A. Zahn

Identification of extratropical two‐way troposphere‐stratosphere mixing based on CARIBIC measurements of O3, CO, and ultrafine particles

Simultaneous measurements of O3, CO, and ultrafine aerosol particles (UFP), conducted on board of a Boeing 767-ER passenger aircraft flying from Sri Lanka to Germany (project CARIBIC), are used to study two-way cross-tropopause mixing near a subtropical tropopause fold. On the equatorward side of the fold, downward mixing of stratospheric air into the upper troposphere is identified by enhanced concentrations of O3 and 14CO. Very high UFP number concentrations of up to 1.5×104 cm−3 (STP) were encountered inside the poleward half of the fold. This accumulation of small particles is explained by recent extensive aerosol nucleation, most likely triggered by the mixing of stratospheric air with tropospheric air injected into the fold. Further, nine particle formation events were observed outside the fold which are attributed to isolated cells of deep convection and to rising air parcels under cyclonic conditions that mix with surrounding air. In the upper troposphere O3 and CO were found to be correlated with high ΔO3/ΔCO ratios of 0.6 to 1.5. In the fold the correlation was strongly negative with ΔO3/ΔCO; = −3.5; but the high CO mixing ratios of 100 ppb at O3 mixing ratios of 250 ppb point to earlier injection of tropospheric air, in agreement with the UFP measurements.

ML-CIRRUS - The airborne experiment on natural cirrus and contrail cirrus with the high-altitude long-range research aircraft HALO

AbstractThe Midlatitude Cirrus experiment (ML-CIRRUS) deployed the High Altitude and Long Range Research Aircraft (HALO) to obtain new insights into nucleation, life cycle, and climate impact of natural cirrus and aircraft-induced contrail cirrus. Direct observations of cirrus properties and their variability are still incomplete, currently limiting our understanding of the clouds’ impact on climate. Also, dynamical effects on clouds and feedbacks are not adequately represented in today’s weather prediction models.Here, we present the rationale, objectives, and selected scientific highlights of ML-CIRRUS using the G-550 aircraft of the German atmospheric science community. The first combined in situ–remote sensing cloud mission with HALO united state-of-the-art cloud probes, a lidar and novel ice residual, aerosol, trace gas, and radiation instrumentation. The aircraft observations were accompanied by remote sensing from satellite and ground and by numerical simulations.In spring 2014, HALO performed 16 f...

Influence of the 2008 Kasatochi volcanic eruption on sulfurous and carbonaceous aerosol constituents in the lower stratosphere

Influences on stratospheric aerosol during the first four months following the eruption of Kasatochi volcano (Alaska) were studied using observations at 10700 +/- 600 m altitude from the CARIBIC platform. Collected aerosol samples were analyzed for elemental constituents. Particle number concentrations were recorded in three size intervals together with ozone mixing ratios and slant column densities of SO2. The eruption increased particulate sulfur concentrations by a factor of up to 10 compared to periods before the eruption (1999-2002 and 2005-August 2008). Three to four months later, the concentration was still elevated by a factor of 3 in the lowermost stratosphere at northern midlatitudes. Besides sulfur, the Kasatochi aerosol contained a significant carbonaceous component and ash that declined in time after the eruption. The carbon-to-sulfur mass concentration ratio of the volcanic aerosol was 2.6 seven days after the eruption and reached 1.2 after 3 - 4 months. Citation: Martinsson, B. G., C. A. M. Brenninkmeijer, S. A. Carn, M. Hermann, K.-P. Heue, P. F. J. van Velthoven, and A. Zahn (2009), Influence of the 2008 Kasatochi volcanic eruption on sulfurous and carbonaceous aerosol constituents in the lower stratosphere, Geophys. Res. Lett., 36, L12813, doi: 10.1029/2009GL038735. (Less)

Significant radiative impact of volcanic aerosol in the lowermost stratosphere.

Despite their potential to slow global warming, until recently, the radiative forcing associated with volcanic aerosols in the lowermost stratosphere (LMS) had not been considered. Here we study volcanic aerosol changes in the stratosphere using lidar measurements from the NASA CALIPSO satellite and aircraft measurements from the IAGOS-CARIBIC observatory. Between 2008 and 2012 volcanism frequently affected the Northern Hemisphere stratosphere aerosol loadings, whereas the Southern Hemisphere generally had loadings close to background conditions. We show that half of the global stratospheric aerosol optical depth following the Kasatochi, Sarychev and Nabro eruptions is attributable to LMS aerosol. On average, 30% of the global stratospheric aerosol optical depth originated in the LMS during the period 2008–2011. On the basis of the two independent, high-resolution measurement methods, we show that the LMS makes an important contribution to the overall volcanic forcing.

Deuterium, oxygen-18, and tritium as tracers for water vapour transport in the lower stratosphere and tropopause region

Simultaneous measurements of the three rare isotopes Deuterium (D), Tritium (T), and Oxygen-18 (18O) in water vapour were made for the first time in the vicinity of the northern hemisphere tropopause. In contrast to expectation, high D/H and 18O/16O ratios, but relatively low T/H ratios, were found within the lowermost stratosphere. Since water vapour in the low-latitude upper troposphere shows a similar isotopic signature, we conclude that in the mid-latitudes considerable amounts of tropospheric water vapour are injected into the lowermost stratosphere, probably resulting in a hydration of the lower stratosphere. In addition, T can serve as tracer for precipitation of water containing stratospheric aerosol particles, because the T/H ratio in stratospheric water vapour is orders of magnitude higher than in the upper troposphere. Thus, even a small contribution of water of stratospheric origin should be detectable in the tropopause region. In our measurements performed in the Arctic we did not find isotopic evidence for sedimentation of PSC particles down to the tropopause. This may be caused by the low spatial and temporal coverage of our observations; however, it may also be due to the much weaker wintertime dehydration of the Arctic vortex compared to the Antarctic.

Chemical composition and morphology of individual aerosol particles from a CARIBIC flight at 10 km altitude between 50°N and 30°S

Analysis of individual particles by analytical electron microscopy as well as quantitative analysis using particle-induced X-ray emission (PIXE) and particle elastic scattering analysis (PESA) were carried out on samples collected from a flight at 10 km altitude between 50 degrees N and 30 degrees S as part of the Civil Aircraft for Regular Investigation of the atmosphere Based on an Instrument Container (CARIBIC) project (http://www.caribic-atmospheric.com). Particle morphology showed large variability with sampling latitude. Complicated branched structures dominated the large particles of the extratropical northern and southern hemisphere and the northern tropics. Particles in the tropics of the southern hemisphere were small in size and large in number concentration, whereas particles in or close to the intertropical convergence zone were few and small in size. Particles in the lowermost stratosphere were found to have similar structures but more branched than the ones found in the upper troposphere of the extratropics. Quantitative analysis revealed that the sulfur concentration varied by a factor of 50 in the nine samples analyzed in this study. The carbon-to-sulfur mass concentration ratio was lowest in the lowermost stratosphere (0.5) and highest in the tropics of the southern hemisphere (3.5). The elemental distribution of carbon and sulfur in individual particles was mapped by energy-filtered transmission electron microscopy (EFTEM). Almost all particles analyzed contained a mixture of carbonaceous and sulfurous matter. Particles with satellites were found by EFTEM to contain both carbon and sulfur in the central particle, whereas in the satellite particles only carbonaceous material was detected. (Less)

Characteristics and origin of lowermost stratospheric aerosol at northern midlatitudes under volcanically quiescent conditions based on CARIBIC observations

Characteristics and origin of the aerosol in the lowermost stratosphere at northern midlatitudes were studied using measurements from a passenger aircraft ( Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container, or CARIBIC). Aerosol samples were collected during 60 intercontinental flights during 1999 - 2002 and analyzed for elemental composition with particle-induced X-ray emission ( PIXE). Concurrent measurements of trace gases were used to interpret the aerosol measurements. It was found that particulate sulfur concentration increased steadily in the potential vorticity ( PV) region of 2 - 7 PVU, whereas particulate potassium and iron showed no such dependence. The variability in concentration of the latter two elements was mainly connected with season, similar to their variation in the upper troposphere, whereas PV dominated the particulate sulfur variability. An ozone-based model was developed to quantitatively determine the mixing of stratospheric and tropospheric air masses. A significant dependence on PV was found, and the stratospheric fraction of the air peaked during spring. It was found that the particulate sulfur concentration was strongly dependent on the origin of the air masses. The concentration increased by a factor of 3 over the lowermost stratosphere. A discontinuity in the concentration over the tropopause indicated particle formation from sulfur dioxide transported across the tropopause. The concentration at the top of the lowermost stratosphere was used to estimate that the particulate sulfur production in the stratosphere is 0.066 Tg S/yr with approximately half of the amount transported across the top of the lowermost stratosphere originating in carbonyl sulfide. (Less)

Influence of clouds on aerosol particle number concentrations in the upper troposphere

Atmospheric Chemistry Dep., Max Planck Institute for Chemistry, 55128, Mainz, Germany Keywords: aerosol-cloud interaction, particle formation From 1997 to 2001, 47 flights between Colombo (Sri Lanka) or Male (Maldives) and Germany were conducted as part of the CARIBIC project (Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrumentation Container, Brenninkmeijer et al., 1999, http://www.caribic- atmospheric.com). During these flights several trace gas and aerosol parameters were measured. Aerosol number concentrations for three different size ranges were obtained using three condensation particle counters (CPCs), i.e., ultrafine particles (4 to 12 nm, N

Biomass burning and fossil fuel signatures in the upper troposphere observed during a CARIBIC flight from Namibia to Germany

During a CARIBIC flight from Namibia to Germany in July 2000, air influenced by recent convective injection of biomass burning emissions was intersected in the vicinity of the ITCZ at an altitude of 10 km. The observed CO enhancement ratios for non-methane hydrocarbons (NMHCs) and methyl halides are consistent with those reported for fresh biomass burning plumes. Air masses affected by transcontinental transport of natural gas emissions, most probably from the Gulf of Mexico, were encountered over the Mediterranean Sea. These are one of the few observations of deep convection of biomass burning emissions to the upper troposphere and of long range transport of natural gas emissions reported so far. The observations demonstrate the importance of deep convection for the chemistry of the upper troposphere and the potential of commercial aircraft for atmospheric research.

Processes controlling water vapor in the upper troposphere/lowermost stratosphere: An analysis of 8 years of monthly measurements by the IAGOS-CARIBIC observatory

An extensive set of in situ water vapor (H2O) data obtained by the IAGOS-CARIBIC passenger aircraft at 10–12 km altitude over 8 years (2005–2013) is analyzed. A multifaceted description of the vertical distribution of H2O from the upper troposphere (UT) via the extratropical tropopause mixing layer (exTL) into the lowermost stratosphere (LMS) is given. Compared to longer-lived trace gases, H2O is highly variable in the UT and exTL. It undergoes considerable seasonal variation, with maxima in summer and in phase from the UT up to ~4 km above the tropopause. The transport and dehydration pathways of air starting at the Earths surface and ending at 10–12 km altitude are reconstructed based upon (i) potential temperature (θ), (ii) relative humidity with respect to ice (RHi), and (iii) back trajectories as a function of altitude relative to the tropopause. RHi of an air mass was found to be primarily determined by its temperature change during recent vertical movement, i.e., cooling during ascent/expansion and warming during descent/compression. The data show, with great clarity, that H2O and RHi at 10–12 km altitude are controlled by three dominant transport/dehydration pathways: (i) the Hadley circulation, i.e., convective uplift in the tropics and poleward directed subsidence drying from the tropical tropopause layer with observed RHi down to 2%; (ii) warm conveyor belts and midlatitude convection transporting moist air into the UT with observed RHi usually above 60%; and (iii) the Brewer–Dobson shallow and deep branches with observed RHi down to 1%.