Ozone Depletion Events Research Articles

Bromide and other ions in the snow, firn air, and atmospheric boundary layer at Summit during GSHOX

Abstract. Measurements of gas phase soluble bromide in the boundary layer and in firn air, and Br− in aerosol and snow, were made at Summit, Greenland (72.5° N, 38.4° W, 3200 m a.s.l.) as part of a larger investigation into the influence of Br chemistry on HOx cycling. The soluble bromide measurements confirm that photochemical activation of Br− in the snow causes release of active Br to the overlying air despite trace concentrations of Br− in the snow (means 15 and 8 nmol Br− kg−1 of snow in 2007 and 2008, respectively). Mixing ratios of soluble bromide above the snow were also found to be very small (mean <1 ppt both years, with maxima of 3 and 4 ppt in 2007 and 2008, respectively), but these levels clearly oxidize and deposit long-lived gaseous elemental mercury and may perturb HOx partitioning. Concentrations of Br− in surface snow tended to increase/decrease in parallel with the specific activities of the aerosol-associated radionuclides 7Be and 210Pb. Earlier work has shown that ventilation of the boundary layer causes simultaneous increases in 7Be and 210Pb at Summit, suggesting there is a pool of Br in the free troposphere above Summit in summer time. Speciation and the source of this free tropospheric Br− are not well constrained, but we suggest it may be linked to extensive regions of active Br chemistry in the Arctic basin which are known to cause ozone and mercury depletion events shortly after polar sunrise. If this hypothesis is correct, it implies persistence of the free troposphere Br− for several months after peak Br activation in March/April. Alternatively, there may be a ubiquitous pool of Br− in the free troposphere, sustained by currently unknown sources and processes.

Aircraft observations of enhancement and depletion of black carbon mass in the springtime Arctic

Abstract. Understanding the processes controlling black carbon (BC) in the Arctic is crucial for evaluating the impact of anthropogenic and natural sources of BC on Arctic climate. Vertical profiles of BC mass loadings were observed from the surface to near 7-km altitude in April 2008 using a Single-Particle Soot Photometer (SP2) during flights on the NOAA WP-3D research aircraft from Fairbanks, Alaska. These measurements were conducted during the NOAA-sponsored Aerosol, Radiation, and Cloud Processes affecting Arctic Climate (ARCPAC) project. In the free troposphere, the Arctic air mass was influenced by long-range transport from biomass-burning and anthropogenic source regions at lower latitudes especially during the latter part of the campaign. Average BC mass mixing ratios peaked at about 150 ng BC (kg dry air )−1 near 5.5 km altitude in the aged Arctic air mass and 250 ng kg−1 at 4.5 km in biomass-burning influenced air. BC mass loadings were enhanced by up to a factor of 5 in biomass-burning influenced air compared to the aged Arctic air mass. At the bottom of some of the profiles, positive vertical gradients in BC were observed over the sea-ice. The vertical profiles generally occurred in the vicinity of open leads in the sea-ice. In the aged Arctic air mass, BC mass loadings more than doubled with increasing altitude within the ABL and across the boundary layer transition while carbon monoxide (CO) remained constant. This is evidence for depletion of BC mass in the ABL. BC mass loadings were positively correlated with O3 in ozone depletion events (ODEs) for all the observations in the ABL. Since bromine catalytically destroys ozone in the ABL after being released as molecular bromine in regions of new sea-ice formation at the surface, the BC–O3 correlation suggests that BC particles were removed by a surface process such as dry deposition. We develop a box model to estimate the dry deposition flux of BC mass to the snow constrained by the vertical profiles of BC mass in the ABL. Open leads in the sea-ice may increase vertical mixing and entrainment of pollution from the free troposphere possibly enhancing the deposition of BC aerosol to the snow.

Spectroscopic studies of the heterogeneous reaction between O3(g) and halides at the surface of frozen salt solutions

Springtime ozone depletion events have been linked to elevated levels of bromine in the polar boundary layer. It has also been suggested that iodine may play an important role in boundary layer chemistry. However, the mechanism(s) for initial halogen activation is not well understood. We report results of experiments using glancing‐angle laser‐induced fluorescence (LIF) and glancing‐angle Raman spectroscopy to investigate the heterogeneous reaction of gas‐phase ozone with bromide and iodide ions at the surface of frozen salt solutions. The results suggest that halides that have been excluded to the ice surface during the freezing process react in the dark and under non‐acidified conditions to produce molecular halogen species. At environmentally relevant bromide concentrations, the reaction proceeds ∼60 times more rapidly at ice surfaces than at the surface of aqueous solutions. At an ozone concentration of ∼8 × 1015 molec cm−3, and sea water concentrations of bromide and iodide, we estimate uptake coefficients γ(Br−) = (1.3 ± 0.5) × 10−8 and γ(I−) = (1.6 ± 0.5) × 10−9. We estimate that at atmospheric ozone concentrations, the uptake coefficients will be approximately a factor of ten greater.

Vertical structure of Antarctic tropospheric ozone depletion events: characteristics and broader implications

Abstract. The majority of tropospheric ozone depletion event (ODE) studies have focussed on time-series measurements, with comparatively few studies of the vertical component. Those that exist have almost exclusively used free-flying balloon-borne ozonesondes and almost all have been conducted in the Arctic. Here we use measurements from two separate Antarctic field experiments to examine the vertical profile of ozone during Antarctic ODEs. We use tethersonde data to probe details in the lowest few hundred meters and find considerable structure in the profiles associated with complex atmospheric layering. The profiles were all measured at wind speeds less than 7 ms−1, and on each occasion the lowest inversion height lay between 10 m and 40 m. We also use data from a free-flying ozonesonde study to select events where ozone depletion was recorded at altitudes >1 km above ground level. Using ERA-40 meteorological charts, we find that on every occasion the high altitude depletion was preceded by an atmospheric low pressure system. An examination of limited published ozonesonde data from other Antarctic stations shows this to be a consistent feature. Given the link between BrO and ODEs, we also examine ground-based and satellite BrO measurements and find a strong association between atmospheric low pressure systems and enhanced BrO that must arise in the troposphere. The results suggest that, in Antarctica, such depressions are responsible for driving high altitude ODEs and for generating the large-scale BrO clouds observed from satellites. In the Arctic, the prevailing meteorology differs from that in Antarctica, but, while a less common effect, major low pressure systems in the Arctic can also generate BrO clouds. Such depressions thus appear to be fundamental when considering the broader influence of ODEs, certainly in Antarctica, such as halogen export and the radiative influence of ozone-depleted air masses.

Snow-sourced bromine and its implications for polar tropospheric ozone

Abstract. In the last two decades, significant depletion of boundary layer ozone (ozone depletion events, ODEs) has been observed in both Arctic and Antarctic spring. ODEs are attributed to catalytic destruction by bromine radicals (Br plus BrO), especially during bromine explosion events (BEs), when high concentrations of BrO periodically occur. However, neither the exact source of bromine nor the mechanism for sustaining the observed high BrO concentrations is completely understood. Here, by considering the production of sea salt aerosol from snow lying on sea ice during blowing snow events and the subsequent release of bromine, we successfully simulate the BEs using a global chemistry transport model. We find that heterogeneous reactions play an important role in sustaining a high fraction of the total inorganic bromine as BrO. We also find that emissions of bromine associated with blowing snow contribute significantly to BrO at mid-latitudes. Modeled tropospheric BrO columns generally compare well with the tropospheric BrO columns retrieved from the GOME satellite instrument (Global Ozone Monitoring Experiment). The additional blowing snow bromine source, identified here, reduces modeled high latitude lower tropospheric ozone amounts by up to an average 8% in polar spring.

Introducing the bromide/alkalinity ratio for a follow-up discussion on "Precipitation of salts in freezing seawater and ozone depletion events: a status report", by Morin et al., published in Atmos. Chem. Phys., 8, 7317–7324, 2008

Abstract. Sander et al. (2006) proposed that CaCO3 precipitation can be an important factor in triggering tropospheric ozone depletion events. Recently, Morin et al. (2008b) presented calculations with the FREZCHEM model and concluded that their results and interpretation cast doubt on the validity of this hypothesis. In this joint publication, we have re-analyzed the implications of the FREZCHEM results and show how they can be reconciled with the proposal of Sander et al. (2006). The chemical predictions of both approaches are consistent. Although an interpretation solely based on the alkalinity change in the brine does not support the conclusion of Sander et al. (2006), we show that the bromide/alkalinity ratio (which increases during the cooling of the brine) can be used as an indicator of the potential for triggering bromine explosions.

Satellite observations of long range transport of a large BrO plume in the Arctic

Abstract. Ozone Depletion Events (ODE) during polar springtime are a well known phenomenon in the Arctic and Antarctic boundary layer. They are caused by the catalytic destruction of ozone by halogens producing reactive halogen oxides like bromine monoxide (BrO). The key halogen bromine can be rapidly transferred into the gas phase in an autocatalytic process – the so called "Bromine Explosion". However, the exact mechanism, which leads to an initial bromine release as well as the influence of transport and chemical processes on BrO, is still not clearly understood. In this study, BrO measurements from the satellite instrument GOME-2 are used together with model calculations with the dispersion model FLEXPART to study an arctic BrO event in March 2007, which could be tracked over several days and a large area. Full BrO activation was observed within one day east of Siberia with subsequent transport to Hudson Bay. The event was linked to a cyclone with very high surface wind speeds, which could have been involved in the production and lifting of aerosols or blowing snow. Considering the short life time of BrO, transported aerosols or snow can also provide the surface for BrO recycling within the plume for several days. The evolution of the BrO plume could be reproduced by FLEXPART simulations of a passive tracer indicating that the activated air mass was transported all the way from Siberia to Hudson Bay. To localise the most probable transport height, model runs initialised in different heights have been performed showing similar transport patterns throughout the troposphere but best agreement with the measurements between the surface and 3 km. The influence of changes in tropopause height on measured BrO values has been considered, but cannot completely explain the observed high BrO values. Backward trajectories from the area of BrO initialisation show upward lifting from the surface up to 3 km and no indication for intrusion of stratospheric air. These observations are consistent with a scenario in which bromine in the air mass was activated on the surface within the cyclone, lifted upwards and transported over several thousand kilometres to Hudson Bay.

Trends in long-term gaseous mercury observations in the Arctic and effects of temperature and other atmospheric conditions

Abstract. Gaseous elemental mercury (GEM) measurements at Alert, Canada, from 1995 to 2007 were analyzed for statistical time trends and for correlations with meteorological and climate data. A significant decreasing trend in annual GEM concentration is reported at Alert, with an estimated slope of −0.0086 ng m−3 yr−1 (−0.6% yr−1) over this 13-year period. It is shown that there has been a shift in the month of minimum mean GEM concentration from May to April due to a change in the timing of springtime atmospheric mercury depletion events (AMDEs). These AMDEs are found to decrease with increasing local temperature within each month, both at Alert and at Amderma, Russia. These results support the temperature dependence suggested by previous experimental results and theoretical kinetic calculations on both bromine generation and mercury oxidation and highlight the potential for changes in Arctic mercury chemistry with climate. A correlation between total monthly AMDEs at Alert and the Polar/Eurasian Teleconnection Index was observed only in March, perhaps due to higher GEM inputs in early spring in those years with a weak polar vortex. A correlation of AMDEs at Alert with wind direction supports the origin of mercury depletion events over the Arctic Ocean, in agreement with a previous trajectory study of ozone depletion events. Interannual variability in total monthly depletion event frequency at Alert does not appear to correlate significantly with total or first-year northern hemispheric sea ice area or with other major teleconnection patterns. Nor do AMDEs at either Alert or Amderma correlate with local wind speed, as might be expected if depletion events are sustained by stable, low-turbulence atmospheric conditions. The data presented here – both the change in timing of depletion events and their relationship with temperature – can be used as additional constraints to improve the ability of models to predict the cycling and deposition of mercury in the Arctic.

Source identification of short-lived air pollutants in the Arctic using statistical analysis of measurement data and particle dispersion model output

Abstract. As a part of the IPY project POLARCAT (Polar Study using Aircraft, Remote Sensing, Surface Measurements and Models, of Climate Chemistry, Aerosols and Transport), this paper studies the sources of equivalent black carbon (EBC), sulphate, light-scattering aerosols and ozone measured at the Arctic stations Zeppelin, Alert, Barrow and Summit during the years 2000–2007. These species are important pollutants and climate forcing agents, and sulphate and EBC are main components of Arctic haze. To determine where these substances originate, the measurement data were combined with calculations using FLEXPART, a Lagrangian particle dispersion model. The climatology of atmospheric transport from surrounding regions on a twenty-day time scale modelled by FLEXPART shows that the stations Zeppelin, Alert and Barrow are highly sensitive to surface emissions in the Arctic and to emissions in high-latitude Eurasia in winter. Emission sensitivities over southern Asia and southern North America are small throughout the year. The high-altitude station Summit is an order of magnitude less sensitive to surface emissions in the Arctic whereas emissions in the southern parts of the Northern Hemisphere continents are more influential relative to the other stations. Our results show that for EBC and sulphate measured at Zeppelin, Alert and Barrow, northern Eurasia is the dominant source region. For sulphate, Eastern Europe and the metal smelting industry in Norilsk are particularly important. For EBC, boreal forest fires also contribute in summer. No evidence for any substantial contribution to EBC from sources in southern Asia is found. European air masses are associated with low ozone concentrations in winter due to titration by nitric oxides, but are associated with high ozone concentrations in summer due to photochemical ozone formation. There is also a strong influence of ozone depletion events in the Arctic boundary layer on measured ozone concentrations in spring and summer. These results will be useful for developing emission reduction strategies for the Arctic.

Modelling the multiphase near-surface chemistry related to ozone depletions in polar spring

Near-total depletions of ozone have been observed in the Arctic spring since the mid 1980s. The autocatalytic reaction cycles involving reactive halogens are now recognized to be of main importance for ozone depletion events in the polar boundary layer. We present sensitivity studies using the model MISTRA in the box-model mode on the influence of chemical species on these ozone depletion processes. In order to test the sensitivity of the chemistry under polar conditions, we compared base runs undergoing fluxes of either Br2, BrCl, or Cl2 to induce ozone depletions, with similar runs including a modification of the chemical conditions. The role of HCHO, H2O2, DMS, Cl2, C2H6, HONO, NO2, and RONO2 was investigated. Cases with elevated mixing ratios of HCHO, H2O2, DMS, Cl2, and HONO induced a shift in bromine speciation from Br / BrO to HOBr/HBr, while high mixing ratios of C2H6 induced a shift from HOBr/HBr to Br/BrO. The shifts from Br/BrO to HOBr/HBr accelerated the aerosol debromination, but also increased the total amount of deposited bromine at the surface (mainly via increased deposition of HOBr). For all NOy species studied (HONO, NO2, RONO2) the chemistry is characterized by an increased bromine deposition on snow reducing the amount of reactive bromine in the air. Ozone is less depleted under conditions of high mixing ratios of NOx. The production of HNO3 led to the acid displacement of HCl, and the release of chlorine out of salt aerosol (Cl2 or BrCl) increased.

Year‐round record of surface ozone at coastal (Dumont d'Urville) and inland (Concordia) sites in East Antarctica

Surface ozone is measured since 2004 at the coastal East Antarctic station of Dumont d'Urville (DDU) and since 2007 at the Concordia station located on the high East Antarctic plateau. Ozone levels at Concordia reach a maximum of 35 ppbv in July and a minimum of 21 ppbv in February. From November to January, sudden increases of the ozone level, up to 15–20 ppbv above average, often take place. They are attributed to local photochemical ozone production as previously seen at the South Pole. The detailed examination of the diurnal ozone record in summer at Concordia suggests a local photochemical ozone production of around 0.2 ppbv h−1 during the morning. The ozone record at DDU exhibits a maximum of 35 ppbv in July and a minimum of 18 ppbv in January. Mixing ratios at DDU are always higher than those at Neumayer (NM), another coastal Antarctic station. A noticeable difference in the ozone records at the two coastal sites lies in the larger ozone depletion events occurring from July to September at NM compared to DDU, likely due to stronger BrO episodes in relation with a larger sea ice coverage offshore that site. A second difference is the large day‐to‐day fluctuations which are observed from November to January at DDU but not at NM. That is attributed to a stronger impact at DDU than at NM of air masses coming from the Antarctic plateau. The consequences of such a high oxidizing property of the atmosphere over East Antarctica are discussed with regard to the dimethylsulfide (DMS) chemistry.

A DFT study of the reactions of O 3 with Hg° or Br −

In the mid 1980s the study of ozone reactivity gained a significant interest with the discoveries of the stratospheric ozone hole ( Farman et al., 1985) and of the ozone depletion events in the polar boundary layer ( Oltmans et al., 1989). In the stratosphere, the mechanism involves heterogeneous reactions on polar stratospheric clouds that lead to chlorine activation ( Solomon et al., 1986). In contrast, tropospheric ozone depletion occurring during polar springtime rather involves reactive bromine species. They are released during a series of photochemical and heterogeneous reactions often called the bromine explosion (see the review of Simpson et al., 2007). In this reaction sequence, an essential step is the generation of photolyzable Br 2, the precursor of two Br atoms, via the multiphasic reaction (1): (1) HOBr + Br − + H + → H 2O + Br 2 The production of reactive HOBr could occur with the oxidation of BrO by HO 2.

Modelling chemistry over the Dead Sea: bromine and ozone chemistry

Abstract. Measurements of O3 and BrO concentrations over the Dead Sea indicate that Ozone Depletion Events (ODEs), widely known to happen in polar regions, are also occuring over the Dead Sea due to the very high bromine content of the Dead Sea water. However, we show that BrO and O3 levels as they are detected cannot solely be explained by high Br− levels in the Dead Sea water and the release of gas phase halogen species out of sea borne aerosol particles and their conversion to reactive halogen species. It is likely that other sources for reactive halogen compounds are needed to explain the observed concentrations for BrO and O3. To explain the chemical mechanism taking place over the Dead Sea leading to BrO levels of several pmol/mol we used the one-dimensional model MISTRA which calculates microphysics, meteorology, gas and aerosol phase chemistry. We performed pseudo Lagrangian studies by letting the model column first move over the desert which surrounds the Dead Sea region and then let it move over the Dead Sea itself. To include an additional source for gas phase halogen compounds, gas exchange between the Dead Sea water and the atmosphere is treated explicitly. Model calculations indicate that this process has to be included to explain the measurements.

BrO, blizzards, and drivers of polar tropospheric ozone depletion events

Abstract. The source of bromine that drives polar boundary layer ozone depletion events (ODEs) is still open to some debate. While ODEs are generally noted to form under conditions of a shallow stable boundary layer, observations of depleted air under high wind conditions are taken as being transport-related. Here we report observations from Antarctica in which an unusually large cloud of BrO formed over the Weddell Sea. The enhanced BrO was observed over Halley station in coastal Antarctica, providing an opportunity to probe the conditions within an active "bromine explosion" event. On this occasion, enhanced BrO and depleted boundary layer ozone coincided with high wind speeds and saline blowing snow. We derive a simple model to consider the environmental conditions that favour ODEs and find two maxima, one at low wind/stable boundary layer and one at high wind speeds with blowing snow. Modelling calculations aiming to reproduce the wider regional or global impacts of ODEs, either via radiative effects or as a halogen source, will also need to account for high wind speed mechanisms.

A three‐dimensional model study on the production of BrO and Arctic boundary layer ozone depletion

A three‐dimensional model (GEM‐AQ/Arctic) was developed to study the chemistry and processes involved in the ozone depletion events (ODEs) in the Arctic boundary layer (ABL), which included a comprehensive mechanism of multiphase halogen chemistry in the ABL and our current understanding of the ODEs. Assuming that frost flower‐derived aerosols were the primary halogen source, the areas potentially covered by frost flowers were determined. The carbonate precipitation scheme was taken into account for triggering the bromine explosions in the model. A comparison of the simulations with GOME satellite measurements in springs of 2000 and 2001 showed that the spatial structure and temporal evolution of tropospheric BrO clouds were well predicted by the model. The majority of the springtime ODEs observed at three arctic stations was reasonably reproduced. An analysis on the model results indicated that most periods of simulated ozone depletion (O3 < 1 nmol mol−1) occurred in a layer 300 to 400 m deep at the Arctic sites. It is found that the halogen chemistry in the marine boundary layer (MBL) contributed substantially to the spring time ODEs, but atmospheric temperature and circulations as well as the transported air pollution in the ABL were also responsible for the ODEs. For springs of 2000 and 2001, two source regions with low surface O3 levels were identified: the Siberian/Beaufort Arctic and the Canadian Arctic, broadly corresponding to areas of enhanced BrO levels and accompanied by the Arctic anticyclones. Dominant trans‐Arctic transport pathways were also investigated for the ODEs at Alert, Barrow and Zeppelinfjellet.

Precipitation of salts in freezing seawater and ozone depletion events: a status report

Abstract. In springtime, the polar marine boundary layer exhibits drastic ozone depletion events (ODEs), associated with elevated bromine oxide (BrO) mixing ratios. The current interpretation of this peculiar chemistry requires the existence of acid and bromide-enriched surfaces to heterogeneously promote and sustain ODEs. Sander et al. (2006) have proposed that calcium carbonate (CaCO3) precipitation in any seawater-derived medium could potentially decrease its alkalinity, making it easier for atmospheric acids such as HNO3 and H2SO4 to acidify it. We performed simulations using the state-of-the-art FREZCHEM model, capable of handling the thermodynamics of concentrated electrolyte solutions, to try to reproduce their results, and found that when ikaite (CaCO3·6H2O) rather than calcite (CaCO3) precipitates, there is no such effect on alkalinity. Given that ikaite has recently been identified in Antarctic brines (Dieckmann et al., 2008), our results show that great caution should be exercised when using the results of Sander et al. (2006), and reveal the urgent need of laboratory investigations on the actual link(s) between bromine activation and the pH of the surfaces on which it is supposed to take place at subzero temperature. In addition, the evolution of the Cl/Br ratio in the brine during freezing was computed using FREZCHEM, taking into account Br substitutions in Cl–containing salts.

Simulation of atmospheric mercury depletion events (AMDEs) during polar springtime using the MECCA box model

Abstract. Atmospheric mercury depletion events (AMDEs) during polar springtime are closely correlated with bromine-catalyzed tropospheric ozone depletion events (ODEs). To study gas- and aqueous-phase reaction kinetics and speciation of mercury during AMDEs, we have included mercury chemistry into the box model MECCA (Module Efficiently Calculating the Chemistry of the Atmosphere), which enables dynamic simulation of bromine activation and ODEs. We found that the reaction of Hg with Br atoms dominates the loss of gaseous elemental mercury (GEM). To explain the experimentally observed synchronous depletion of GEM and O3, the reaction rate of Hg+BrO has to be much lower than that of Hg+Br. The synchronicity is best reproduced with rate coefficients at the lower limit of the literature values for both reactions, i.e. kHg+Br≈3×10−13 and kHg+BrO≤1×10−15 cm3 molecule−1 s−1, respectively. Throughout the simulated AMDEs, BrHgOBr was the most abundant reactive mercury species, both in the gas phase and in the aqueous phase. The aqueous-phase concentrations of BrHgOBr, HgBr2, and HgCl2 were several orders of magnitude larger than that of Hg(SO3)22−. Considering chlorine chemistry outside depletion events (i.e. without bromine activation), the concentration of total divalent mercury in sea-salt aerosol particles (mostly HgCl42−) was much higher than in dilute aqueous droplets (mostly Hg(SO3)22−), and did not exhibit a diurnal cycle (no correlation with HO2 radicals).

UVR-induced photosynthetic inhibition dominates over DNA damage in marine dinoflagellates exposed to fluctuating solar radiation regimes

The combined effect of solar radiation (UV-B (280-315 nm), UV-A (315-400 nm) and PAR (400-700 nm)) and vertical mixing (i.e., fluctuating radiation regimes) on the marine dinoflagellates Gymnodinium chlorophorum, Heterocapsa triquetra and Prorocentrum micans was investigated during the austral spring in Patagonia, Argentina. Photosynthesis, measured as radiocarbon incorporation, and accumulation of DNA damage, as cyclobutane pyrimidine dimers (CPDs), were investigated under simulated mixed and non-mixed water column conditions using 3 h incubations centered at local noon. Static samples had significant UVR-induced photoinhibition that was higher in H. triquetra as compared to the other two species. Increasing mixing speed significantly increased UVR-induced inhibition of carbon fixation in G. chlorophorum and H. triquetra. No significant UVR effect was observed in P. micans under any of the mixing regimes. Most of the loss in carbon fixation in G. chlorophorum was due to UV-B while in H. triquetra it was due to UV-A. Part of these responses may be associated to the presence of UV-absorbing compounds which were abundant in P. micans, and low in H. triquetra and in G. chlorophorum. However, other variables such as cell size and active repair might have also influenced our results. We did not detect CPD accumulation in any of the species, probably because of the low solar angle that resulted in very low levels of DNA effective UV-B dose. Our results indicate that exposure to solar UVR in the Patagonia area during spring time (even during ozone depletion events) has a clear impact on photosynthesis and much less or negligible on DNA in the three studied species.

Evaluation of the chemical transport model Oslo CTM2 with focus on arctic winter ozone depletion

The global chemical transport model Oslo CTM2, driven by European Centre for Medium‐Range Weather Forecasts international Integrated Forecast System meteorology and including tropospheric and stratospheric chemistry, is described and evaluated against measurements by satellite‐based instruments, ozone sondes, and aircraft. A new heterogeneous chemistry and microphysics scheme capable of reproducing severe ozone depletion events is described. The Oslo CTM2 simulates ozone measurements well, overestimating by less than 20% in the lowermost stratosphere, while underestimating slightly at higher altitudes. In the upper troposphere, the spatial and temporal variation is well simulated, although a 30–50% overestimation is seen. In order to demonstrate ozone depletion in the Arctic by heterogeneous processes, the ozone loss during the cold 2004/2005 Arctic winter stratosphere is compared to the warm 2000/2001 Arctic winter. Enhanced ozone loss is shown to have occurred at eight Arctic stations. From January to March the accumulated ozone loss amounts to up to 1 ppmv, with a maximum ozone loss rate of 32 ppb/d. Ozone sonde measurements are well reproduced for both years.

The potential importance of frost flowers, recycling on snow, and open leads for ozone depletion events

Abstract. We present model studies with the one-dimensional model MISTRA to investigate the potential role of frost flowers, recycling on snow, and open leads in the depletion of tropospheric ozone in the Arctic spring. In our model, we assumed frost flower aerosols to be the major source of bromine. We show that a major ozone depletion event can be satisfactorily reproduced only if the recycling on snow of deposited bromine into gas phase bromine is assumed. In the model, this cycling is more efficient than the bromine explosion process and maintains sufficiently high levels of bromine to deplete ozone down to few nmol mol−1 within four days. We assessed the influence of different surface combinations (open lead/frost flowers) on the chemistry in the model. Results showed noticeable modifications affecting the composition of aerosols and the deposition velocities. A model run with a series of coupled frost flower fields and open leads, separated by large areas of snow, showed results comparable with field observations. In addition, we studied the effects of modified temperature of either the frost flower field or the ambient airmass. A warmer frost flower field increases the relative humidity and the aerosol deposition rate. The deposition/re-emission process gains in importance, inducing more reactive bromine in the gas phase, and a stronger ozone depletion. A decrease of 1K in airmass temperature shows in our model that the aerosol uptake capacities of all gas phase species substantially increases, leading to enhanced uptake of acids from the gas phase. Consequently, the so-called bromine explosion accelerated and O3 mixing ratios decreased. In our model representation, variations in wind speed affected the aerosol source function and influenced the amount of bromine in the atmosphere and thus the ozone depletion strength. Recent studies have suggested the important role of the precipitation of calcium carbonate (CaCO3) out of the brine layer for the possible acidification of the liquid phase by acid uptake. Our investigation showed that this precipitation is a crucial process for the timing of the bromine explosion in aerosols. Nevertheless, model runs with either 50% precipitation or complete precipitation displayed a relatively weak difference in ozone mixing ratios after four simulated days. By considering conditions typical for "Arctic Haze" pollution events at the start of the run we obtained a low pH in frost flower aerosols due to a greater mixing ratio of SO2, and a strong recycling efficiency via large aerosol number concentration. The aerosol acidification during a haze event most likely intensifies the ozone depletion strength and occurrence. The comparison between our modeled deposition on snow and sampled snow at Barrow (Alaska) shows that approximately 75% of deposited bromine may be re-emitted into the gas phase as Br2/BrCl. Among several non-halogen fluxes from the snow, model simulations showed that only HONO affects the chemistry. Finally, we investigated the release of Br2 potentially produced by heterogeneous reactions directly on frost flowers. In this case, we obtained unrealistic results of aerosol compositions and deposition rates on snow compared to observations in the Arctic.