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Tropospheric ozone is a potent greenhouse gas and air pollutant harmful to humans and vegetation. This atmospheric monitoring at Toolik Field Station is one of the very few ongoing surface ozone monitoring programs located in a tundra environment. This dataset contains the mole fraction of ozone measured in the atmospheric surface layer at Toolik Field Station (68◦38′ N, 149◦36′ W) across four years (2019-2023) at one minute sampling frequency. Measurements were performed using a Thermo Scientific model 49C ultraviolet (UV) absorption analyzer. 

Abstract Atmospheric gaseous elemental mercury (GEM) concentrations in the Arctic exhibit a clear summertime maximum, while the origin of this peak is still a matter of debate in the community. Based on summertime observations during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition and a modeling approach, we further investigate the sources of atmospheric Hg in the central Arctic. Simulations with a generalized additive model (GAM) show that long-range transport of anthropogenic and terrestrial Hg from lower latitudes is a minor contribution (~2%), and more than 50% of the explained GEM variability is caused by oceanic evasion. A potential source contribution function (PSCF) analysis further shows that oceanic evasion is not significant throughout the ice-covered central Arctic Ocean but mainly occurs in the Marginal Ice Zone (MIZ) due to the specific environmental conditions in that region. Our results suggest that this regional process could be the leading contributor to the observed summertime GEM maximum. In the context of rapid Arctic warming and the observed increase in width of the MIZ, oceanic Hg evasion may become more significant and strengthen the role of the central Arctic Ocean as a summertime source of atmospheric Hg. 

Abstract Chlorine radicals are strong atmospheric oxidants known to play an important role in the depletion of surface ozone and the degradation of methane in the Arctic troposphere. Initial oxidation processes of chlorine produce chlorine oxides, and it has been speculated that the final oxidation steps lead to the formation of chloric (HClO 3 ) and perchloric (HClO 4 ) acids, although these two species have not been detected in the atmosphere. Here, we present atmospheric observations of gas-phase HClO 3 and HClO 4 . Significant levels of HClO 3 were observed during springtime at Greenland (Villum Research Station), Ny-Ålesund research station and over the central Arctic Ocean, on-board research vessel Polarstern during the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) campaign, with estimated concentrations up to 7 × 10 6 molecule cm −3 . The increase in HClO 3 , concomitantly with that in HClO 4 , was linked to the increase in bromine levels. These observations indicated that bromine chemistry enhances the formation of OClO, which is subsequently oxidized into HClO 3 and HClO 4 by hydroxyl radicals. HClO 3 and HClO 4 are not photoactive and therefore their loss through heterogeneous uptake on aerosol and snow surfaces can function as a previously missing atmospheric sink for reactive chlorine, thereby reducing the chlorine-driven oxidation capacity in the Arctic boundary layer. Our study reveals additional chlorine species in the atmosphere, providing further insights into atmospheric chlorine cycling in the polar environment. 

Abstract Despite the key role of the Arctic in the global Earth system, year-round in-situ atmospheric composition observations within the Arctic are sparse and mostly rely on measurements at ground-based coastal stations. Measurements of a suite of in-situ trace gases were performed in the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. These observations give a comprehensive picture of year-round near-surface atmospheric abundances of key greenhouse and trace gases, i.e., carbon dioxide, methane, nitrous oxide, ozone, carbon monoxide, dimethylsulfide, sulfur dioxide, elemental mercury, and selected volatile organic compounds (VOCs). Redundancy in certain measurements supported continuity and permitted cross-evaluation and validation of the data. This paper gives an overview of the trace gas measurements conducted during MOSAiC and highlights the high quality of the monitoring activities. In addition, in the case of redundant measurements, merged datasets are provided and recommended for further use by the scientific community. 

Dry deposition to the surface is one of the main removal pathways of tropospheric ozone (O3). We quantified for the first time the impact of O3 deposition to the Arctic sea ice on the planetary boundary layer (PBL) O3 concentration and budget using year-round flux and concentration observations from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) campaign and simulations with a single-column atmospheric chemistry and meteorological model (SCM). Based on eddy-covariance O3 surface flux observations, we find a median surface resistance on the order of 20,000 s m−1, resulting in a dry deposition velocity of approximately 0.005 cm s−1. This surface resistance is up to an order of magnitude larger than traditionally used values in many atmospheric chemistry and transport models. The SCM is able to accurately represent the yearly cycle, with maxima above 40 ppb in the winter and minima around 15 ppb at the end of summer. However, the observed springtime ozone depletion events are not captured by the SCM. In winter, the modelled PBL O3 budget is governed by dry deposition at the surface mostly compensated by downward turbulent transport of O3 towards the surface. Advection, which is accounted for implicitly by nudging to reanalysis data, poses a substantial, mostly negative, contribution to the simulated PBL O3 budget in summer. During episodes with low wind speed (<5 m s−1) and shallow PBL (<50 m), the 7-day mean dry deposition removal rate can reach up to 1.0 ppb h−1. Our study highlights the importance of an accurate description of dry deposition to Arctic sea ice in models to quantify the current and future O3 sink in the Arctic, impacting the tropospheric O3 budget, which has been modified in the last century largely due to anthropogenic activities. 

Abstract. During the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) campaign from 21 July to 3 August 2016,field experiments on leaf-level trace gas exchange of nitric oxide (NO), nitrogen dioxide (NO2), and ozone (O3) were conducted for thefirst time on the native American tree species Pinus strobus (eastern white pine), Acer rubrum (redmaple), Populus grandidentata (bigtooth aspen), and Quercus rubra (red oak) in a temperate hardwood forest inMichigan, USA. We measured the leaf-level trace gas exchange rates andinvestigated the existence of an NO2 compensation point, hypothesizedbased on a comparison of a previously observed average diurnal cycle ofNOx (NO2+NO) concentrations with that simulated using amulti-layer canopy exchange model. Known amounts of trace gases wereintroduced into a tree branch enclosure and a paired blank referenceenclosure. The trace gas concentrations before and after the enclosures weremeasured, as well as the enclosed leaf area (single-sided) and gas flow rate to obtain the trace gas fluxes with respect to leaf surface. There was nodetectable NO uptake for all tree types. The foliar NO2 and O3uptake largely followed a diurnal cycle, correlating with that of the leafstomatal conductance. NO2 and O3 fluxes were driven by theirconcentration gradient from ambient to leaf internal space. The NO2 loss rate at the leaf surface, equivalently the foliar NO2 deposition velocity toward the leaf surface, ranged from 0 to 3.6 mm s−1 for bigtooth aspen and from 0 to 0.76 mm s−1 for red oak, both of which are∼90 % of the expected values based on the stomatalconductance of water. The deposition velocities for red maple and white pineranged from 0.3 to 1.6 and from 0.01 to 1.1 mm s−1, respectively, and were lower than predicted from the stomatal conductance, implying amesophyll resistance to the uptake. Additionally, for white pine, theextrapolated velocity at zero stomatal conductance was 0.4±0.08 mm s−1, indicating a non-stomatal uptake pathway. The NO2compensation point was ≤60 ppt for all four tree species andindistinguishable from zero at the 95 % confidence level. This agrees withrecent reports for several European and California tree species butcontradicts some earlier experimental results where the compensation pointswere found to be on the order of 1 ppb or higher. Given that the sampledtree types represent 80 %–90 % of the total leaf area at this site, theseresults negate the previously hypothesized important role of a leaf-scaleNO2 compensation point. Consequently, to reconcile these findings,further detailed comparisons between the observed and simulated in- and above-canopy NOx concentrations and the leaf- and canopy-scaleNOx fluxes, using the multi-layer canopy exchange model withconsideration of the leaf-scale NOx deposition velocities as well asstomatal conductances reported here, are recommended. 

Abstract. Atmospheric non-methane hydrocarbons (NMHCs) play an important role in theformation of secondary organic aerosols and ozone. After a multidecadalglobal decline in atmospheric mole fractions of ethane and propane – themost abundant atmospheric NMHCs – previous work has shown a reversal ofthis trend with increasing atmospheric abundances from 2009 to 2015 in theNorthern Hemisphere. These concentration increases were attributed to theunprecedented growth in oil and natural gas (O&NG) production in NorthAmerica. Here, we supplement this trend analysis building on the long-term(2008–2010; 2012–2020) high-resolution (∼3 h) record ofambient air C2–C7 NMHCs from in situ measurements at the GreenlandEnvironmental Observatory at Summit station (GEOSummit, 72.58 ∘ N,38.48 ∘ W; 3210 m above sea level). We confirm previous findingsthat the ethane mole fraction significantly increased by +69.0 [+47.4,+73.2; 95 % confidence interval] ppt yr−1 from January 2010 toDecember 2014. Subsequent measurements, however, reveal a significantdecrease by −58.4 [−64.1, −48.9] ppt yr−1 from January 2015 to December2018. A similar reversal is found for propane. The upturn observed after2019 suggests, however, that the pause in the growth of atmospheric ethaneand propane might only have been temporary. Discrete samples collected atother northern hemispheric baseline sites under the umbrella of the NOAAcooperative global air sampling network show a similar decrease in 2015–2018and suggest a hemispheric pattern. Here, we further discuss the potentialcontribution of biomass burning and O&NG emissions (the main sources ofethane and propane) and conclude that O&NG activities likely played arole in these recent changes. This study highlights the crucial need forbetter constrained emission inventories. 

Abstract Unlike bromine, the effect of iodine chemistry on the Arctic surface ozone budget is poorly constrained. We present ship-based measurements of halogen oxides in the high Arctic boundary layer from the sunlit period of March to October 2020 and show that iodine enhances springtime tropospheric ozone depletion. We find that chemical reactions between iodine and ozone are the second highest contributor to ozone loss over the study period, after ozone photolysis-initiated loss and ahead of bromine.