Chemical composition and source attribution of sub-micrometre aerosol particles in the summertime Arctic lower troposphere

Abstract. Aerosol particles impact the Arctic climate system both directly and indirectly by modifying cloud properties, yet our understanding of their vertical distribution, chemical composition, mixing state, and sources in the summertime Arctic is incomplete. In situ vertical observations of particle properties in the high Arctic combined with modelling analysis on source attribution are in short supply, particularly during summer. We thus use airborne measurements of aerosol particle composition to demonstrate the strong contrast between particle sources and composition within and above the summertime Arctic boundary layer. In situ measurements from two complementary aerosol mass spectrometers, the Aircraft-based Laser Ablation Aerosol Mass Spectrometer (ALABAMA) and an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), are presented alongside black carbon measurements from an single particle soot photometer (SP2). Particle composition analysis was complemented by trace gas measurements, satellite data, and air mass history modelling to attribute particle properties to particle origin and air mass source regions. Particle composition above the summertime Arctic boundary layer was dominated by chemically aged particles, containing elemental carbon, nitrate, ammonium, sulfate, and organic matter. From our analysis, we conclude that the presence of these particles was driven by transport of aerosol and precursor gases from mid-latitudes to Arctic regions. Specifically, elevated concentrations of nitrate, ammonium, and organic matter coincided with time spent over vegetation fires in northern Canada. In parallel, those particles were largely present in high CO environments (> 90 ppbv). Additionally, we observed that the organic-to-sulfate ratio was enhanced with increasing influence from these fires. Besides vegetation fires, particle sources in mid-latitudes further include anthropogenic emissions in Europe, North America, and East Asia. The presence of particles in the Arctic lower free troposphere, particularly sulfate, correlated with time spent over populated and industrial areas in these regions. Further, the size distribution of free tropospheric particles containing elemental carbon and nitrate was shifted to larger diameters compared to particles present within the boundary layer. Moreover, our analysis suggests that organic matter, when present in the Arctic free troposphere, can partly be identified as low molecular weight dicarboxylic acids (oxalic, malonic, and succinic acid). Particles containing dicarboxylic acids were largely present when the residence time of air masses outside Arctic regions was high. In contrast, particle composition within the marine boundary layer was largely driven by Arctic regional processes. Air mass history modelling demonstrated that alongside primary sea spray particles, marine biogenic sources contributed to secondary aerosol formation via trimethylamine, methanesulfonic acid, sulfate, and other organic species. Our findings improve our knowledge of mid-latitude and Arctic regional sources that influence the vertical distribution of particle chemical composition and mixing state in the Arctic summer.

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Chemical composition and source attribution of submicron aerosol particles in the summertime Arctic lower troposphere

10.5194/acp-2020-742 ◽

2020 ◽

Author(s):

Franziska Köllner ◽

Johannes Schneider ◽

Megan D. Willis ◽

Hannes Schulz ◽

Daniel Kunkel ◽

...

Keyword(s):

Boundary Layer ◽

Organic Matter ◽

Elemental Carbon ◽

Dicarboxylic Acids ◽

The Arctic ◽

Arctic Boundary Layer ◽

Air Mass ◽

Particle Composition ◽

Arctic Regions ◽

Vegetation Fires

Abstract. We use airborne measurements of aerosol particle composition to demonstrate the strong contrast between particle sources and composition within and above the summertime Arctic boundary layer. In-situ measurements from two complementary aerosol mass spectrometers, the ALABAMA and the HR-ToF-AMS, with black carbon measurements from an SP2 are presented. Particle composition analysis was complemented by trace gas measurements, satellite data, and air mass history modeling to attribute particle properties to particle origin and air mass source regions. Particle composition above the summertime Arctic boundary layer was dominated by chemically aged particles, containing elemental carbon, nitrate, ammonium, sulfate, and organic matter. From our analysis, we conclude that the presence of these particles was driven by transport of aerosol and precursor gases from mid-latitudes to Arctic regions. Particularly, elevated concentrations of nitrate, ammonium, and organic matter coincided with time spent over vegetation fires in northern Canada. In parallel, those particles were largely present in high CO environments (> 90 ppbv). Additionally, we observed that the organic-to-sulfate ratio was enhanced with increasing influence from these fires. Besides vegetation fires, particle sources in mid-latitudes further include anthropogenic emissions in Europe, North America, and East Asia. The presence of particles in the Arctic lower free troposphere correlated with time spent over populated and industrial areas in these regions. Further, the size distribution of free tropospheric particles containing elemental carbon and nitrate was shifter to larger diameters compared to particles present within the boundary layer. Moreover, our analysis suggests that organic matter when present in the Arctic free troposphere can partly be identified as low-molecular weight dicarboxylic acids (oxalic, malonic, and succinic acid). Particles containing dicarboxylic acids were largely present when the residence time of air masses outside Arctic regions was high. In contrast, particle composition within the marine boundary layer was largely driven by Arctic regional processes. Air mass history modeling demonstrated that alongside primary sea spray particles, marine-biogenic sources contributed to secondary aerosol formation by trimethylamine, methanesulfonic acid, sulfate, and other organic species.

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Particulate trimethylamine in the summertime Canadian high Arctic lower troposphere

10.5194/acp-2017-505 ◽

2017 ◽

Cited By ~ 1

Author(s):

Franziska Köllner ◽

Johannes Schneider ◽

Megan D. Willis ◽

Thomas Klimach ◽

Frank Helleis ◽

...

Keyword(s):

Boundary Layer ◽

Chemical Composition ◽

Single Particle ◽

High Arctic ◽

The Arctic ◽

Particle Analysis ◽

Sea Spray ◽

Canadian High Arctic ◽

Arctic Boundary Layer ◽

Sea Surface Microlayer

Abstract. Size-resolved and vertical profile measurements of single particle chemical composition (sampling altitude range 50–3000 m) were conducted in July 2014 in the Canadian high Arctic during the aircraft-based measurement campaign NETCARE 2014. We deployed the single particle laser ablation aerosol mass spectrometer ALABAMA (vacuum aerodynamic diameter range approximately 200–1000 nm) to identify different particle types and their mixing states. On basis of the single particle analysis, we found that a significant fraction (23 %) of all analyzed particles (in total: 7412) contained trimethylamine (TMA). The identification of TMA in ambient mass spectra was confirmed by laboratory measurements. From the maximum occurrence of particulate TMA in the Arctic boundary layer and the higher abundance of smaller TMA-containing particles (maximum in the size distribution at 300 nm), we conclude that the TMA component of these particles resulted from emissions within the Arctic boundary layer. Air mass history according to FLEXPART backward simulations and associated wind data give evidence of a marine-biogenic influence on particulate TMA. The marine influence on particle chemical composition in the summertime Arctic is further demonstrated by the existence of larger sodium and chloride (Na/Cl-) containing particles which are mainly abundant in the boundary layer. Some of these sea spray particles were internally mixed with carbohydrates (e.g., cellulose) which likely originated from a sea surface microlayer enriched with microorganisms and organic compounds. The external mix of sea spray particles and TMA-containing particles suggests the latter result from secondary conversion of precursor gases from marine inner-Arctic sources. In contrast to TMA- and Na/Cl-containing aerosol types, particles with biomass-burning markers (such as levoglucosan) showed a higher fraction at higher altitudes, thereby indicating long-range transport as their source. Our measurements highlight the importance of natural, marine inner-Arctic sources for summertime Arctic aerosol.

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Physical aerosol properties and their relation to air mass origin at Monte Cimone (Italy) during the first MINATROC campaign

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-5-1067-2005 ◽

2005 ◽

Vol 5 (1) ◽

pp. 1067-1114 ◽

Cited By ~ 3

Author(s):

R. Van Dingenen ◽

J.-P. Putaud ◽

S. Martins-Dos Santos ◽

F. Raes

Keyword(s):

Boundary Layer ◽

Organic Matter ◽

The Arctic ◽

Size Distributions ◽

Air Mass ◽

Air Masses ◽

Accumulation Mode ◽

Hygroscopic Properties ◽

Hygroscopic Particles ◽

Air Mass Origin

Abstract. Aerosol physical properties were measured at the Monte Cimone Observatory (Italy) from 1 June till 6 July 2000. The measurement site is located in the transition zone between continental boundary layer and the free troposphere (FT), at the border between the Mediterranean area and Central Europe, and is exposed to a variety of air masses. Sub-micrometer number size distributions, aerosol hygroscopicity at 90% RH, refractory size distribution at 270°C and black carbon mass were continuously measured. Number size distributions and hygroscopic properties indicate that the site is exposed to aged continental air masses, however during daytime it is also affected by upslope winds. The mixing of this transported polluted boundary layer air masses with relatively clean FT air leads to frequent nucleation events around local noon. Night-time size distributions including fine and coarse fractions for each air mass episode have been parameterized by a 3-modal lognormal distribution. Number and volume concentrations in the sub-micrometer modes are strongly affected by the air mass origin, with highest levels in NW-European air masses, versus very clean air in the ''Arctic'' episode. During the dust episode, the coarse mode is clearly enhanced. The observed hygroscopic behavior of the aerosol is consistent with the chemical composition described by Putaud et al. (2004a), but no closure could be made because the hygroscopic properties of the water-soluble organic matter is not known. The data suggest that WSOM is slightly-to-moderately hygroscopic, and that this property may well depend on the air mass origin and history. Although externally mixing is observed in all air masses, the occurrence of ''less'' hygroscopic particles has mostly such a low occurrence rate that the average growth factor distribution mostly appears as a single mode. This is not the case for the dust episode, where the external mixing between less hygroscopic and more hygroscopic particles is very prominent, and indicating clearly the occurrence of a dust accumulation mode, extending down to 50 nm particles, along with an anthropogenic pollution mode. The presented physical measurements finally allow us to provide a partitioning of the sub-µm aerosol in four non-overlapping fractions (soluble + volatile, non-soluble + volatile, refractory + non-BC, BC) which can be roughly associated with separate groups of chemical compounds (ions, organic matter, dust, BC). For what concerns the relative contributions of the fractions, all air masses except the free-tropospheric (FT) and Dust Episodes show a similar composition within the uncertainty of the data. The latter two have a significantly higher refractory fraction, which in the FT air mass is attributed to carbonaceous particles, and in the dust episode to a sub-µm accumulation mode of dust.

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An aircraft-borne chemical ionization – ion trap mass spectrometer (CI-ITMS) for fast PAN and PPN measurements

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-3-4313-2010 ◽

2010 ◽

Vol 3 (5) ◽

pp. 4313-4354

Author(s):

A. Roiger ◽

H. Aufmhoff ◽

P. Stock ◽

F. Arnold ◽

H. Schlager

Keyword(s):

Boundary Layer ◽

Mass Spectrometer ◽

Chemical Ionization ◽

Ion Trap ◽

The Arctic ◽

Arctic Boundary Layer ◽

Online Calibration ◽

Mixing Ratios ◽

Ion Trap Mass Spectrometer ◽

Research Aircraft

Abstract. An airborne chemical ionization ion trap mass spectrometer instrument (CI-ITMS) has been developed for tropospheric and stratospheric fast in-situ measurements of PAN (peroxyacetyl nitrate) and PPN (peroxypropionyl nitrate). The first scientific deployment of the FASTPEX instrument (FASTPEX = Fast Measurement of Peroxyacyl nitrates) took place in the Arctic during 18 missions aboard the DLR research aircraft Falcon, within the framework of the POLARCAT-GRACE campaign in the summer of 2008. The FASTPEX instrument is described and characteristic properties of the employed ion trap mass spectrometer are discussed. Atmospheric data obtained at altitudes of up to ~12 km are presented, from the boundary layer to the lowermost stratosphere. Data were sampled with a time resolution of 2 s and a 2σ detection limit of 25 pmol mol−1. An isotopically labelled standard was used for a permanent online calibration. For this reason the accuracy of the PAN measurements is better than ±10% for mixing ratios greater than 200 pmol mol−1. PAN mixing ratios in the summer Arctic troposphere were in the order of a few hundred pmol mol−1 and generally correlated well with CO. In the Arctic boundary layer and lowermost stratosphere smaller PAN mixing ratios were observed due to a combination of missing local sources of PAN precursor gases and efficient removal processes (thermolysis/photolysis). PPN, the second most abundant PAN homologue, was measured simultanously. Observed PPN/PAN ratios range between ~0.03 and 0.3.

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The Arctic Boundary Layer Expedition (ABLE 3A): July–August 1988

Journal of Geophysical Research Atmospheres ◽

10.1029/91jd02109 ◽

1992 ◽

Vol 97 (D15) ◽

pp. 16383 ◽

Cited By ~ 68

Author(s):

R. C. Harriss ◽

S. C. Wofsy ◽

D. S. Bartlett ◽

M. C. Shipham ◽

D. J. Jacob ◽

...

Keyword(s):

Boundary Layer ◽

The Arctic ◽

Arctic Boundary Layer

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Combined effects of boundary layer dynamics and atmospheric chemistry on aerosol composition during new particle formation periods

10.5194/acp-2018-538 ◽

2018 ◽

Author(s):

Liqing Hao ◽

Olga Garmash ◽

Mikael Ehn ◽

Pasi Miettinen ◽

Paola Massoli ◽

...

Keyword(s):

Boundary Layer ◽

Chemical Composition ◽

Atmospheric Chemistry ◽

Mass Fraction ◽

Organic Aerosols ◽

Organic Aerosol ◽

Sulfate Aerosol ◽

Organic Mass ◽

Night Time ◽

Aerosol Mass

Abstract. Characterizing aerosol chemical composition in response to meteorological changes and atmospheric chemistry is important to gain insights into new particle formation mechanisms. A BAECC (Biogenic Aerosols-Effects on Clouds and Climate) campaign was conducted during the spring 2014 at SMEAR II station (Station for Measuring Forest Ecosystem-Aerosol Relations) in Finland. The particles were characterized by a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). A PBL (planetary boundary layer) dilution model was developed to assist interpreting the measurement results. Right before nucleation events, the mass concentrations of organic and sulfate aerosol species were both decreased rapidly along with the growth of PBL heights. However, the mass fraction of sulfate aerosol of the total aerosol mass was increased, in contrast to a decrease for the organic mass fraction. Meanwhile, an increase of LVOOA (low-volatility oxygenated organic aerosol) mass fraction of the total organic mass was observed, in distinct comparison to a reduction of SVOOA (semi-volatile OOA) mass fraction. Our results demonstrate that, at the beginning of nucleation events, the observed sulfate aerosol mass was mainly driven by vertical turbulent mixing of sulfate-rich aerosols between the residual layer and the newly formed boundary layer, while the condensation of sulfuric acid played a minor role in interpreting the measured sulfate mass concentration. For the measured organic aerosols, their temporal profiles were mainly driven by dilution from PBL development, organic aerosol mixing in different boundary layers and/or condensation of organic vapors, but accurate measurements of organic vapor concentrations and characterization on the spatial aerosol chemical composition are required. In general, the observed aerosol particles by AMS are subjected to joint effects of PBL dilution, atmospheric chemistry and aerosol mixing in different boundary layers. During aerosol growth periods in the night time, the mass concentrations of organic aerosols and organic nitrate aerosols were both increased. The increase of SVOOA mass correlated well with the calculated increase of condensed HOMs (highly oxygenated organic molecules) mass. To our knowledge, our results are the first atmospheric observations showing a connection between increase in SVOOA and condensed HOMs during the night time.

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Ozone variability and halogen oxidation within the Arctic and sub-Arctic springtime boundary layer

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-10223-2010 ◽

2010 ◽

Vol 10 (21) ◽

pp. 10223-10236 ◽

Cited By ~ 92

Author(s):

J. B. Gilman ◽

J. F. Burkhart ◽

B. M. Lerner ◽

E. J. Williams ◽

W. C. Kuster ◽

...

Keyword(s):

Boundary Layer ◽

North Atlantic ◽

Marine Boundary Layer ◽

Transport Model ◽

Arctic Basin ◽

The Arctic ◽

Arctic Boundary Layer ◽

Air Masses ◽

The North ◽

The North Atlantic

Abstract. The influence of halogen oxidation on the variabilities of ozone (O3) and volatile organic compounds (VOCs) within the Arctic and sub-Arctic atmospheric boundary layer was investigated using field measurements from multiple campaigns conducted in March and April 2008 as part of the POLARCAT project. For the ship-based measurements, a high degree of correlation (r = 0.98 for 544 data points collected north of 68° N) was observed between the acetylene to benzene ratio, used as a marker for chlorine and bromine oxidation, and O3 signifying the vast influence of halogen oxidation throughout the ice-free regions of the North Atlantic. Concurrent airborne and ground-based measurements in the Alaskan Arctic substantiated this correlation and were used to demonstrate that halogen oxidation influenced O3 variability throughout the Arctic boundary layer during these springtime studies. Measurements aboard the R/V Knorr in the North Atlantic and Arctic Oceans provided a unique view of the transport of O3-poor air masses from the Arctic Basin to latitudes as far south as 52° N. FLEXPART, a Lagrangian transport model, was used to quantitatively determine the exposure of air masses encountered by the ship to first-year ice (FYI), multi-year ice (MYI), and total ICE (FYI+MYI). O3 anti-correlated with the modeled total ICE tracer (r = −0.86) indicating that up to 73% of the O3 variability measured in the Arctic marine boundary layer could be related to sea ice exposure.

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Signature of Arctic surface ozone depletion events in the isotope anomaly (Δ<sup>17</sup>O) of atmospheric nitrate

Atmospheric Chemistry and Physics ◽

10.5194/acp-7-1451-2007 ◽

2007 ◽

Vol 7 (5) ◽

pp. 1451-1469 ◽

Cited By ~ 72

Author(s):

S. Morin ◽

J. Savarino ◽

S. Bekki ◽

S. Gong ◽

J. W. Bottenheim

Keyword(s):

Boundary Layer ◽

Ozone Depletion ◽

Surface Ozone ◽

The Arctic ◽

Arctic Boundary Layer ◽

Mixing Ratios ◽

Atmospheric Nitrate ◽

Boundary Layer Ozone ◽

Oxidation Of No ◽

Arctic Surface

Abstract. We report the first measurements of the oxygen isotope anomaly of atmospheric inorganic nitrate from the Arctic. Nitrate samples and complementary data were collected at Alert, Nunavut, Canada (82°30 ' N, 62°19 ' W) in spring 2004. Covering the polar sunrise period, characterized by the occurrence of severe boundary layer ozone depletion events (ODEs), our data show a significant correlation between the variations of atmospheric ozone (O3) mixing ratios and Δ17O of nitrate (Δ17O(NO−3)). This relationship can be expressed as: Δ17O(NO−3)/‰, =(0.15±0.03)×O3/(nmol mol–1)+(29.7±0.7), with R2=0.70(n=12), for Δ17O(NO−3) ranging between 29 and 35 ‰. We derive mass-balance equations from chemical reactions operating in the Arctic boundary layer, that describe the evolution of Δ17O(NO−3) as a function of the concentrations of reactive species and their isotopic characteristics. Changes in the relative importance of O3, RO2 and BrO in the oxidation of NO during ODEs, and the large isotope anomalies of O3 and BrO, are the driving force for the variability in the measured Δ17O(NO−3) . BrONO2 hydrolysis is found to be a dominant source of nitrate in the Arctic boundary layer, in agreement with recent modeling studies.

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Chemical characteristics of submicron particles at the central Tibetan Plateau: insights from aerosol mass spectrometry

Atmospheric Chemistry and Physics ◽

10.5194/acp-18-427-2018 ◽

2018 ◽

Vol 18 (1) ◽

pp. 427-443 ◽

Cited By ~ 19

Author(s):

Jianzhong Xu ◽

Qi Zhang ◽

Jinsen Shi ◽

Xinlei Ge ◽

Conghui Xie ◽

...

Keyword(s):

Chemical Composition ◽

High Resolution ◽

Tibetan Plateau ◽

Mass Transport ◽

Mass Concentration ◽

Organic Aerosol ◽

Early Morning ◽

Monsoon Period ◽

Air Mass ◽

Aerosol Mass

Abstract. Recent studies have revealed a significant influx of anthropogenic aerosol from South Asia to the Himalayas and Tibetan Plateau (TP) during pre-monsoon period. In order to characterize the chemical composition, sources, and transport processes of aerosol in this area, we carried out a field study during June 2015 by deploying a suite of online instruments including an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-AMS) and a multi-angle absorption photometer (MAAP) at Nam Co station (90∘57′ E, 30∘46′ N; 4730 m a.s.l.) at the central of the TP. The measurements were made at a period when the transition from pre-monsoon to monsoon occurred. The average ambient mass concentration of submicron particulate matter (PM1) over the whole campaign was ∼ 2.0 µg m−3, with organics accounting for 68 %, followed by sulfate (15 %), black carbon (8 %), ammonium (7 %), and nitrate (2 %). Relatively higher aerosol mass concentration episodes were observed during the pre-monsoon period, whereas persistently low aerosol concentrations were observed during the monsoon period. However, the chemical composition of aerosol during the higher aerosol concentration episodes in the pre-monsoon season was on a case-by-case basis, depending on the prevailing meteorological conditions and air mass transport routes. Most of the chemical species exhibited significant diurnal variations with higher values occurring during afternoon and lower values during early morning, whereas nitrate peaked during early morning in association with higher relative humidity and lower air temperature. Organic aerosol (OA), with an oxygen-to-carbon ratio (O ∕ C) of 0.94, was more oxidized during the pre-monsoon period than during monsoon (average O ∕ C ratio of 0.72), and an average O ∕ C was 0.88 over the entire campaign period, suggesting overall highly oxygenated aerosol in the central TP. Positive matrix factorization of the high-resolution mass spectra of OA identified two oxygenated organic aerosol (OOA) factors: a less oxidized OOA (LO-OOA) and a more oxidized OOA (MO-OOA). The MO-OOA dominated during the pre-monsoon period, whereas LO-OOA dominated during monsoon. The sensitivity of air mass transport during pre-monsoon with synoptic process was also evaluated with a 3-D chemical transport model.

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Analysis of reactive bromine production and ozone depletion in the Arctic boundary layer using 3-D simulations with GEM-AQ: inference from synoptic-scale patterns

Atmospheric Chemistry and Physics ◽

10.5194/acp-11-3949-2011 ◽

2011 ◽

Vol 11 (8) ◽

pp. 3949-3979 ◽

Cited By ~ 53

Author(s):

K. Toyota ◽

J. C. McConnell ◽

A. Lupu ◽

L. Neary ◽

C. A. McLinden ◽

...

Keyword(s):

Boundary Layer ◽

Sea Ice ◽

Ozone Depletion ◽

Surface Ozone ◽

High Arctic ◽

Heterogeneous Reactions ◽

The Arctic ◽

Synoptic Scale ◽

Arctic Boundary Layer ◽

Surface Snow

Abstract. Episodes of high bromine levels and surface ozone depletion in the springtime Arctic are simulated by an online air-quality model, GEM-AQ, with gas-phase and heterogeneous reactions of inorganic bromine species and a simple scheme of air-snowpack chemical interactions implemented for this study. Snowpack on sea ice is assumed to be the only source of bromine to the atmosphere and to be capable of converting relatively stable bromine species to photolabile Br2 via air-snowpack interactions. A set of sensitivity model runs are performed for April 2001 at a horizontal resolution of approximately 100 km×100 km in the Arctic, to provide insights into the effects of temperature and the age (first-year, FY, versus multi-year, MY) of sea ice on the release of reactive bromine to the atmosphere. The model simulations capture much of the temporal variations in surface ozone mixing ratios as observed at stations in the high Arctic and the synoptic-scale evolution of areas with enhanced BrO column amount ("BrO clouds") as estimated from satellite observations. The simulated "BrO clouds" are in modestly better agreement with the satellite measurements when the FY sea ice is assumed to be more efficient at releasing reactive bromine to the atmosphere than on the MY sea ice. Surface ozone data from coastal stations used in this study are not sufficient to evaluate unambiguously the difference between the FY sea ice and the MY sea ice as a source of bromine. The results strongly suggest that reactive bromine is released ubiquitously from the snow on the sea ice during the Arctic spring while the timing and location of the bromine release are largely controlled by meteorological factors. It appears that a rapid advection and an enhanced turbulent diffusion associated with strong boundary-layer winds drive transport and dispersion of ozone to the near-surface air over the sea ice, increasing the oxidation rate of bromide (Br−) in the surface snow. Also, if indeed the surface snowpack does supply most of the reactive bromine in the Arctic boundary layer, it appears to be capable of releasing reactive bromine at temperatures as high as −10 °C, particularly on the sea ice in the central and eastern Arctic Ocean. Dynamically-induced BrO column variability in the lowermost stratosphere appears to interfere with the use of satellite BrO column measurements for interpreting BrO variability in the lower troposphere but probably not to the extent of totally obscuring "BrO clouds" that originate from the surface snow/ice source of bromine in the high Arctic. A budget analysis of the simulated air-surface exchange of bromine compounds suggests that a "bromine explosion" occurs in the interstitial air of the snowpack and/or is accelerated by heterogeneous reactions on the surface of wind-blown snow in ambient air, both of which are not represented explicitly in our simple model but could have been approximated by a parameter adjustment for the yield of Br2 from the trigger.

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