Bromine, Ozone, And Mercury Experiment Research Articles

Intercomparison of snow depth retrievals over Arctic sea ice from radar data acquired by Operation IceBridge

Abstract. Since 2009, the ultra-wideband snow radar on Operation IceBridge (OIB; a NASA airborne mission to survey the polar ice covers) has acquired data in annual campaigns conducted during the Arctic and Antarctic springs. Progressive improvements in radar hardware and data processing methodologies have led to improved data quality for subsequent retrieval of snow depth. Existing retrieval algorithms differ in the way the air–snow (a–s) and snow–ice (s–i) interfaces are detected and localized in the radar returns and in how the system limitations are addressed (e.g., noise, resolution). In 2014, the Snow Thickness On Sea Ice Working Group (STOSIWG) was formed and tasked with investigating how radar data quality affects snow depth retrievals and how retrievals from the various algorithms differ. The goal is to understand the limitations of the estimates and to produce a well-documented, long-term record that can be used for understanding broader changes in the Arctic climate system. Here, we assess five retrieval algorithms by comparisons with field measurements from two ground-based campaigns, including the BRomine, Ozone, and Mercury EXperiment (BROMEX) at Barrow, Alaska; a field program by Environment and Climate Change Canada at Eureka, Nunavut; and available climatology and snowfall from ERA-Interim reanalysis. The aim is to examine available algorithms and to use the assessment results to inform the development of future approaches. We present results from these assessments and highlight key considerations for the production of a long-term, calibrated geophysical record of springtime snow thickness over Arctic sea ice.

Molecular Halogens Above the Arctic Snowpack: Emissions, Diurnal Variations, and Recycling Mechanisms

AbstractElevated levels of reactive bromine and chlorine species in the springtime Arctic boundary layer contribute to ozone depletion and mercury oxidation, as well as reactions with volatile organic compounds. Recent laboratory and field studies have revealed that snowpack photochemistry leads to Br2 and Cl2 production, the mechanisms of which remain poorly understood. In this work, we use a photochemical box model, with a simplified snow module, to examine the halogen chemistry occurring during the March 2012 Bromine, Ozone, and Mercury Experiment (BROMEX) near Utqiaġvik (Barrow), Alaska. Elevated daytime Br2 levels (e.g., 6–30 parts per trillion (ppt) at around local noon) reported in previous studies and in this work may be explained by Br + BrNO2/BrONO2 reactions under conditions of depleted O3 (<~10 ppb) and background NO2 (10–100 ppt). Even at low background NOx levels at Utqiaġvik, ClONO2 is predicted to be important in the production of Cl2 via multiphase reaction with Cl−. In the late afternoon, photolysis alone cannot explain the rapid decrease of Cl2 observed in the Arctic boundary layer. Heterogeneous reactions of Cl2 on aerosol particles and surface snowpack are suggested to play a key role in atmospheric Cl2 removal and possible BrCl production. Given the importance of the snowpack in the multiphase chemistry of the Arctic boundary layer, future measurements should focus on vertically resolved measurements of NOx and reactive halogens, as well as simultaneous particulate and snow halide measurements, to further evaluate and isolate the halogen production and vertical propagation mechanisms through one‐dimensional modeling.

Horizontal and vertical structure of reactive bromine events probed by bromine monoxide MAX-DOAS

Abstract. Heterogeneous photochemistry converts bromide (Br−) to reactive bromine species (Br atoms and bromine monoxide, BrO) that dominate Arctic springtime chemistry. This phenomenon has many impacts such as boundary-layer ozone depletion, mercury oxidation and deposition, and modification of the fate of hydrocarbon species. To study environmental controls on reactive bromine events, the BRomine, Ozone, and Mercury EXperiment (BROMEX) was carried out from early March to mid-April 2012 near Barrow (Utqiaġvik), Alaska. We measured horizontal and vertical gradients in BrO with multiple-axis differential optical absorption spectroscopy (MAX-DOAS) instrumentation at three sites, two mobile and one fixed. During the campaign, a large crack in the sea ice (an open lead) formed pushing one instrument package ∼ 250 km downwind from Barrow (Utqiaġvik). Convection associated with the open lead converted the BrO vertical structure from a surface-based event to a lofted event downwind of the lead influence. The column abundance of BrO downwind of the re-freezing lead was comparable to upwind amounts, indicating direct reactions on frost flowers or open seawater was not a major reactive bromine source. When these three sites were separated by ∼ 30 km length scales of unbroken sea ice, the BrO amount and vertical distributions were highly correlated for most of the time, indicating the horizontal length scales of BrO events were typically larger than ∼ 30 km in the absence of sea ice features. Although BrO amount and vertical distribution were similar between sites most of the time, rapid changes in BrO with edges significantly smaller than this ∼ 30 km length scale episodically transported between the sites, indicating BrO events were large but with sharp edge contrasts. BrO was often found in shallow layers that recycled reactive bromine via heterogeneous reactions on snowpack. Episodically, these surface-based events propagated aloft when aerosol extinction was higher (> 0.1 km−1); however, the presence of aerosol particles aloft was not sufficient to produce BrO aloft. Highly depleted ozone (< 1 nmol mol−1) repartitioned reactive bromine away from BrO and drove BrO events aloft in cases. This work demonstrates the interplay between atmospheric mixing and heterogeneous chemistry that affects the vertical structure and horizontal extent of reactive bromine events.

Constraints on Arctic Atmospheric Chlorine Production through Measurements and Simulations of Cl2 and ClO

During springtime, unique halogen chemistry involving chlorine and bromine atoms controls the prevalence of volatile organic compounds, ozone, and mercury in the Arctic lower troposphere. In situ measurements of the chlorine monoxide radical, ClO, and its precursor, Cl2, along with BrO and Br2, were conducted using chemical ionization mass spectrometry (CIMS) during the Bromine, Ozone, and Mercury Experiment (BROMEX) near Barrow, Alaska, in March 2012. To our knowledge, these data represent the first ClO measurements made using CIMS. A maximum daytime ClO concentration of 28 ppt was observed following an early morning peak of 75 ppt of Cl2. A zero-dimensional photochemistry model was constrained to Cl2 observations and used to simulate ClO during a 7-day period of the field campaign. The model simulates ClO within the measurement uncertainty, and the model results highlight the importance of chlorine chemistry participation in bromine radical cycling, as well as the dependence of halogen chemistry on NOx levels. The ClO measurements and simulations are consistent with Cl2 being the dominant Cl atom source in the Arctic boundary layer. Simulated Cl atom concentrations, up to ∼1 × 106 molecules cm-3, highlight the importance of chlorine chemistry in the degradation of volatile organic compounds, including the greenhouse gas methane.

The role of open lead interactions in atmospheric ozone variability between Arctic coastal and inland sites

Abstract Boundary layer atmospheric ozone depletion events (ODEs) are commonly observed across polar sea ice regions following polar sunrise. During March-April 2005 in Alaska, the coastal site of Barrow and inland site of Atqasuk experienced ODEs (O3< 10 nmol mol-1) concurrently for 31% of the observations, consistent with large spatial scale ozone depletion. However, 7% of the time ODEs were exclusively observed inland at Atqasuk. This phenomenon also occurred during one of nine flights during the BRomine, Ozone, and Mercury EXperiment (BROMEX), when atmospheric vertical profiles at both sites showed near-surface ozone depletion only at Atqasuk on 28 March 2012. Concurrent in-flight BrO measurements made using nadir scanning differential optical absorption spectroscopy (DOAS) showed the differences in ozone vertical profiles at these two sites could not be attributed to differences in locally occurring halogen chemistry. During both studies, backward air mass trajectories showed that the Barrow air masses observed had interacted with open sea ice leads, causing increased vertical mixing and recovery of ozone at Barrow and not Atqasuk, where the air masses only interacted with tundra and consolidated sea ice. These observations suggest that, while it is typical for coastal and inland sites to have similar ozone conditions, open leads may cause heterogeneity in the chemical composition of the springtime Arctic boundary layer over coastal and inland areas adjacent to sea ice regions.

The NO<sub><i>x</i></sub> dependence of bromine chemistry in the Arctic atmospheric boundary layer

Abstract. Arctic boundary layer nitrogen oxides (NOx = NO2 + NO) are naturally produced in and released from the sunlit snowpack and range between 10 to 100 pptv in the remote background surface layer air. These nitrogen oxides have significant effects on the partitioning and cycling of reactive radicals such as halogens and HOx (OH + HO2). However, little is known about the impacts of local anthropogenic NOx emission sources on gas-phase halogen chemistry in the Arctic, and this is important because these emissions can induce large variability in ambient NOx and thus local chemistry. In this study, a zero-dimensional photochemical kinetics model was used to investigate the influence of NOx on the unique springtime halogen and HOx chemistry in the Arctic. Trace gas measurements obtained during the 2009 OASIS (Ocean – Atmosphere – Sea Ice – Snowpack) field campaign at Barrow, AK were used to constrain many model inputs. We find that elevated NOx significantly impedes gas-phase halogen radical-based depletion of ozone, through the production of a variety of reservoir species, including HNO3, HO2NO2, peroxyacetyl nitrate (PAN), BrNO2, ClNO2 and reductions in BrO and HOBr. The effective removal of BrO by anthropogenic NOx was directly observed from measurements conducted near Prudhoe Bay, AK during the 2012 Bromine, Ozone, and Mercury Experiment (BROMEX). Thus, while changes in snow-covered sea ice attributable to climate change may alter the availability of molecular halogens for ozone and Hg depletion, predicting the impact of climate change on polar atmospheric chemistry is complex and must take into account the simultaneous impact of changes in the distribution and intensity of anthropogenic combustion sources. This is especially true for the Arctic, where NOx emissions are expected to increase because of increasing oil and gas extraction and shipping activities.

Uncertainties of Temperature Measurements on Snow-Covered Land and Sea Ice from In Situ and MODIS Data during BROMEX

AbstractThe Bromine, Ozone, and Mercury Experiment (BROMEX) was conducted in March and April of 2012 near Barrow, Alaska, to investigate impacts of Arctic sea ice reduction on chemical processes. During BROMEX, multiple sensors were deployed to measure air and surface temperature. The uncertainties in temperature measurement on snow-covered land and sea ice surfaces were examined using in situ data and temperature measurements that were derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) and are part of the Terra and Aqua ice-surface temperature and land-surface temperature (LST) standard data products. Following an ~24-h cross-calibration study, two Thermochrons (small temperature-sensing devices) were deployed at each of three field sites: a sea ice site in the Chukchi Sea, a mixed-cover site, and a homogeneous tundra site. At each site, one Thermochron was shielded from direct sunlight and one was left unshielded, and they were placed on top of the snow or ice. The best agreement between the Thermochron- and MODIS-derived temperatures was found between the shielded Thermochrons and the Aqua MODIS LSTs, with an average agreement of 0.6° ± 2.0°C (sample size of 84) at the homogeneous tundra site. The results highlight some uncertainties associated with obtaining consistent air and surface temperature measurements in the harsh Arctic environment, using both in situ and satellite sensors. It is important to minimize uncertainties that could introduce biases in long-term temperature trends.

The Heidelberg Airborne Imaging DOAS Instrument (HAIDI) – a novel imaging DOAS device for 2-D and 3-D imaging of trace gases and aerosols

Abstract. Many relevant processes in tropospheric chemistry take place on rather small scales (e.g., tens to hundreds of meters) but often influence areas of several square kilometer. Thus, measurements of the involved trace gases with high spatial resolution are of great scientific interest. In order to identify individual sources and sinks and ultimately to improve chemical transport models, we developed a new airborne instrument, which is based on the well established Differential Optical Absorption Spectroscopy (DOAS) method. The Heidelberg Airborne Imaging DOAS Instrument (HAIDI) is a passive imaging DOAS spectrometer, which is capable of recording horizontal and vertical trace gas distributions with a resolution of better than 100 m. Observable species include NO2, HCHO, C2H2O2, H2O, O3, O4, SO2, IO, OClO and BrO. Here we give a technical description of the instrument including its custom-built spectrographs and CCD detectors. Also first results from measurements with the new instrument are presented. These comprise spatial resolved SO2 and BrO in volcanic plumes, mapped at Mt. Etna (Sicily, Italy), NO2 emissions in the metropolitan area of Indianapolis (Indiana, USA) as well as BrO and NO2 distributions measured during arctic springtime in context of the BRomine, Ozone, and Mercury EXperiment (BROMEX) campaign, which was performed 2012 in Barrow (Alaska, USA).

Interdecadal changes in snow depth on Arctic sea ice

AbstractSnow plays a key role in the growth and decay of Arctic sea ice. In winter, it insulates sea ice from cold air temperatures, slowing sea ice growth. From spring to summer, the albedo of snow determines how much insolation is absorbed by the sea ice and underlying ocean, impacting ice melt processes. Knowledge of the contemporary snow depth distribution is essential for estimating sea ice thickness and volume, and for understanding and modeling sea ice thermodynamics in the changing Arctic. This study assesses spring snow depth distribution on Arctic sea ice using airborne radar observations from Operation IceBridge for 2009–2013. Data were validated using coordinated in situ measurements taken in March 2012 during the Bromine, Ozone, and Mercury Experiment (BROMEX) field campaign. We find a correlation of 0.59 and root‐mean‐square error of 5.8 cm between the airborne and in situ data. Using this relationship and IceBridge snow thickness products, we compared the recent results with data from the 1937, 1954–1991 Soviet drifting ice stations. The comparison shows thinning of the snowpack, from 35.1 ± 9.4 to 22.2 ± 1.9 cm in the western Arctic, and from 32.8 ± 9.4 to 14.5 ± 1.9 cm in the Beaufort and Chukchi seas. These changes suggest a snow depth decline of 37 ± 29% in the western Arctic and 56 ± 33% in the Beaufort and Chukchi seas. Thinning is negatively correlated with the delayed onset of sea ice freezeup during autumn.