Historic temperature observations on Nordaustlandet, north-east Svalbard

Long-term meteorological data for the Arctic are sparse. One of the longest quasi-continuous temperature time series in the High Arctic is the extended Svalbard Airport series, providing daily temperature data from 1898 until the present. Here, I derive an adjustment to historic temperature observations on the island of Nordaustlandet, north-east Svalbard, in order to link these to the extended Svalbard Airport series. This includes the Haudegen observations at Rijpfjorden during 1944/45 and a previously unrecognized data set obtained by the Norwegian hunters and trappers Gunnar Knoph and Henry Rudi during their wintering at Rijpfjorden in 1934/35. The adjustment is based on data from an automatic weather station at Rijpfjorden during 2014–16 and verified with other independent historic temperature observations on Nordaustlandet. An analysis of the Haudegen radiosonde data indicates that the surface temperature observations at Rijpfjorden are generally well correlated with the free tropospheric temperatures at 850 hPa, but occasionally show the occurrence of boundary-layer inversions during winter, where local temperatures fall substantially below what is expected from the regression. The adjusted historic observations from Nordaustlandet can, therefore, be used to fill remaining gaps in the extended Svalbard Airport series

Polar boundary layer bromine explosion and ozone depletion events in the chemistry-climate model EMAC v2.52: Implementation and evaluation of AirSnow algorithm

Ozone depletion events (ODEs) in the polar boundary layer have been observed frequently during springtime. They are related to events of boundary layer enhancement of bromine. Consequently, increased amounts of boundary layer volume mixing ratio (VMR) and vertical column densities (VCDs) of BrO have been observed by in situ observation, ground-based as well as airborne remote sensing, and from satellites. These so-called bromine explosion (BE) events have been discussed serving as a source of tropospheric BrO at high latitudes, which has been underestimated in global models so far. We have implemented a treatment of bromine release and recycling on sea-ice- and snow-covered surfaces in the global chemistry–climate model EMAC (ECHAM/MESSy Atmospheric Chemistry) based on the scheme of Toyota et al. (2011). In this scheme, dry deposition fluxes of HBr, HOBr, and BrNO3 over ice- and snow-covered surfaces are recycled into Br2 fluxes. In addition, dry deposition of O3, dependent on temperature and sunlight, triggers a Br2 release from surfaces associated with first-year sea ice. Many aspects of observed bromine enhancements and associated episodes of near-complete depletion of boundary layer ozone, both in the Arctic and in the Antarctic, are reproduced by this relatively simple approach. We present first results from our global model studies extending over a full annual cycle, including comparisons with Global Ozone Monitoring Experiment (GOME) satellite BrO VCDs and surface ozone observations

Radiative and dynamical contributions to past and future Arctic stratospheric temperature trends

Arctic stratospheric ozone depletion is closely linked to the occurrence of low stratospheric temperatures. There are indications that cold winters in the Arctic stratosphere have been getting colder, raising the question if and to what extent a cooling of the Arctic stratosphere may continue into the future. We use meteorological reanalyses from the European Centre for Medium Range Weather Forecasts (ECMWF) ERA-Interim and NASA's Modern-Era Retrospective-Analysis for Research and Applications (MERRA) for the past 32 yr together with calculations of the chemistry-climate model (CCM) ECHAM/MESSy Atmospheric Chemistry (EMAC) and models from the Chemistry-Climate Model Validation (CCMVal) project to infer radiative and dynamical contributions to long-term Arctic stratospheric temperature changes. For the past three decades the reanalyses show a warming trend in winter and cooling trend in spring and summer, which agree well with trends from the Radiosonde Innovation Composite Homogenization (RICH) adjusted radiosonde data set. Changes in winter and spring are caused by a corresponding change of planetary wave activity with increases in winter and decreases in spring. During winter the increase of planetary wave activity is counteracted by a residual radiatively induced cooling. Stratospheric radiatively induced cooling is detected throughout all seasons, being highly significant in spring and summer. This means that for a given dynamical situation, according to ERA-Interim the annual mean temperature of the Arctic lower stratosphere has been cooling by −0.41 ± 0.11 K decade−1 at 50 hPa over the past 32 yr. Calculations with state-of-the-art models from CCMVal and the EMAC model qualitatively reproduce the radiatively induced cooling for the past decades, but underestimate the amount of radiatively induced cooling deduced from reanalyses. There are indications that this discrepancy could be partly related to a possible underestimation of past Arctic ozone trends in the models. The models project a continued cooling of the Arctic stratosphere over the coming decades (2001–2049) that is for the annual mean about 40% less than the modeled cooling for the past, due to the reduction of ozone depleting substances and the resulting ozone recovery. This projected cooling in turn could offset between 15 and 40% of the Arctic ozone recovery

Brominated VSLS and their influence on ozone under a changing climate

Very short-lived substances (VSLS) contribute as source gases significantly to the tropospheric and stratospheric bromine loading. At present, an estimated 25 % of stratospheric bromine is of oceanic origin. In this study, we investigate how climate change may impact the ocean–atmosphere flux of brominated VSLS, their atmospheric transport, and chemical transformations and evaluate how these changes will affect stratospheric ozone over the 21st century. Under the assumption of fixed ocean water concentrations and RCP6.0 scenario, we find an increase of the ocean–atmosphere flux of brominated VSLS of about 8–10 % by the end of the 21st century compared to present day. A decrease in the tropospheric mixing ratios of VSLS and an increase in the lower stratosphere are attributed to changes in atmospheric chemistry and transport. Our model simulations reveal that this increase is counteracted by a corresponding reduction of inorganic bromine. Therefore the total amount of bromine from VSLS in the stratosphere will not be changed by an increase in upwelling. Part of the increase of VSLS in the tropical lower stratosphere results from an increase in the corresponding tropopause height. As the depletion of stratospheric ozone due to bromine depends also on the availability of chlorine, we find the impact of bromine on stratospheric ozone at the end of the 21st century reduced compared to present day. Thus, these studies highlight the different factors influencing the role of brominated VSLS in a future climate

Biomass burning pollution in the South Atlantic upper troposphere: GLORIA trace gas observations and evaluation of the CAMS model

In this study, we present simultaneous airborne measurements of peroxyacetyl nitrate (PAN), ethane (C2H6), formic acid (HCOOH), methanol (CH3OH), and ethylene (C2H4) above the South Atlantic in September and October 2019. Observations were obtained from the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA), as two-dimensional altitude cross sections along the flight path. The flights were part of the SouthTRAC (Transport and Composition in the Southern Hemisphere Upper Troposphere/Lower Stratosphere) campaign with the German High Altitude and Long Range Research Aircraft (HALO). On two flights (8 September 2019 and 7 October 2019), large enhancements of all these substances were found between 7 and 14 km altitude with maximum volume mixing ratios (VMRs) of 1000 pptv for PAN, 1400 pptv for C2H6, 800 pptv for HCOOH, 4500 pptv for CH3OH, and 200 pptv for C2H4. One flight showed a common filamentary structure in the trace gas distributions, while the second flight is characterized by one large plume. Using backward trajectories, we show that measured pollutants likely reached upper troposphere and lower stratosphere (UTLS) altitudes above South America and central Africa, where elevated PAN VMRs are visible at the surface layer of the Copernicus Atmosphere Monitoring Service (CAMS) model during the weeks before both measurements. In comparison to results of the CAMS reanalysis interpolated onto the GLORIA measurement geolocations, we show that the model is able to reproduce the overall structure of the measured pollution trace gas distributions. For PAN, the absolute VMRs are in agreement with the GLORIA measurements. However, C2H6 and HCOOH are generally underestimated by the model, while CH3OH and C2H4, the species with the shortest atmospheric lifetimes of the pollution trace gases discussed, are overestimated by CAMS. The good agreement between model and observations for PAN suggests that the general transport pathways and emissions locations are well captured by the model. The poorer agreement for other species is therefore most likely linked to model deficiencies in the representation of loss processes and emission strength

The Michelson Interferometer for Passive Atmospheric Sounding global climatology of BrONO2 2002–2012: a test for stratospheric bromine chemistry

We present the first observational dataset of vertically resolved global stratospheric BrONO2 distributions from July 2002 until April 2012 and compare them to results of the atmospheric chemical climate model ECHAM/MESSy Atmospheric Chemistry (EMAC). The retrieved distributions are based on space-borne measurements of infrared limb-emission spectra recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat. The derived vertical profiles of BrONO2 volume mixing ratios represent 10∘ latitude bins and 3 d means, separated into sunlit observations and observations in the dark. The estimated uncertainties are around 1–4 pptv, caused by spectral noise for single profiles as well as for further parameter and systematic errors which may not improve by averaging. Vertical resolutions range from 3 to 8 km between 15 and 35 km altitude. All leading modes of spatial and temporal variability of stratospheric BrONO2 in the observations are well replicated by the model simulations: the large diurnal variability, the low values during polar winter as well as the maximum values at mid and high latitudes during summer. Three major differences between observations and model results are observed: (1) a model underestimation of enhanced BrONO2 in the polar winter stratosphere above about 30 km of up to 15 pptv, (2) up to 8 pptv higher modelled values than observed globally in the lower stratosphere up to 25 km, most obvious during night, and (3) up to 5 pptv lower modelled concentrations at tropical latitudes between 27 and 32 km during sunlit conditions. (1) is explained by the model missing enhanced NOx produced in the mesosphere and lower thermosphere subsiding at high latitudes in winter. This is the first time that observational evidence for enhancement of BrONO2 caused by mesospheric NOx production is reported. The other major inconsistencies (2, 3) between EMAC model results and observations are studied by sensitivity runs with a 1D model. These tentatively hint at a model underestimation of heterogeneous loss of BrONO2 in the lower stratosphere, a simulated production of BrONO2 that is too low during the day as well as strongly underestimated BrONO2 volume mixing ratios when loss via reaction with O(3P) is considered in addition to photolysis. However, considering the uncertainty ranges of model parameters and of measurements, an unambiguous identification of the causes of the differences remains difficult. The observations have also been used to derive the total stratospheric bromine content relative to years of stratospheric entry between 1997 and 2007. With an average value of 21.2±1.4 pptv of Bry at mid latitudes where the modelled adjustment from BrONO2 to Bry is smallest, the MIPAS data agree with estimates of Bry derived from observations of BrO as well as from MIPAS-Balloon measurements of BrONO2.</p

MIPAS observations of volcanic sulfate aerosol and sulfur dioxide in the stratosphere

Volcanic eruptions can increase the stratospheric sulfur loading by orders of magnitude above the background level and are the most important source of variability in stratospheric sulfur. We present a set of vertical profiles of sulfate aerosol volume densities and derived liquid-phase H2SO4 (sulfuric acid) mole fractions for 2005–2012, retrieved from infrared limb emission measurements performed with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board of the Environmental Satellite (Envisat). Relative to balloon-borne in situ measurements of aerosol at Laramie, Wyoming, the MIPAS aerosol data have a positive bias that has been corrected, based on the observed differences to the in situ data. We investigate the production of stratospheric sulfate aerosol from volcanically emitted SO2 for two case studies: the eruptions of Kasatochi in 2008 and Sarychev in 2009, which both occurred in the Northern Hemisphere midlatitudes during boreal summer. With the help of chemical transport model (CTM) simulations for the two volcanic eruptions we show that the MIPAS sulfate aerosol and SO2 data are qualitatively and quantitatively consistent with each other. Further, we demonstrate that the lifetime of SO2 is explained well by its oxidation by hydroxyl radicals (OH). While the sedimentation of sulfate aerosol plays a role, we find that the long-term decay of stratospheric sulfur after these volcanic eruptions in midlatitudes is mainly controlled by transport via the Brewer–Dobson circulation. Sulfur emitted by the two midlatitude volcanoes resides mostly north of 30° N at altitudes of ∼ 10–16 km, while at higher altitudes ( ∼  18–22 km) part of the volcanic sulfur is transported towards the Equator where it is lifted into the stratospheric “overworld” and can further be transported into both hemispheres