Abstract. Transport from the surface to the lowermost stratosphere (LMS) can occur on timescales of a few months or less, making it possible for short-lived tropospheric pollutants to influence stratospheric composition and chemistry. Models used to study this influence must demonstrate the credibility of their chemistry and transport in the upper troposphere and lower stratosphere (UT/LS). Data sets from satellite and aircraft instruments measuring CO, O3, N2O, and CO2 in the UT/LS are used to create a suite of diagnostics for the seasonally-varying transport into and within the lowermost stratosphere, and of the coupling between the troposphere and stratosphere in the extratropics. The diagnostics are used to evaluate a version of the Global Modeling Initiative (GMI) Chemistry and Transport Model (CTM) that uses a combined tropospheric and stratospheric chemical mechanism and meteorological fields from the GEOS-4 general circulation model. The diagnostics derived from N2O and O3 show that the model lowermost stratosphere has realistic input from the overlying high latitude stratosphere in all seasons. Diagnostics for the LMS show two distinct layers. The upper layer begins ~30 K potential temperature above the tropopause and has a strong annual cycle in its composition. The lower layer is a mixed region ~30 K thick near the tropopause that shows no clear seasonal variation in the degree of tropospheric coupling. Diagnostics applied to the GMI CTM show credible seasonally-varying transport in the LMS and a tropopause layer that is realistically coupled to the UT in all seasons. The vertical resolution of the GMI CTM in the UT/LS, ~1 km, is sufficient to realistically represent the extratropical tropopause layer. This study demonstrates that the GMI CTM has the transport credibility required to study the impact of tropospheric emissions on the stratosphere.
<p>The region around Southern Argentina and the Antarctic peninsula is known as the world&#8217;s strongest hotspot of stratospheric gravity wave activity. In this region, large tropospheric winds are perturbed by the orography of the Andes and the Antarctic peninsula resulting in the excitation of mountain waves which might propagate all the way up into the upper mesosphere when the polar night jet is intact. In addition, satellite observations also show large stratospheric wave activity in the region of the Drake passage, i.e., in between the Andes and the Antarctic peninsula, and along the corresponding latitudinal circle of 60&#176;S. The origin of these waves is currently not entirely understood. Several hypotheses are currently being investigated, like for example the idea that the mountain waves that were originally excited over the Andes and the Antarctic peninsula propagate horizontally to 60&#176;S and along the latitudinal circle. In order to investigate this and other hypotheses the German research aircraft HALO was deployed to Rio Grande, Tierra del Fuego, at the Southern Tip of Argentina in September and November 2019 in the frame of the SOUTHTRAC (Southern hemisphere Transport, Dynamics, and Chemistry) research mission. A total of 6 dedicated research flights with a typical length of 7000km were conducted to obtain gravity wave observations with the newly developed ALIMA (ALIMA=Airborne LIdar for Middle Atmosphere research)-instrument and the GLORIA (GLORIA=Gimballed Limb Observer for Radiance Imaging of the Atmosphere) limb sounder. While ALIMA measures temperatures and temperature perturbations in the altitude range from 20-90 km, GLORIA observations allow to characterize wave perturbations in temperatures and trace gas concentrations below flight level (<~14 km). This paper gives an overview of the mission objectives, the prevailing atmospheric conditions during the HALO deployment, and highlights some outstanding initial results of the gravity wave observations.</p>
Abstract In the tropopause region turbulence exhibits a common feature with posing a threat for aviation as well as presenting a pathway for cross tropopause exchange. However, current numerical weather prediction and in particular climate models still struggle to resolve a large part of the dynamics responsible for turbulence generation. This renders high resolution observations of turbulence to be very valuable to identify the spatio‐temporal distribution of turbulence. In this study we use aircraft observations along with reanalysis data over the North Atlantic during winter season to analyze turbulence occurrence. Using a tropopause‐based coordinate system, we found that turbulence occurs within a sharp unimodal distribution which maximizes just below the tropopause. Turbulence also significantly affects the first 2 km above the tropopause which highlights its potential impact on the formation of the so called extratropical mixing layer around the tropopause.
Abstract. Size-resolved and vertical profile measurements of single particle chemical composition (sampling altitude range 50–3000 m) were conducted in July 2014 in the Canadian high Arctic during the aircraft-based measurement campaign NETCARE 2014. We deployed the single particle laser ablation aerosol mass spectrometer ALABAMA (vacuum aerodynamic diameter range approximately 200–1000 nm) to identify different particle types and their mixing states. On basis of the single particle analysis, we found that a significant fraction (23 %) of all analyzed particles (in total: 7412) contained trimethylamine (TMA). The identification of TMA in ambient mass spectra was confirmed by laboratory measurements. From the maximum occurrence of particulate TMA in the Arctic boundary layer and the higher abundance of smaller TMA-containing particles (maximum in the size distribution at 300 nm), we conclude that the TMA component of these particles resulted from emissions within the Arctic boundary layer. Air mass history according to FLEXPART backward simulations and associated wind data give evidence of a marine-biogenic influence on particulate TMA. The marine influence on particle chemical composition in the summertime Arctic is further demonstrated by the existence of larger sodium and chloride (Na/Cl-) containing particles which are mainly abundant in the boundary layer. Some of these sea spray particles were internally mixed with carbohydrates (e.g., cellulose) which likely originated from a sea surface microlayer enriched with microorganisms and organic compounds. The external mix of sea spray particles and TMA-containing particles suggests the latter result from secondary conversion of precursor gases from marine inner-Arctic sources. In contrast to TMA- and Na/Cl-containing aerosol types, particles with biomass-burning markers (such as levoglucosan) showed a higher fraction at higher altitudes, thereby indicating long-range transport as their source. Our measurements highlight the importance of natural, marine inner-Arctic sources for summertime Arctic aerosol.
The lowermost stratosphere (LMS) as part of the extratropical UTLS can be divided into dynamically and chemically distinct regions. A layer of mixed tropospheric and stratospheric tracer characteristics in the proximity of the extratropical tropopause: the extratropical tropopause transition layer (ExTL). This chemically distinct layer roughly coincides with a layer of strongly enhanced thermal stratification: the tropopause inversion layer (TIL) (Birner, 2006). The LMS above the ExTL, also named the free LMS (Bonisch et al., 2009), is less coupled to the local extratropical troposphere. Simultaneous in-situ measurements of CO2 and SF6 have been used to calculate mean transport time from the troposphere to the measurement location in the free LMS (Bonisch et al., 2009) which is on the order of months.
Abstract. The vertical distribution of black carbon (BC) particles in the Arctic atmosphere is one of the key parameters controlling its radiative forcing. Hence, this work investigates the presence and properties of BC over the high Canadian Arctic. Airborne campaigns were performed as part of the NETCARE project and provided insights into the variability of the vertical distributions of BC particles in summer 2014 and spring 2015. The observation periods covered evolutions of cyclonic disturbances to the polar dome that caused and changed transport of air pollution into the High–Arctic, as otherwise the airmass boundary largely impedes entrainment of pollution from lower latitudes. A total of 48 vertical profiles of refractory BC (rBC) mass concentration and particle size, extending from 0.1 to 5.5 km altitude, were obtained with a Single–Particle Soot Photometer (SP2). Generally, the rBC mass concentration decreased from spring to summer by a factor 10. Such depletion was associated with a decrease of the mean rBC particle diameter, from approximately 200 nm to 130 nm at low altitude. Due to the very low number fraction, rBC particles did not substantially contribute to the total aerosol population in summer. Profiles analysed with potential temperature as vertical coordinate revealed characteristic variability patterns due to different balances of supply and removal of rBC in specific levels of the stable atmosphere. Kinematic back–trajectories were used to investigate transport pathways into these levels. The lower polar dome was influenced by low–level transport from sources within the cold central and marginal Arctic. During the spring campaign, a cold air outbreak over eastern Europe additionally caused northward transport of air from a corridor over western Russia to Central Asia that was affected by emissions from gas flaring, industrial activity and wildfires. This caused rBC concentrations between about 500 to 1800 m altitude to gradually increase from 32 to 49 ng m−3. The temporal development of transport to the level above, at around 2500 m, caused the initially low concentration to increase from
Abstract. To estimate the impact of emissions by road, aircraft and ship traffic on ozone and OH of the present-day atmosphere seven different atmospheric chemistry models simulated the atmospheric composition of the year 2003. Based on newly developed global emission inventories for road, maritime and aircraft emission data sets each model performed a series of five simulations: A base scenario using the full set of emissions, three sensitivity studies with each individual sector of transport reduced by 5% and one simulation with all traffic related emissions reduced by 5%. The approach minimizes non-linearities in atmospheric chemical effects and are later scaled to 100%. The global annual mean impact of ship emissions on ozone in the boundary layer leads to an increase of ozone of 1.2%, followed by road (0.87%) and aircraft emissions (0.3%). In the upper troposphere between 200–300 hPa both road and ship traffic affect ozone by 1.1%, whereas aircraft emissions contribute 0.9%. However, the sensitivity of ozone formation per NOx molecule emitted is highest for aircraft exhausts. The local maximum effect of the summed traffic emissions on the ozone column predicted by the models is 4.0 DU and occurs over the northern subtropical Atlantic. The impact of traffic emissions on total ozone in the Southern Hemisphere is approximately half of the northern hemispheric perturbation. Below 800 hPa both ozone and OH respond most sensitively to ship emissions in the marine boundary layer over the Atlantic, where the effect can exceed 10% (zonal mean) which is 80% of the total traffic induced ozone perturbation. In the Southern Hemisphere ship emissions contribute relatively strongly to the total ozone perturbation by 60%–80% throughout the year (equivalent to 1–1.5 ppbv). Road emissions have the strongest impact on ozone in the continental boundary layer and the free troposphere in summer. They also affect the upper troposphere particularly during northern summer associated with strong convection in mid latitudes. Ozone perturbations due to road traffic show the strongest seasonal cycle in the northern troposphere, and can even change sign in the continental boundary layer during winter. The OH concentration in the boundary layer is most strongly affected by ship emissions, which has a significant influence on the lifetime of many trace gases including methane. Methane lifetime changes due to ship emissions amount to 4.1%, followed by road (1.6%) and air traffic (1.0%).
