Levels Of BrO Research Articles

Modeling the observed tropospheric BrO background: Importance of multiphase chemistry and implications for ozone, OH, and mercury

AbstractAircraft and satellite observations indicate the presence of ppt (ppt ≡ pmol/mol) levels of BrO in the free troposphere with important implications for the tropospheric budgets of ozone, OH, and mercury. We can reproduce these observations with the GEOS‐Chem global tropospheric chemistry model by including a broader consideration of multiphase halogen (Br‐Cl) chemistry than has been done in the past. Important reactions for regenerating BrO from its nonradical reservoirs include HOBr + Br−/Cl− in both aerosols and clouds, and oxidation of Br− by ClNO3 and ozone. Most tropospheric BrO in the model is in the free troposphere, consistent with observations and originates mainly from the photolysis and oxidation of ocean‐emitted CHBr3. Stratospheric input is also important in the upper troposphere. Including production of gas phase inorganic bromine from debromination of acidified sea salt aerosol increases free tropospheric Bry by about 30%. We find HOBr to be the dominant gas‐phase reservoir of inorganic bromine. Halogen (Br‐Cl) radical chemistry as implemented here in GEOS‐Chem drives 14% and 11% decreases in the global burdens of tropospheric ozone and OH, respectively, a 16% increase in the atmospheric lifetime of methane, and an atmospheric lifetime of 6 months for elemental mercury. The dominant mechanism for the Br‐Cl driven tropospheric ozone decrease is oxidation of NOx by formation and hydrolysis of BrNO3 and ClNO3.

Measurement-based modelling of bromine-induced oxidation of mercury above the Dead Sea

Abstract. Atmospheric mercury depletion events (AMDEs) outside the polar region – driven by high levels of gaseous Br and BrO (i.e., BrOx) – were observed recently in the warm Dead Sea boundary layer. The efficient oxidation of gaseous elemental mercury (GEM) under temperate conditions by BrOx was unexpected considering that the thermal back dissociation reaction of HgBr is about 2.5 orders of magnitude higher under Dead Sea temperatures compared to polar temperatures, and hence was expected to significantly slow down GEM oxidation under warm temperatures. The goal of this modelling study was to improve understanding of the interaction of reactive bromine and mercury during Dead Sea AMDEs using numerical simulations based on a comprehensive measurement campaign in summer 2009. Our analysis is focused on daytime AMDE when chemical processes dominate concentration changes. Best agreements between simulations and observations were achieved using rate constants for kHg+Br and kHg+BrO of 2.7 × 10−13 cm3 molecule−1 s−1 and 1.5 × 10−13 cm3 molecule−1 s−1, respectively. Our model also predicted that a rate constant kHg+BrO of 5.0 × 10−14 cm3 molecule−1 s−1 may be considered as a minimum, which is higher than most reported values. These rate constants suggest that BrO could be a more efficient oxidant than Br in the troposphere as long as [Br]/[BrO] ratios are smaller than ~0.2 to 0.5. Under Dead Sea conditions, these kinetics demonstrate a high efficiency and central role of BrOx for AMDEs, with relative contributions to GEM depletion of more than ~90%. Unexpectedly, BrO was found to be the dominant oxidant with relative contributions above 80%. The strong contribution of BrO could explain why the efficiency of GEM oxidation at the Dead Sea does not critically depend on Br and, therefore, is comparable to that in cold polar regions. In order to confirm the suggested kinetics, additional studies, particularly for temperature-dependence of rate constants, are required.

Temperature and sunlight controls of mercury oxidation and deposition atop the Greenland ice sheet

Abstract. We conducted the first ever mercury speciation measurements atop the Greenland ice sheet at Summit Station (Latitude 72.6° N, Longitude 38.5° W, Altitude 3200 m) in the Spring and Summer of 2007 and 2008. These measurements were part of the collaborative Greenland Summit Halogen-HOx experiment (GSHOX) campaigns investigating the importance of halogen chemistry in this remote environment. Significant levels of BrO (1–5 pptv) in the near surface air were often accompanied by diurnal dips in gaseous elemental mercury (GEM), and in-situ production of reactive gaseous mercury (RGM). While halogen (i.e. Br) chemistry is normally associated with marine boundary layers, at Summit, Greenland, far from any marine source, we have conclusively detected bromine and mercury chemistry in the near surface air. The likely fate of the formed mercury-bromine radical (HgBr) is further oxidation to stable RGM (HgBr2, HgBrOH, HgBrCl...), or thermal decomposition. These fates appear to be controlled by the availability of Br, OH, Cl, etc. to produce RGM (Hg(II)), versus the lifetime of HgBr by thermal dissociation. At Summit, the production of RGM appears to require a sun elevation angle of >5 degrees, and an air temperature of 5 degrees, while the formation of RGM from HgBr requires a temperature

Toward a better quantitative understanding of polar stratospheric ozone loss

Previous studies have shown that observed large O3 loss rates in cold Arctic Januaries cannot be explained with current understanding of the loss processes, recommended reaction kinetics, and standard assumptions about total stratospheric chlorine and bromine. Studies based on data collected during recent field campaigns suggest faster rates of photolysis and thermal decomposition of ClOOCl and higher stratospheric bromine concentrations than previously assumed. We show that a model accounting for these kinetic changes and higher levels of BrO can largely resolve the January Arctic O3 loss problem and closely reproduces observed Arctic O3 loss while being consistent with observed levels of ClO and ClOOCl. The model also suggests that bromine catalysed O3 loss is more important relative to chlorine catalysed loss than previously thought.

Fast Ozone Loss Around the Polar Vortex During 2002/2003 Arctic Winter Deep Minihole Event

A three-dimensional (3D) off-line chemical transport model (CTM) has been used to study an Arctic ozone minihole event in early December 2002, when the meteorological conditon were severely colder than in recent years. The minimum temperatures were below 188 K on 6 December 2002 and remained below 195 K for the entire month. Such temperatures are low enough for the formation of PSCs (polar stratospheric clouds) which were observed by a visible/near infrared solar occultation instrument by the Polar Ozone and Aerosol Measurement (POAM) III experiment. The SLIMCAT model compares well with the depth and evolution of the ozone distribution as observed by the second European Remote Sensing Satellite (ERS-2) Global Ozone Monitoring Experiment (GOME). The combined effect of a very strong and large intrusion of sub-tropical air, and uplifting of air masses of low ozone concentrations from the troposphere plays important role in the fomation of the ozone minihole. The modelled chemical ozone loss rate was relatively fast in the lower stratosphere in the region of frequent PSCs and higher levels of ClO and BrO. The observed PSCs in the vicinity of the minihole supports the model-predicted location of PSCs and chlorine activation. However, ozone miniholes have been attributed almost entirely to dynamical effects due to its short duration.

Dynamics and chemistry of tropospheric bromine explosion events in the Antarctic coastal region

We investigate chemistry and dynamics of bromine explosion events during Antarctic spring of 1999 and 2000 using ground‐based differential optical absorption spectroscopy (DOAS) observations of BrO, surface ozone, and ozone sounding measurements performed at the German research station Neumayer, located at the coast of the Antarctic continent (70°39′S, 8°15′W). BrO maps from the GOME satellite instrument show huge areas of elevated BrO above the sea ice around Antarctica, covering several thousand square kilometers. Although the Neumayer station is located at a distance of only 7 km to the coast, bromine‐activated air masses are only detected by our ground‐based DOAS instrument under particular meteorological conditions. We use trajectory calculations in high temporal and spatial resolution in combination with sea ice maps to determine the source of air masses arriving at Neumayer. A comparison of these model calculations with measurements is in very good agreement with the theory that reactive bromine is heterogeneously released by sea‐salt surfaces: in the majority of cases, enhanced levels of BrO are observed whenever a part of the probed air masses was previously in contact with sea ice surfaces, while depletion of surface ozone occurs only if the air at ground comes from the ice covered ocean. After being uplifted by advection processes, bromine‐activated and ozone‐depleted air masses are sometimes observed at high altitudes (>4000 m). This vertical transport of reactive bromine may have an impact on the free tropospheric ozone budget and on local climate in the Antarctic coastal region during springtime. BrO enhancements are usually accompanied by strong increases in light path (as detected by O4) owing to multiple scattering on aerosols, supporting the assumption that sea‐salt particles, on which BrO recycling can take place, are present at the observation site.

In situ observations of ClO near the winter polar tropopause

Significant abundances of chlorine oxide (ClO) were observed throughout the lowermost stratosphere at high latitudes during winter from the NASA DC‐8 aircraft during the SAGE III—Ozone Loss and Validation Experiment and Third European Stratospheric Experiment on Ozone 2000 (SOLVE/THESEO 2000) campaign. Mixing ratios of ClO averaging 15–20 parts per trillion by volume (pptv) were observed near the tropopause, a region where ClO abundances are usually only a pptv or less at lower latitudes. The ratio of ClO to inferred inorganic chlorine ([ClO]/[Cly]) was found to be largest (∼7%) in air characterized by low abundances of ozone (∼100–250 parts per billion by volume (ppbv)). This was the region where cirrus clouds were also observed occasionally during the measurement period, although abundances of ClO directly within cirrus clouds were not significantly different than background abundances. Nonzero instrument signals during darkness are attributed to detection of ∼5–15 pptv of OClO. BrO mixing ratios between 2 and 4 pptv are sufficient to produce these amounts of OClO, assuming daytime mixing ratios of ClO between 15 and 20 pptv. At these levels of ClO and BrO, approximately 10% of the ozone at these altitudes is chemically destroyed per month in springtime by reactions of ClO and BrO, representing an effective loss process for ozone near the high‐latitude tropopause.

DOAS-observation of halogen radical-catalysed arctic boundary layer ozone destruction during the ARCTOC-campaigns 1995 and 1996 in Ny-Ålesund, Spitsbergen

During two field campaigns in spring 1995 and 1996, boundary layer concentration levels of BrO, ClO, IO, SO2, NO2, HNO2, CH2O and O3, were observed at a time resolution of better than 1.5 h by Differential Optical Absorption Spectroscopy (DOAS). Up to 30 ppt of both, BrO and ClO, were found to coincide with low ozone events (LOE). During periods of normal ozone, average levels of BrO and CIO were close to zero. Average CIO levels during LOE were considerably higher in 1995 (21 ppt) than in 1996 (3.3 ppt). Generally, only upper limits for NO2 and SO2 of 50 ppt can be given, however, a series of concentration spikes (up to a few ppb) observed are in the case of NO2 most likely due to local pollution, while the origin of the SO2 spikes remains unclear. Analysis of the data suggests that the ozone loss is most likely caused by a combination of BrOx (= Br + BrO)- and BrOx+ ClOx- catalysed ozone destruction (with the BrO self reaction and ClO + BrO reaction being the rate limiting steps, respectively). The presence of IO at levels exceeding 1–2 ppt and thus its contribution to ozone destruction is unlikely, but cannot be completely excluded. The observed levels of BrO and ClO are sufficiently high to destroy 1–2 ppb O3 per hour, thus completely eliminating 40 ppb of initial O3 within 1–2 days. However airmass dynamics is also likely to be important, in particular observed O3 decrease rates of up to 7 ppb/h can only be attributed to advection of air already depleted in ozone. The Cl-atom and Br-atom concentrations deduced from ClO and BrO measurements and estimates of ClO/CL and BrO/Br ratios, are compared with data derived from “hydrocarbon clock” observations in the arctic. While both sets of data are compatible for BrO and 1996 ClO data, in 1995 the DOAS ClO data indicate much higher Cl-atom levels than seen by hydrocarbon clock observations.

Measurements of arctic sunrise surface ozone depletion events at Kangerlussuaq, Greenland (67°N, 51°W)

In situ measurements of surface ozone were conducted from 30 January through 22 May 1995, at the Sondrestrom Incoherent Scatter Radar Facility near Kangerlussuaq, Greenland. Several periods of depleted ozone were observed, representing the lowest latitude measurements of springtime surface polar ozone depletion in the northern hemisphere to date. The most severe ozone depletion occurred during 14-17 March, when surface ozone levels abruptly fell to below 10 ppbv from typical values of about 40 ppbv. Simultaneous near-ultraviolet and visible total column spectroscopic measurements were made with ground-based instruments, yielding total column values for BrO and OClO. Elevated levels of BrO were observed at the onset of ozone depletion episodes and the behaviour of BrO slant column values through the morning twilight of 15 March indicated that the BrO column must reside in the boundary layer. The boundary layer BrO mixing ratio was estimated at greater than 13 pptv during this episode. OClO levels showed no correlation with surface ozone depletion nor clear evidence of a large tropospheric burden.

Spectroscopic measurement of the free radicals NO3, BRO, IO, and OH in the troposphere

Free radicals are the driving force for most chemical processes in the atmosphere. In particular OH- and nitrate radicals (NO3) play a central role in the troposphere. However, recent investigations indicate an important influence of the halogen-oxide radicals BrO, IO, and possibly ClO on tropospheric chemistry. Therefore, the knowledge of the concentration of those species in the atmosphere is a key requirement for the investigation of atmospheric chemistry. Unfortunately the low concentration of free radicals makes measurements particularly difficult. Among several techniques applied to the problem UV/visible differential absorption spectroscopy (also known as DOAS) appears to be the most successful for the observation of the above species. Detection limits of the order of 106 to 107 molec/cm3 have been reached, which are sufficiently low to resolve the diurnal variation profiles of the observed radicals. As examples tropospheric DOAS measurements of NO3- and OH radicals at mid latitudes are presented, as well as observations of BrO in the Canadian Arctic. The latter measurements are discussed with a scenario combining BrO catalyzed ozone loss and atmospheric dispersion to explain the observed rapid changes in ozone with the measured levels of BrO.

Surface ozone depletion in Arctic spring sustained by bromine reactions on aerosols

NEAR-TOTAL depletion of the ozone in surface air is often observed in the Arctic spring, coincident with high atmospheric concentrations of inorganic bromine1–5. Barrie et al.1 suggested that the ozone depletion was due to a catalytic cycle involving the radicals Br and BrO (ref. 6); however, these species are rapidly converted to the nonradical species HBr, HOBr and BrNO3, quenching ozone loss. McConnell et al.7 proposed that cycling of inorganic bromine between aerosols and the gas phase could maintain sufficiently high levels of Br and BrO to destroy ozone, but they did not specify a mechanism for aerosol-phase production of active bromine species. Here we propose such a mechanism, based on known aqueous-phase chemistry, which rapidly converts HBr, HOBr and BrNO3 back to Br and BrO radicals. This mechanism should be particularly efficient in the presence of the high concentrations of sulphuric acid aerosols observed during ozone depletion events3.