Ozone Decrease Research Articles

Ozone attributed to Madrid and Barcelona on-road transport emissions: Characterization of plume dynamics over the Iberian Peninsula

Despite the ~30% emission decrease of the main tropospheric ozone (O3) precursors in Spain in the 2001–2012 period, the O3 concentration in summer still exceeds the target value for the protection of the human health of the Air Quality Directive (2008/50/EC). On-road transport is the main anthropogenic contributor to O3 precursor's emissions in Madrid and Barcelona metropolitan areas (65%/59% of NOx, 40%/33% of NMVOC, and 67%/85% of CO emissions) but this contribution to O3 formation is not well understood. The present work aims at increasing the understanding on the role of on-road transport emissions from main Spanish urban areas in O3 dynamics over Spain under typical circulation types. For that purpose, the Integrated Source Apportionment Method is used within the CALIOPE modelling system (WRF/CMAQ/HERMES/BSC-DREAM8b). The results indicate that the daily maximum O3 concentration attributed to the on-road transport emissions from Madrid (O3T-MAD) and Barcelona metropolitan areas (O3T-BCN) contribute up to 24% and 8% to total O3 concentration, respectively, within an area of influence of 200km. The contribution of O3T-MAD and O3T-BCN is particularly significant (up to 80–100μgm−3 in an hour) to the O3 concentration peak during the central hours of the day in the high O3 concentration season (April–September). The maximum O3T-MAD concentration is calculated within the metropolitan area of Madrid but the plume, channelled by the Tajo and the Henares valleys, affects large areas of the Iberian Peninsula. The O3T-BCN plume is more driven by sea-land and mountain-valley breezes than by the synoptic advection and its maximum concentration is usually registered over the Mediterranean Sea. The O3 concentration transported long-range to the Iberian Peninsula is significant in the area of influence of Madrid and Barcelona, being maxima under cold (70–96%) and minima in warm circulation types (35–70%).

Seasonal relationship between meteorological conditions and surface ozone in Korea based on an offline chemistry–climate model

Tropospheric ozone plays an important role in climate variation and air quality, and it has seen a dramatic rise in East Asia due to the region's rapid economic growth. The relationship between meteorological conditions and surface ozone in Korea varies seasonally, and to obtain a better understanding of this process, this study performed offline simulations using a climate–chemistry model. The model represented the observed annual cycle of surface ozone over East Asia well, including the spring/autumn peaks and summer troughs. Increases in ozone were associated primarily with the westerly wind anomaly during spring and with surface warming during the autumn and summer. Moreover, a decrease in ozone during the summer likely resulted from the transportation of ozone-depleted air masses by anomalous southeasterly winds. Reduced cloud cover increased ozone levels significantly during all seasons except winter. The relationship between the El Niño and Southern Oscillation and ozone concentrations in Korea was also examined. Spring ozone levels tended to be elevated following mature-phase El Niño winters, whereas elevated levels during summer and autumn followed La Niña winters.

Influence of synoptic patterns on surface ozone variability over the eastern United States from 1980 to 2012

Abstract. We investigate the effect of synoptic-scale weather patterns on observed maximum daily 8-hour average (MDA8) surface ozone over the eastern United States during 1980–2012 in summer (June–August, JJA). Zonally averaged, the relative standard deviation (SD) of daily MDA8 JJA ozone shows a bimodal structure, with peaks at 28–32 and 40–45° N, and we show that those regions are most influenced by the variability in daily weather. We apply empirical orthogonal functions (EOFs) to understand the causes of this structure. The first three leading EOF patterns explain 53 % of the total variance in detrended surface ozone, displaying (1) a widespread response of ozone in the eastern United States associated with north–south movement of jet wind latitude, (2) a north–south pattern linked to the Bermuda High system when its west boundary is located along the east coast, and (3) an east–west pattern characteristic of a westward extension of the Bermuda High and an enhanced Great Plains low level jet (GPLLJ). The northern peak of ozone relative SD can be explained by polar jet activity, while the southern peak appears related to variability in the Bermuda High and GPLLJ. We define a new metric polar jet frequency as the total number of days the jet traverses the Midwest and northeast each summer. In the Midwest and northeast, we find that the correlation coefficient r between detrended mean JJA MDA8 ozone and the polar jet frequency ranges between −0.76 and −0.93 over 1980–2012 depending on the time period selected, suggesting that polar jet frequency could provide a simple metric to predict ozone variability in future climate regimes. In the southeast, the influence of the Bermuda High on mean JJA MDA8 ozone depends on the location of its west edge. For those summers when the average position of the west edge is located west of ~ 85.4° W, a westward shift in the Bermuda High west edge increases ozone in the southeast by ~ 1 ppbv deg−1 in longitude. For all summers, a northward shift in the Bermuda High west edge increases ozone over the entire eastern United States by 1–2 ppbv deg−1 in latitude. None of the synoptic patterns identified in this study show a significant trend from 1980 to 2012, confirming that the observed ozone decrease over the eastern United States during this time period is mainly caused by emission controls. Our work underscores the impact of synoptic patterns on ozone variability and suggests that a combination of changing local and synoptic meteorology together with trends in background ozone will determine the influence of climate change on US ozone air quality in future decades. The observed relationships of US surface ozone and synoptic circulations in this study can also be used to validate models of atmospheric chemistry.

INVESTIGATION OF EFFICIENCY OF AIR CLEANING FROM ACETONE USING A SEGMENTAL CONSTRUCTION BIOFILTER / ACETONO VALYMO IŠ ORO EFEKTYVUMO TYRIMAI, TAIKANT SEGMENTINĖS KONSTRUKCIJOS BIOFILTRĄ

Volatile organic compounds, e. g. acetone, have a direct impact on climate change, decrease of ozone in the air, and on the growth of greenhouse effect. One of the most popular air purifying methods from VOC is a biological air cleaning. Experimental investigations were conducted to determine the efficiency of the new structure of biofilter with polypropylene plates segments. During the investigations the efficiency of segmental construction biofilter of air purification at different initial concentrations of pollutants was determined. Different concentrations of pollutants were estimated during the acetone dilution with water. During the tests the efficiency of biofilter air purification from acetone vapor and its change under different concentrations of vapors was set. Based on test results, the maximum efficiency of biofilter air purification was up to 93%. Studies have shown that increasing the allowable pollutant concentration, the efficiency of air purification unit decreases. Increasing the concentration of supplied acetone vapor into the biofilter from 232 to 701 mg/m3, cleaning efficiency decreased from 92.8 to 82.3%. Since microorganisms fail to oxidize organic compounds, the filter works better at lower initial concentrations of pollutants. Lakieji organiniai junginiai, tokie kaip acetonas, turi tiesioginę įtaką klimato kaitai, ozono kiekio ore mažėjimui, šiltnamio efekto susidarymui. Vienas populiariausių oro valymo nuo LOJ metodų yra biologinis oro valymas. Eksperimentiniai tyrimai atlikti siekiant nustatyti naujos konstrukcijos biofiltro, užkrauto polipropileno plokštelių segmentais, efektyvumą. Tyrimų metu buvo nustatinėjamas segmentinės konstrukcijos oro valymo biofiltro efektyvumas, esant skirtingoms pradinėms teršalo koncentracijoms. Skirtingos teršalo koncentracijos gautos acetoną skiedžiant vandeniu. Bandymų metu nustatytas biofiltro oro valymo nuo acetono garų efektyvumas, jo kaita esant skirtingoms garų koncentracijoms. Remiantis bandymų rezultatais, didžiausias nustatytas biofiltro oro valymo efektyvumas siekė 93 %. Tyrimų metu nustatyta, kad didinant leidžiamo teršalo koncentraciją, įrenginio valymo efektyvumas mažėja. Padidinus į biofiltrą tiekiamų acetono garų koncentraciją nuo 232 iki 701 mg/m3, valymo efektyvumas sumažėjo nuo 92,8 iki 82,3 %. Kadangi mikroorganizmai nespėja oksiduoti organinių junginių, filtras geriau veikia esant mažesnėms pradinėms teršalų koncentracijoms.

Impact of climate and land cover changes on tropospheric ozone air quality and public health in East Asia between 1980 and 2010

Abstract. Understanding how historical climate and land cover changes have affected tropospheric ozone in East Asia would help constrain the large uncertainties associated with future East Asian air quality projections. We perform a series of simulations using a global chemical transport model driven by assimilated meteorological data and a suite of land cover and land use data to examine the public health effects associated with changes in climate, land cover, land use, and anthropogenic emissions between the 5-year periods 1981–1985 and 2007–2011 in East Asia. We find that between these two periods land cover change alone could lead to a decrease in summertime surface ozone by up to 4 ppbv in East Asia and ~ 2000 fewer ozone-related premature deaths per year, driven mostly by enhanced dry deposition resulting from climate- and CO2-induced increase in vegetation density, which more than offsets the effect of reduced isoprene emission arising from cropland expansion. Climate change alone could lead to an increase in summertime ozone by 2–10 ppbv in most regions of East Asia and ~ 6000 more premature deaths annually, mostly attributable to warming. The combined impacts (−2 to +12 ppbv) show that while the effect of climate change is more pronounced, land cover change could offset part of the climate effect and lead to a previously unknown public health benefit. While the changes in anthropogenic emissions remain the largest contributor to deteriorating ozone air quality in East Asia over the past 30 years, we show that climate change and land cover changes could lead to a substantial modification of ozone levels, and thus should come into consideration when formulating future air quality management strategies. We also show that the sensitivity of surface ozone to land cover change is more dependent on dry deposition than on isoprene emission in most of East Asia, leading to ozone responses that are quite distinct from that in North America, where most ozone-vegetation sensitivity studies to date have been conducted.

Estimate of biogenic VOC emissions in Japan and their effects on photochemical formation of ambient ozone and secondary organic aerosol

A new gridded database has been developed to estimate the amount of isoprene, monoterpene, and sesquiterpene emitted from all the broadleaf and coniferous trees in Japan with the Model of Emissions of Gases and Aerosols from Nature (MEGAN). This database reflects the vegetation specific to Japan more accurately than existing ones. It estimates much lower isoprene emitted from other vegetation than trees, and higher sesquiterpene emissions mainly emitted from Cryptomeria japonica, which is the most abundant plant type in Japan. Changes in biogenic emissions result in the decrease in ambient ozone and increase in organic aerosol simulated by the air quality simulation over the Tokyo Metropolitan Area in Japan. Although newly estimated biogenic emissions contribute to a better model performance on overestimated ozone and underestimated organic aerosol, they are not a single solution to solve problems associated with the air quality simulation.

Impacts of stratospheric ozone depletion and recovery on wave propagation in the boreal winter stratosphere

AbstractThis paper uses a state‐of‐the‐art general circulation model to study the impacts of the stratospheric ozone depletion from 1980 to 2000 and the expected partial ozone recovery from 2000 to 2020 on the propagation of planetary waves in December, January, and February. In the Southern Hemisphere (SH), the stratospheric ozone depletion leads to a cooler and stronger Antarctic stratosphere, while the stratospheric ozone recovery has the opposite effects. In the Northern Hemisphere (NH), the impacts of the stratospheric ozone depletion on polar stratospheric temperature are not opposite to that of the stratospheric ozone recovery; i.e., the stratospheric ozone depletion causes a weak cooling and the stratospheric ozone recovery causes a statistically significant cooling. The stratospheric ozone depletion leads to a weakening of the Arctic polar vortex, while the stratospheric ozone recovery leads to a strengthening of the Arctic polar vortex. The cooling of the Arctic polar vortex is found to be dynamically induced via modulating the planetary wave activity by stratospheric ozone increases. Particularly interesting is that stratospheric ozone changes have opposite effects on the stationary and transient wave fluxes in the NH stratosphere. The analysis of the wave refractive index and Eliassen‐Palm flux in the NH indicates (1) that the wave refraction in the stratosphere cannot fully explain wave flux changes in the Arctic stratosphere and (2) that stratospheric ozone changes can cause changes in wave propagation in the northern midlatitude troposphere which in turn affect wave fluxes in the NH stratosphere. In the SH, the radiative cooling (warming) caused by stratospheric ozone depletion (recovery) produces a larger (smaller) meridional temperature gradient in the midlatitude upper troposphere, accompanied by larger (smaller) zonal wind vertical shear and larger (smaller) vertical gradients of buoyancy frequency. Hence, there are more (fewer) transient waves propagating into the stratosphere. The dynamical warming (cooling) caused by stratospheric ozone decreases (increases) partly offsets their radiative cooling (warming).

Hydroxyl layer: trend of number density and intra-annual variability

Abstract. The layer of vibrationally excited hydroxyl (OH*) near the mesopause in Earth's atmosphere is widely used to derive the temperature at this height and to observe dynamical processes such as gravity waves. The concentration of OH* is controlled by the product of atomic hydrogen, with ozone creating a layer of enhanced concentration in the mesopause region. However, the basic influences on the OH* layer are atomic oxygen and temperature. The long-term monitoring of this layer provides information on a changing atmosphere. It is important to know which proportion of a trend results from anthropogenic impacts on the atmosphere and which proportion reflects natural variations. In a previous paper (Grygalashvyly et al., 2014), the trend of the height of the layer and the trend in temperature were investigated particularly in midlatitudes on the basis of our coupled dynamic and chemical transport model LIMA (Leibniz Institute Middle Atmosphere). In this paper we consider the trend for the number density between the years 1961 and 2009 and analyze the reason of the trends on a global scale. Further, we consider intra-annual variations. Temperature and wind have the strongest impacts on the trend. Surprisingly, the increase in greenhouse gases (GHGs) has no clear influence on the chemistry of OH*. The main reason for this lies in the fact that, in the production term of OH*, if atomic hydrogen increases due to increasing humidity of the middle atmosphere by methane oxidation, ozone decreases. The maximum of the OH* layer is found in the mesopause region and is very variable. The mesopause region is a very intricate domain marked by changeable dynamics and strong gradients of all chemically active minor constituents determining the OH* chemistry. The OH* concentration responds, in part, very sensitively to small changes in these parameters. The cause for this behavior is given by nonlinear reactions of the photochemical system being a nonlinear enforced chemical oscillator driven by the diurnal-periodic solar insolation. At the height of the OH* layer the system operates in the vicinity of chemical resonance. The solar cycle is mirrored in the data, but the long-term behavior due to the trend in the Lyman-α radiation is very small. The number density shows distinct hemispheric differences. The calculated OH* values show sometimes a step around a certain year. We introduce a method to find out the date of this step and discuss a possible reason for such behavior.

Anomalies of Northern Hemisphere ozone associated with a tropopause‐lower stratosphere teleconnection during summer

ABSTRACTUsing the ozone data set from the satellite observations and the atmospheric reanalysis data sets during 1979–2005, we examine the variability of a tropopause‐lower stratosphere (TS) teleconnection a and its relationship with the ozone change over the Northern Hemisphere (NH) during summer. The TS teleconnection has an approximate opposite phase to that in the upper‐troposphere Asian‐Pacific Oscillation (APO), and the TS teleconnection index (the TS index) is also related well to the APO index. Corresponding to a higher TS index, the ozone generally decreases in the NH mid‐high latitudes (near 40°N and the Far East of Russia). This link between the TS teleconnection and the ozone is physically supported by the changes of the background atmospheric circulation. When the TS index is higher, the decrease of the ozone is closely associated with an increase of the local tropopause height. The upward and downward motion variations in the upper troposphere and the lower stratosphere in the east and west parts of the anomalous anticyclones associated with the TS index strengthen the exchanges of the ozone between the troposphere and the lower stratosphere, which may cause a decrease of the ozone. The horizontal advection of the ozone could also contribute to the ozone decreases over these regions.

Modelling aerosol–cloud–meteorology interaction: A case study with a fully coupled air quality model (GEM-MACH)

A fully coupled on-line air quality forecast model, GEM-MACH, was used to study aerosol–cloud interactions for a case of an urban-industrial plume impacting stratocumulus. The aerosol effect on the cloud microphysics was achieved by the use of parameterization of cloud droplet nucleation predicted from the on-line size- and composition-resolved aerosols and coupled with a double-moment cloud microphysics parameterization. The model simulations with and without the on-line aerosol effect on cloud microphysics were compared and evaluated against in-situ aerosol and cloud observations from ICARTT 2004. Inclusion of the on-line aerosol interaction with cloud resulted in an increase in modelled cloud amount and cloud liquid water content (LWC) due to increased cloud droplet number concentration (Nd), a decrease in cloud droplet size and a reduction in warm precipitation. The modelled LWC and Nd agreed more closely with the observations when the on-line aerosol was allowed to affect the cloud than when aerosol effects on cloud were not explicitly simulated. The increased cloud amount due to the aerosol effects reduced the modelled downward shortwave radiative flux and air temperature at the surface, contributing to a decrease in ozone over the region of enhanced cloud and an increase in particle sulphate from an increased capacity for aqueous-phase production. Aerosol activation is shown to have a significant influence on the cloud microphysics and cloud processing of trace gases and aerosols. The importance of reasonable parameterization of cloud updraft speed is demonstrated.

The decrease in mid-stratospheric tropical ozone since 1991

Abstract. While global stratospheric O3 has begun to recover, there are localized regions where O3 has decreased since 1991. Specifically, we use measurements from the Halogen Occultation Experiment (HALOE) for the period 1991–2005 and the NASA Aura Microwave Limb Sounder (MLS) for the period 2004–2013 to demonstrate a significant decrease in O3 near ~ 10 hPa in the tropics. O3 in this region is very sensitive to variations in NOy, and the observed decrease can be understood as a spatially localized, yet long-term increase in NOy. In turn, using data from MLS and from the Atmospheric Chemistry Experiment (ACE), we show that the NOy variations are caused by decreases in N2O which are likely linked to long-term variations in dynamics. To illustrate how variations in dynamics can affect N2O and O3, we show that by decreasing the upwelling in the tropics, more of the N2O can photodissociate with a concomitant increase in NOy production (via N2O + O(1D) → 2NO) at 10 hPa. Ultimately, this can cause an O3 decrease of the observed magnitude.

Simulating the impact of emissions of brominated very short lived substances on past stratospheric ozone trends

AbstractBromine from very short lived substances (VSLS), primarily from natural oceanic sources, contributes substantially to the stratospheric bromine loading. This source of stratospheric bromine has so far been ignored in most chemistry climate model calculations of stratospheric ozone trends. Here we present a transient simulation with the chemistry climate model EMAC for the period 1960–2005 including emissions of the five brominated VSLS CHBr3, CH2Br2, CH2BrCl, CHBrCl2, and CHBr2Cl. The emissions lead to a realistic stratospheric bromine loading of about 20 pptv for present‐day conditions. Comparison with a standard model simulation without VSLS shows large differences in modeled ozone in the extratropical lowermost stratosphere and in the troposphere. Differences in ozone maximize in the Antarctic Ozone Hole, resulting in more than 20% less ozone when VSLS are included. Even though the emissions of VSLS are assumed to be constant in time, the model simulation with VSLS included shows a much larger ozone decrease in the lowermost stratosphere during the 1979–1995 period and a faster ozone increase during 1996–2005, in better agreement with observed ozone trends than the standard simulation without VSLS emissions.

Evaluation of Erythemal ultraviolet solar radiation over Saudi Arabia

To characterize and reduce the harmful effects of Erythemal ultraviolet radiation (EUV), it is vital to recognize the spatial distribution and temporal variation of EUV radiation under various meteorological conditions. In light of these requirements, the problem we address here is to understand the results of a study performed to assess the influence of stratospheric ozone and aerosol loading on the receipt of EUV at six sites located in Saudi Arabia. The selected sites were chosen to represent the climatic zones of the north, middle, and south of Saudi Arabia. Intensive satellite-based measurements of the atmosphere over the period 1979–2005 obtained from the Total Ozone Mapping Spectrometer (TOMS) website (ftp://toms.gsfc.nasa.gov) were used, with a four year gap from 1993 to 1996. The results hint that, in spite of the significant decrease in stratospheric ozone, there is a clear decreasing trend in the EUV data series due to the increase in the aerosol loading during summer months. For the other months the correlation is not clear and in all cases there was no decrease in the EUV radiation trend outside summer months, with few exceptions.

О роли извержения вулкана Мерапи в аномальном понижении ОСО над Томском в апреле 2011 г.

О роли извержения вулкана Мерапи в аномальном понижении ОСО над Томском в апреле 2011 г.

Dynamic evaluation of CMAQ part I: Separating the effects of changing emissions and changing meteorology on ozone levels between 2002 and 2005 in the eastern US

A dynamic evaluation of the Community Multiscale Air Quality (CMAQ) modeling system version 5.0.1 was conducted to evaluate the model's ability to predict changes in ozone levels between 2002 and 2005, a time period characterized by emission reductions associated with the EPA's Nitrogen Oxides State Implementation Plan as well as significant reductions in mobile source emissions. Model results for the summers of 2002 and 2005 were compared to simulations from a previous version of CMAQ to assess the impact of model updates on predicted pollutant response. Changes to the model treatment of emissions, meteorology and chemistry had substantial impacts on the simulated ozone concentrations. While the median bias for high summertime ozone decreased in both years compared to previous simulations, the observed decrease in ozone from 2002 to 2005 in the eastern US continued to be underestimated by the model. Additional “cross” simulations were used to decompose the model predicted change in ozone into the change due to emissions, the change due to meteorology and any remaining change not explained individually by these two components. The decomposition showed that the emission controls led to a decrease in modeled high summertime ozone close to twice as large as the decrease attributable to changes in meteorology alone. Quantifying the impact of retrospective emission controls by removing the impacts of meteorology during the control period can be a valuable approach for communicating to policy makers the net benefit of national control measures.

On the hiatus in the acceleration of tropical upwelling since the beginning of the 21st century

Abstract. Chemistry–climate models predict an acceleration of the upwelling branch of the Brewer–Dobson circulation as a consequence of increasing global surface temperatures, resulting from elevated levels of atmospheric greenhouse gases. The observed decrease of ozone in the tropical lower stratosphere during the last decades of the 20th century is consistent with the anticipated acceleration of upwelling. However, more recent satellite observations of ozone reveal that this decrease has unexpectedly stopped in the first decade of the 21st century, challenging the implicit assumption of a continuous acceleration of tropical upwelling. In this study we use three decades of chemistry-transport-model simulations (1980–2013) to investigate this phenomenon and resolve this apparent contradiction. Aside from a high-bias between 1985–1990, our model is able to reproduce the observed tropical lower stratosphere ozone record. A regression analysis identifies a significant decrease in the early period followed by a statistically robust trend-change after 2002, in qualitative agreement with the observations. We demonstrate that this trend-change is correlated with structural changes in the vertical transport, represented in the model by diabatic heating rates taken from the reanalysis product Era-Interim. These changes lead to a hiatus in the acceleration of tropical upwelling between 70–30 hPa and a southward shift of the tropical pipe at 30 and 100 hPa during the past decade, which appear to be the primary causes for the observed trend-change in ozone.

A large ozone-circulation feedback and its implications for global warming assessments.

State-of-the-art climate models now include more climate processes which are simulated at higher spatial resolution than ever1. Nevertheless, some processes, such as atmospheric chemical feedbacks, are still computationally expensive and are often ignored in climate simulations1,2. Here we present evidence that how stratospheric ozone is represented in climate models can have a first order impact on estimates of effective climate sensitivity. Using a comprehensive atmosphere-ocean chemistry-climate model, we find an increase in global mean surface warming of around 1°C (~20%) after 75 years when ozone is prescribed at pre-industrial levels compared with when it is allowed to evolve self-consistently in response to an abrupt 4×CO2 forcing. The difference is primarily attributed to changes in longwave radiative feedbacks associated with circulation-driven decreases in tropical lower stratospheric ozone and related stratospheric water vapour and cirrus cloud changes. This has important implications for global model intercomparison studies1,2 in which participating models often use simplified treatments of atmospheric composition changes that are neither consistent with the specified greenhouse gas forcing scenario nor with the associated atmospheric circulation feedbacks3-5.

Chemistry–Climate Interactions of Stratospheric and Mesospheric Ozone in EMAC Long-Term Simulations with Different Boundary Conditions for CO2, CH4, N2O, and ODS

To evaluate future climate change in the middle atmosphere and the chemistry–climate interaction of stratospheric ozone, we performed a long-term simulation from 1960 to 2050 with boundary conditions from the Intergovernmental Panel on Climate Change A1B greenhouse gas scenario and the World Meteorological Organization Ab halogen scenario using the chemistry–climate model ECHAM5/MESSy Atmospheric Chemistry (EMAC). In addition to this standard simulation we performed five sensitivity simulations from 2000 to 2050 using the rerun files of the simulation mentioned above. For these sensitivity simulations we used the same model setup as in the standard simulation but changed the boundary conditions for carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone-depleting substances (ODS). In the first sensitivity simulation we fixed the mixing ratios of CO2, CH4, and N2O in the boundary conditions to the amounts for 2000. In each of the four other sensitivity simulations we fixed the boundary conditions of only one of CO2, CH4, N2O, or ODS to the year 2000.In our model simulations the future evolution of greenhouse gases leads to significant cooling in the stratosphere and mesosphere. Increasing CO2 mixing ratios make the largest contributions to this radiative cooling, followed by increasing stratospheric CH4, which also forms additional H2O in the upper stratosphere and mesosphere. Increasing N2O mixing ratios makes the smallest contributions to the cooling. The simulated ozone recovery leads to warming of the middle atmosphere.In the EMAC model the future development of ozone is influenced by several factors. 1) Cooler temperatures lead to an increase in ozone in the upper stratosphere. The strongest contribution to this ozone production is cooling due to increasing CO2 mixing ratios, followed by increasing CH4. 2) Decreasing ODS mixing ratios lead to ozone recovery, but the contribution to the total ozone increase in the upper stratosphere is only slightly higher than the contribution of the cooling by greenhouse gases. In the polar lower stratosphere a decrease in ODS is mainly responsible for ozone recovery. 3) Higher NOx and HOx mixing ratios due to increased N2O and CH4 lead to intensified ozone destruction, primarily in the middle and upper stratosphere, from additional NOx; in the mesosphere the intensified ozone destruction is caused by additional HOx. In comparison to the increase in ozone due to decreasing ODS, ozone destruction caused by increased NOx is of similar importance in some regions, especially in the middle stratosphere. 4) In the stratosphere the enhancement of the Brewer-Dobson circulation leads to a change in ozone transport. In the polar stratosphere increased downwelling leads to additional ozone in the future, especially at high northern latitudes. The dynamical impact on ozone development is higher at some altitudes in the polar stratosphere than the ozone increase due to cooler temperatures. In the tropical lower stratosphere increased residual vertical upward transport leads to a decrease in ozone.

On the capabilities and limitations of GCCM simulations of summertime regional air quality: A diagnostic analysis of ozone and temperature simulations in the US using CESM CAM-Chem

We conduct a diagnostic analysis of ozone chemistry simulated by four different configurations of a Global Climate-Chemistry Model (GCCM), the Community Earth System Model (CESM) with detailed tropospheric chemistry. The purpose of this study is to evaluate the ability of GCCMs to simulate future ozone chemistry by evaluating their ability to simulate present-day chemistry. To address this we chose four configurations of the CESM that differ in their meteorology (analyzed versus simulated meteorological fields), number of vertical levels, and the coupling of the ice and ocean models. We apply mixed model statistics to evaluate these different configurations against CASTNET ozone observations within different regions of the US by using various performance metrics relevant to evaluating future ozone changes. These include: mean biases and interannual variability, the ozone response to emission changes, the ozone response to temperature changes and ozone extreme values. Using these metrics, we find that although the configuration using analyzed meteorology best simulates temperatures it does not outperform a configuration with simulated meteorology in other metrics. All configurations are unable to capture observed ozone decreases and the ozone north-south gradient over the eastern US during 1995–2005. We find that the configuration with simulated meteorology with 56 vertical levels is markedly better in capturing observed ozone-temperature relationships and extreme values than a configuration that is identical except that it contains 26 vertical levels. We recommend caution in the use of GCCMs in simulating surface chemistry as differences in a variety of model parameters have a significant impact on the resulting chemical and climate variables. Isoprene emissions depend strongly on surface temperature and the resulting ozone chemistry is dependent on isoprene emissions but also on cloud cover, photolysis, the number of vertical levels, and the choice of meteorology. These dependencies must be accounted for in the interpretation of GCCM results.

How sensitive is the recovery of stratospheric ozone to changes in concentrations of very short-lived bromocarbons?

Abstract. Naturally produced very short-lived substances (VSLS) account for almost a quarter of the current stratospheric inorganic bromine, Bry. Following VSLS oxidation, bromine radicals (Br and BrO) can catalytically destroy ozone. The extent to which possible increases in surface emissions or transport of these VSLS bromocarbons to the stratosphere could counteract the effect of halogen reductions under the Montreal Protocol is an important policy question. Here, by using a chemistry–climate model, UM-UKCA, we investigate the impact of a hypothetical doubling (an increase of 5 ppt Bry) of VSLS bromocarbons on ozone and how the resulting ozone changes depend on the background concentrations of chlorine and bromine. Our model experiments indicate that for the 5 ppt increase in Bry from VSLS, the ozone decrease in the lowermost stratosphere of the Southern Hemisphere (SH) may reach up to 10% in the annual mean; the ozone decrease in the Northern Hemisphere (NH) is smaller (4–6%). The largest impact on the ozone column is found in the Antarctic spring. There is a significantly larger ozone decrease following the doubling of the VSLS burden under a high stratospheric chlorine background than under a low chlorine background, indicating the importance of the inter-halogen reactions. For example, the decline in the high-latitude, lower-stratospheric ozone concentration as a function of Bry is higher by about 30–40% when stratospheric Cly is ~ 3 ppb (present day), compared with Cly of ~ 0.8 ppb (a pre-industrial or projected future situation). Bromine will play an important role in the future ozone layer. However, even if bromine levels from natural VSLS were to increase significantly later this century, changes in the concentration of ozone will likely be dominated by the decrease in anthropogenic chlorine. Our calculation suggests that for a 5 ppt increase in Bry from VSLS, the Antarctic ozone hole recovery date could be delayed by approximately 6–8 years, depending on Cly levels.