC (based on previous studies) and 10 km for the NWT fires yielded the strongest correlations with ground‐based measurements. Applying these injection heights to the model significantly improves the simulated plume transport and agreement with ground‐ and space‐based observations. GEOS‐Chem was also used to estimate the magnitude of secondary in‐plume production of CH3OH and HCOOH; it was found to be an important component (∼18%) of the enhanced HCOOH columns at Eureka. Plain Language Summary: Wildfires are a significant natural source of pollution to the atmosphere. During mid‐August 2017, two extremely large wildfires occurred simultaneously in British Columbia (BC) and the Northwest Territories (NWT) of Canada. These fires produced an exceptional amount of smoke, which was subsequently transported to the Canadian high Arctic region. The atmospheric concentrations of carbon monoxide (CO), peroxyacetyl nitrate (PAN), ethylene (C2H4), methanol (CH3OH), and formic acid (HCOOH) contained within these transported smoke plumes were measured using both a ground‐based infrared spectrometer located at Eureka, Nunavut (80.05°N, 86.42°W) and satellite instruments. The largest concentrations of CO, PAN, C2H4, and HCOOH observed over the 14‐year measurement record (2006–2020) of the ground‐based spectrometer were due to these wildfires. The emissions of these fires were estimated from both the ground‐ and space‐based measurements and were found to be unusually high in comparison to previous studies. The wildfire smoke plumes were also simulated using a chemical transport model, but the model was found to inject the smoke too low into the atmosphere above the fires; after adjustments to the injection scheme were made, the agreement of the model with the measurements was significantly improved. Key Points: The 2017 Canadian wildfires produced the largest PAN, C2H4, and HCOOH column enhancements observed in the high Arctic from 2006 to 2020GEOS‐Chem effectively simulates the observed plume transport, but required modifications to the injection heights of the fire emissionsMid‐latitude wildfires may become an increasingly significant source of reactive VOC species to the high Arctic during the summer months [ABSTRACT FROM AUTHOR]

Using multidecadal time series of ground‐based and satellite Fourier transform infrared measurements of inorganic fluorine (i.e., total fluorine resident in stratospheric fluorine reservoirs), we investigate stratospheric circulation changes over the past 20 years. The representation of these changes in five modern reanalyses is further analyzed through chemical‐transport model (CTM) simulations. From the observations but also from all reanalyses, we show that the inorganic fluorine is accumulating less rapidly in the Southern Hemisphere than in the Northern Hemisphere during the 21st century. Comparisons with a study evaluating the age‐of‐air of these reanalyses using the same CTM allow us to link this hemispheric asymmetry to changes in the Brewer‐Dobson circulation (BDC), with the age‐of‐air of the Southern Hemisphere getting younger relative to that of the Northern Hemisphere. Large differences in simulated total columns and absolute trend values are, nevertheless, depicted between our simulations driven by the five reanalyses. Superimposed on this multidecadal change, we, furthermore, confirm a 5–7‐year variability of the BDC that was first described in a recent study analyzing long‐term time series of hydrogen chloride (HCl) and nitric acid (HNO3). It is important to stress that our results, based on observations and meteorological reanalyses, are in contrast with the projections of chemistry‐climate models in response to the coupled increase of greenhouse gases and decrease of ozone‐depleting substances, calling for further investigations and the continuation of long‐term observations. Plain Language Summary: The overturning circulation of the stratosphere is projected to change in response to increases in greenhouse gases and decreases in ozone‐depleting substances, with the Southern Hemisphere branch expected to get weaker relative to the Northern Hemisphere. Here, we use 30‐year time series of observations and simulations of a long‐lived tracer to investigate stratospheric circulation changes. The observations analyzed are from ground‐based and satellite instruments. We compare them with simulations that use a chemical‐transport model driven by winds from meteorological reanalyses of 1990–2019. A reanalysis assimilates meteorological measurements into a forecast model to simulate the best possible representation of the atmospheric state. All five simulations, which use different reanalyses, agree with the observational analysis showing that the analyzed tracer is accumulating less rapidly in the Southern Hemisphere than in the Northern Hemisphere through 2019. This hemispheric asymmetry is attributed to changes in the stratospheric transport circulation, with the Southern Hemisphere branch getting stronger relative to the Northern Hemisphere. Our results support the conclusions of a recent observational study but do not support circulation changes projected by chemistry‐climate models. This study highlights the crucial importance of long‐term observation time series. Key Points: Fluorine in the stratosphere is accumulating less rapidly in the Southern Hemisphere than in the Northern HemisphereThe observed fluorine trend asymmetry, attributed to transport circulation changes, opposes trends predicted by chemistry‐climate modelsObservations and five modern reanalyses are qualitatively in agreement with these changes, calling for maintaining long‐term observations [ABSTRACT FROM AUTHOR]

Emissions of C2‐C5 alkanes from the U.S. oil and gas sector have changed rapidly over the last decade. We use a nested GEOS‐Chem simulation driven by updated 2011NEI emissions with aircraft, surface, and column observations to (1) examine spatial patterns in the emissions and observed atmospheric abundances of C2‐C5 alkanes over the United States and (2) estimate the contribution of emissions from the U.S. oil and gas industry to these patterns. The oil and gas sector in the updated 2011NEI contributes over 80% of the total U.S. emissions of ethane (C2H6) and propane (C3H8), and emissions of these species are largest in the central United States. Observed mixing ratios of C2‐C5 alkanes show enhancements over the central United States below 2 km. A nested GEOS‐Chem simulation underpredicts observed C3H8 mixing ratios in the boundary layer over several U.S. regions, and the relative underprediction is not consistent, suggesting C3H8 emissions should receive more attention moving forward. Our decision to consider only C4‐C5 alkane emissions as a single lumped species produces a geographic distribution similar to observations. Due to the increasing importance of oil and gas emissions in the United States, we recommend continued support of existing long‐term measurements of C2‐C5 alkanes. We suggest additional monitoring of C2‐C5 alkanes downwind of northeastern Colorado, Wyoming, and western North Dakota to capture changes in these regions. The atmospheric chemistry modeling community should also evaluate whether chemical mechanisms that lump larger alkanes are sufficient to understand air quality issues in regions with large emissions of these species. Key Points: Oil and gas development is the largest source of ethane and propane in the United States; this sector is the third largest source of C4‐C5 alkanesPropane is underpredicted over several U.S. regionsBoundary layer enhancements of C2‐C5 alkanes mixing ratios are largest over the central United States [ABSTRACT FROM AUTHOR]