Airborne, Ground-based, and Satellite Measurements of BrO during ARCTAS and ARCPAC
During the spring 2008 phase of NASA ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) and NOAA ARCPAC (Aerosol, Radiation, and Cloud Processes affecting Arctic Climate), measurements of BrO and related species were obtained in the Arctic region from in situ aircraft instruments, ground-based spectrometers, and satellite sensors. The field measurements were designed to reveal details about the spring-time phenomena of enhanced Arctic BrO that has been termed the "bromine explosion" and about how tropospheric ozone is removed by enhanced halogens. Surprisingly, regions of enhanced BrO and depleted ozone measured by instruments aboard the NASA DC-8 and NOAA P3 aircraft were often not co- located with satellite measurements of elevated total column BrO. This presentation will focus on attempts to reconcile these observations based on three dimensional model simulations that reveal many of the regions of enhanced total column BrO measured by the satellite sensors might be due to bromine above the tropopause, supplied to this region by biogenic halocarbons produced in the tropical ocean.
Preliminary Results from the 2009 OOTI Campaign: Bromine, Ozone and Mercury
As the extent of arctic sea ice changes, it is important to monitor changes in arctic chemical processes as species move back and forth between the ocean, atmosphere, sea-ice and snow. A large player in this story is bromide from sea salt. Once heterogeneously activated on snow or ice surfaces, it can be converted into bromine gas and subsequently interact with tropospheric Ozone and Mercury. Both of these species undergo depletion in the near-surface atmosphere. In the case of Ozone this happens largely by conversion to Bromine Oxide which remains in the atmosphere. For Mercury, it is the conversion from elemental Hg(0) to the inorganic and more soluble Hg(II) form which is more easily removed from the atmosphere entirely. To help quantify the processes which affect bromine, ozone and mercury, we will take an instrumented sled out over the ice (OOTI Sled) north of Barrow, Alaska on the Arctic Ocean in March and April of 2009. This activity is part of the Ocean-Atmospheres-Sea Ice-Snow (OASIS) segment of the International Polar Year. At our location we will measure local concentrations of tropospheric ozone and gaseous elemental mercury as well as bromine oxide using both spectroscopy and sample collection methods. Of particular interest to our campaign will be the observation of so-called Ozone Depletion Events (ODEs) and the related Atmospheric Mercury Depletion Events (AMDEs). We also hope to make direct measurements of bromine oxide and ozone directly above the snow pack as well as in the vicinity of frost flowers out over the ice. To this end, we plan to cut at least one hole in the ice which we hope will freeze over and fill in with fresh frost flowers which will then be available for sampling. We will report on our preliminary results from each of these observational thrusts and compare them to the results obtained from our previous field studies at Kuujjuarapik, Quebec in 2008.
Ground-based BrO measurements above Eureka, Nunavut during spring 2008
Measurements of bromine at high latitudes are scarce, so the current understanding of bromine chemistry is largely based on model calculations. In order to help quantify the amount of bromine in the atmosphere, we measured BrO columns with two ground-based UV-visible spectrometers at the Polar Environment Atmospheric Research Laboratory (PEARL) in Eureka, Nunavut, Canada (80°N, 86°W) in spring 2008. One of these instruments, the UT-GBS (University of Toronto Ground-Based Spectrometer), has been deployed at Eureka during polar sunrise since 1999. The other instrument, the PEARL-GBS (PEARL Ground-Based Spectrometer), was installed permanently in Eureka in August 2006 for year-round operation. The small signal and large diurnal variation of BrO are challenges for ground-based BrO retrievals. With zenith-sky measurements, we can retrieve vertical column densities of BrO, which are primarily sensitive to the stratosphere. We will discuss different methods for these retrievals and will compare our ground-based BrO vertical column density measurements with Ozone Monitoring Instrument on board the NASA Earth Observing System Aura satellite. Additionally, we are working on techniques to retrieve tropospheric partial columns of BrO using a combination of direct-sun measurements and zenith-sky measurements. We will present these retrievals and future plans for tropospheric BrO measurements at Eureka.
Arctic surface ozone depletions from ozone soundings and surface measurements
Episodes of ozone depletion in the lowermost Arctic atmosphere (0-2 km) at polar sunrise are thought to result from catalytic reactions involving bromine derived from sea salt. Arctic sites showed ozone depletion in the surface boundary layer throughout the spring ARC-IONS campaign. This was occasionally severe. However, the frequency of depletion events in the spring of 2008 was relatively low compared to historical norms. The long ozonesonde record at Resolute shows depletions since 1966, but with an increase in their frequency over the period 1966-2008 of 5.8 ± 4.3% per decade (95% confidence limits), while that at Churchill shows much lower frequency overall, but an increase over the period 1974-2008 of 1.9 ± 1.3% per decade. Surface measurements indicate a shift toward increasing frequency earlier in the year. Although satellite observations of high amounts of column BrO over large areas of the Arctic are thought to be an indicator of ozone depletion, depletion events correlate only weakly with satellite BrO, even when integrated over several days by means of back-trajectory calculations.
Estimates of Cl and Br Concentrations in the Springtime Arctic From Hydrocarbon Measurements During ARCTAS
As part of the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) 2008 field campaign, whole air samples were collected on board the NASA DC-8 airborne science platform, and analyzed for a suite of nonmethane hydrocarbons. During the springtime phase of the ARCTAS field campaign, halogen radical concentrations for Cl and Br were estimated during high latitude marine boundary layer ozone depletion events. Nonmethane hydrocarbons are removed from the atmosphere at different rates via photochemical oxidation by OH, Cl, and Br. Using selected hydrocarbons, the presence and abundance of halogen radicals can be determined based on their relative hydrocarbon reaction rates using the hydrocarbon clock method. Previous studies have found that surface layer ozone depletion events are typically associated with evidence of halogen chemistry, and the halogen radical estimates made during the ARCTAS campaign are comparable to measurements made during previous ground based studies at Alert, Nunavut, and Ny Ålesund, Svalbard.
Integrated Analysis of the Impact of Long-Range Transport of Midlatitude Pollution on Ozone Abundances in the Arctic Troposphere
We use the GEOS-Chem global chemical transport model and its adjoint, together with satellite and in situ observation of tropospheric ozone, to assess the impact of transport of pollution from midlatitudes on the abundance of ozone in the Arctic. The model reproduces well the seasonal cycle in the abundances of PAN and ozone as measured at the surface at Alert. However, relative to ozonesonde measurements, the model overestimates ozone in the middle and upper troposphere in spring, while it underestimates ozone in summer. We examine the information gained by assimilating tropospheric ozone profile retrievals from the Tropospheric Emission Spectrometer (TES) satellite instrument at midlatitudes to provide an improved boundary condition for ozone at midlatitudes to better quantify the transport of ozone into the Arctic. We find that the assimilation corrects model biases relative to sondes in the Arctic free troposphere, implying an increase in the net northward ozone flux. We also use the adjoint model to conduct a detailed analysis of the sensitivity of the modeled ozone abundances in the Arctic to midlatitude precursor emissions.
Source-receptor relationship: A case study of the origin of Canadian High Arctic air masses
In summer 2006 measurements at the Polar Environment Atmospheric Research Laboratory (PEARL) located in the Canadian Arctic (80oN, 86oW) observed episodes of high pollutants (sulphate, organic). We have used trajectory and particle dispersion models FLEXTRA and FLEXPART to study the relationship between the air mass arriving at PEARL within the episode period and the contributing sources, as well as the long-range transport pattern. FLEXPART is a Lagrangian particle dispersion model that simulates the long-range transport, diffusion, dry and wet deposition, and radioactive decay of tracers released from point, line, area or volume sources. Backward simulations of both FLEXTRA and FLEXPART indicate that between September 1 and September 4 (one of the episodes) there was substantial contribution from Norilsk, Northern Russia. In fact, The air mass from Norilsk travelled northwards through Chelyuskin, crossed the Arctic Sea and proceeded towards PEARL. Most of the transport was limited to the Arctic (> 80 ) and at low altitude (< 4 km asl). The total time travelled by the air mass from Norilsk was between 10-20 days. Further analysis is required in order to quantify the regional contributions to these Arctic episodes.