Measurements of NO, NOy, N2O, and O3 during SPURT: implications for transport and chemistry in the lowermost stratosphere

We present measurements of NO, NOy, O3, and N2O within the lowermost stratosphere (LMS) over Europe obtained during the SPURT project. The measurements cover all seasons between November 2001 and July 2003. They span a broad band of latitudes from 30° N to 75° N and a potential temperature range from 290 to 380 K. The measurements represent a comprehensive data set of these tracers and reveal atmospheric transport processes that influence tracer distributions in the LMS. Median mixing ratios of stratospheric tracers in equivalent latitude-potential temperature coordinates show a clear seasonal cycle related to the Brewer-Dobson circulation, with highest values in spring and lowest values in autumn. Vertical tracer profiles show strong gradients at the extratropical tropopause, suggesting that vertical (cross-isentropic) mixing is reduced above the tropopause. Pronounced meridional gradients in the tracer mixing ratios are found on potential temperature surfaces in the LMS. This suggests strongly reduced mixing along isentropes. Concurrent large gradients in static stability in the vertical direction, and of PV in the meridional direction, suggest the presence of a mixing barrier. Seasonal cycles were found in the correlation slopes ΔO3/ΔN2O and ΔNOy/ΔN2O well above the tropopause. Absolute slope values are smallest in spring indicating chemically aged stratospheric air originating from high altitudes and latitudes. Larger values were measured in summer and autumn suggesting that a substantial fraction of air takes a "short-cut" from the tropical tropopause region into the extratropical LMS. The seasonal change in the composition of the LMS has direct implications for the ozone chemistry in this region. Comparisons of measured NO with the critical NO value at which net ozone production changes from negative to positive, imply ozone production up to 20 K above the local tropopause in spring, up to 30 K in summer, and up to 40 K in autumn. Above these heights, and in winter, net ozone production is negative

Tracing troposphere-to-stratosphere transport above a mid-latitude deep convective system

Within the project SPURT (trace gas measurements in the tropopause region) a variety of trace gases have been measured in situ in order to investigate the role of dynamical and chemical processes in the extra-tropical tropopause region. In this paper we report on a flight on 10 November 2001 leading from Hohn, Germany (52�N) to Faro, Portugal (37�N) through a strongly developed deep stratospheric intrusion. This streamer was associated with a large convective system over the western Mediterranean with potentially significant troposphere-to-stratosphere transport. Along major parts of the flight we measured unexpectedly high NOy mixing ratios. Also H2O mixing ratios were significantly higher than stratospheric background levels confirming the extraordinary chemical signature of the probed air masses in the interior of the streamer. Backward trajectories encompassing the streamer enable to analyze the origin and physical characteristics of the air masses and to trace troposphere-to-stratosphere transport. Near the western flank of the streamer features caused by long range transport, such as tropospheric filaments characterized by sudden drops in the O3 and NOy mixing ratios and enhanced CO and H2O can be reconstructed in great detail using the reverse domain filling technique. These filaments indicate a high potential for subsequent mixing with the stratospheric air. At the south-western edge of the streamer a strong gradient in the NOy and the O3 mixing ratios coincides very well with a sharp gradient in potential vorticity in the ECMWF fields. In contrast, in the interior of the streamer the observed highly elevated NOy and H2O mixing ratios up to a potential temperature level of 365K and potential vorticity values of maximum 10 PVU cannot be explained in terms of resolved troposphere-to-stratosphere transport along the backward trajectories. Also mesoscale simulations with a High Resolution Model reveal no direct evidence for convective H2O injection up to this level. Elevated H2O mixing ratios in the ECMWF and HRM are seen only up to about tropopause height at 340 hPa and 270 hPa, respectively, well below flight altitude of about 200 hPa. However, forward tracing of the convective influence as identified by satellite brightness temperature measurements and counts of lightning strokes shows that during this part of the flight the aircraft was closely following the border of an air mass which was heavily impacted by convective activity over Spain and Algeria. This is evidence that deep convection at mid-latitudes may have a large impact on the tracer distribution of the lowermost stratosphere reaching well above the thunderstorms anvils as claimed by recent studies using cloud-resolving models

Ice nucleation in aqueous solutions of poly ethylene glycol with different molar mass

Zobrist B, Weers U, Koop T. Ice nucleation in aqueous solutions of poly ethylene glycol with different molar mass. JOURNAL OF CHEMICAL PHYSICS. 2003;118(22):10254-10261.Homogeneous ice nucleation was investigated in aqueous solutions of poly[ethylene glycol] (PEG) with a molar mass between 300 and 6000 g mol(-1). Experiments were performed with a differential scanning calorimeter using emulsified aqueous PEG solutions with concentrations of 0 - 44 wt %. Equilibrium phase transition temperatures are determined and discussed, in particular the simultaneous occurrence of metastable and stable eutectic temperatures. The observed homogeneous freezing temperatures of ice reveal a continuous increase in the supercooling of PEG solutions with increasing molar mass of the PEG. The freezing behavior was investigated within the framework of water-activity-based ice nucleation theory. The latter predicts that homogeneous ice nucleation in aqueous solutions is independent of the nature of the solute, but depends only on the water activity of the solution. Water activity data of various PEG solutions in the stable and supercooled range were compared. It was found that the water activity of PEG solutions decreases with decreasing temperature. This trend is stronger for the PEGs of larger molar mass, and can explain why solutions of larger PEGs supercool more readily. Our study shows that the predictions of water-activity-based ice nucleation theory are consistent with the observed freezing behavior and with the thermodynamic properties of PEG solutions. It is also in agreement with the suggestion of a thermodynamic control of ice nucleation in aqueous systems due to the existence of a liquid phase spinodal. (C) 2003 American Institute of Physics

Homogeneous nucleation of NAD and NAT in liquid stratospheric aerosols: insufficient to explain denitrification

Knopf DA, Koop T, Luo BP, Weers UG, Peter T. Homogeneous nucleation of NAD and NAT in liquid stratospheric aerosols: insufficient to explain denitrification. ATMOSPHERIC CHEMISTRY AND PHYSICS. 2002;2(3):207-214.The nucleation of NAD and NAT from HNO3/H2O and HNO3/H2SO4/H2O solution droplets is investigated both theoretically and experimentally with respect to the formation of polar stratospheric clouds (PSCs). Our analysis shows that homogeneous NAD and NAT nucleation from liquid aerosols is insufficient to explain the number densities of large nitric acid containing particles recently observed in the Arctic stratosphere. This conclusion is based on new droplet freezing experiments employing optical microscopy combined with Raman spectroscopy. The homogeneous nucleation rate coefficients of NAD and NAT in liquid aerosols under polar stratospheric conditions derived from the experiments are < 2 x 10(-5) cm(-3) s(-1) and < 8 x 10(-2) cm(-3) s(-1), respectively. These nucleation rate coefficients are smaller by orders of magnitude than the value of similar to10(3) cm(-3) s(-1) used in a recent denitrification modelling study that is based on a linear extrapolation of laboratory nucleation data to stratospheric conditions (Tabazadeh et al., Science, 291, 2591-2594, 2001). We show that this linear extrapolation is in disagreement with thermodynamics and with experimental data and, therefore, must not be used in microphysical models of PSCs. Our analysis of the experimental data yields maximum hourly production rates of nitric acid hydrate particles per cm 3 of air of about 3 x 10(-10) cm(-3) (air) h(-1) under polar stratospheric conditions. Assuming PSC particle production to proceed at this rate for two months we arrive at particle number densities of < 5 x 10(-7) cm(-3), much smaller than the value of &SIM;10(-4) cm(-3) reported in recent field observations. In addition, the nitric acid hydrate production rate inferred from our data is much smaller than that required to reproduce the observed denitrification in the modelling study mentioned above. This clearly shows that homogeneous nucleation of NAD and NAT from liquid supercooled ternary solution aerosols cannot explain the observed polar denitrification

Supercooling of single H2SO4/H2O aerosols to 158 K: No evidence for the occurrence of the octrahydrate

Krieger UK, Colberg CA, Weers U, Koop T, Peter T. Supercooling of single H2SO4/H2O aerosols to 158 K: No evidence for the occurrence of the octrahydrate. GEOPHYSICAL RESEARCH LETTERS. 2000;27(14):2097-2100.Polar stratospheric clouds (PSCs) are important for the chemical activation of chlorine compounds and subsequent ozone depletion. However, how solid PSCs form is still subject to controversial discussion. Recently, the octahydrate of sulfuric acid (H2SO4. 8H(2)O) has been proposed as a solid phase for PSC particles. In our experiment single H2SO4/H2O aerosols levitated in an electrodynamic balance are cooled to 158 K along the ice-liquid equilibrium line. This coexistence curve has been measured for the first time in the temperature interval between 200 K and 158 K. Three independent methods are used for measuring the H2SO4 concentration of the aerosol. No phase transition is observed over a time period of 24 hours and longer at these or higher temperatures. There is no indication for the occurrence of the octahydrate in our experiments. Since the product of experimental volume and suspension time is equal to that of stratospheric droplets over an entire winter, the homogeneous formation of the octahydrate at quasi-thermodynamic equilibrium can be practically excluded

Balloon-borne match measurements of midlatitude cirrus clouds

Observations of high supersaturations with respect to ice inside cirrus clouds with high ice water content (> 0.01 g kg−1) and high crystal number densities (> 1 cm−3) are challenging our understanding of cloud microphysics and of climate feedback processes in the upper troposphere. However, single measurements of a cloudy air mass provide only a snapshot from which the persistence of ice supersaturation cannot be judged. We introduce here the "cirrus match technique" to obtain information about the evolution of clouds and their saturation ratio. The aim of these coordinated balloon soundings is to analyze the same air mass twice. To this end the standard radiosonde equipment is complemented by a frost point hygrometer, "SnowWhite", and a particle backscatter detector, "COBALD" (Compact Optical Backscatter AerosoL Detector). Extensive trajectory calculations based on regional weather model COSMO (Consortium for Small-Scale Modeling) forecasts are performed for flight planning, and COSMO analyses are used as a basis for comprehensive microphysical box modeling (with grid scale of 2 and 7 km, respectively). Here we present the results of matching a cirrus cloud to within 2–15 km, realized on 8 June 2010 over Payerne, Switzerland, and a location 120 km downstream close to Zurich. A thick cirrus cloud was detected over both measurement sites. We show that in order to quantitatively reproduce the measured particle backscatter ratios, the small-scale temperature fluctuations not resolved by COSMO must be superimposed on the trajectories. The stochastic nature of the fluctuations is captured by ensemble calculations. Possibilities for further improvements in the agreement with the measured backscatter data are investigated by assuming a very slow mass accommodation of water on ice, the presence of heterogeneous ice nuclei, or a wide span of (spheroidal) particle shapes. However, the resulting improvements from these microphysical refinements are moderate and comparable in magnitude with changes caused by assuming different regimes of temperature fluctuations for clear-sky or cloudy-sky conditions, highlighting the importance of proper treatment of subscale fluctuations. The model yields good agreement with the measured backscatter over both sites and reproduces the measured saturation ratios with respect to ice over Payerne. Conversely, the 30% in-cloud supersaturation measured in a massive 4 km thick cloud layer over Zurich cannot be reproduced, irrespective of the choice of meteorological or microphysical model parameters. The measured supersaturation can only be explained by either resorting to an unknown physical process, which prevents the ice particles from consuming the excess humidity, or – much more likely – by a measurement error, such as a contamination of the sensor housing of the SnowWhite hygrometer by a precipitation drop from a mixed-phase cloud just below the cirrus layer or from some very slight rain in the boundary layer. This uncertainty calls for in-flight checks or calibrations of hygrometers under the special humidity conditions in the upper troposphere

Uptake of nitric acid in ice crystals in persistent contrails

This is a short version of Schäuble et al. (2009) published in ACPD. In November 2006 cirrus clouds and almost 40 persistent contrails were probed with in situ instruments over Germany and Northern Europe during the CIRRUS-III campaign. At altitudes between 10 and 11.5 km and temperatures of 211 to 220 K contrails with ages up to 8 hours were detected. These contrails had a larger ice phase fraction of total nitric acid (HNO3,ice/HNO3,tot = 6 %) than the ambient cirrus layers (3 %). The differences in ice phase fractions between developing contrails and cirrus are likely caused by high plume concentrations of HNO3 prior to contrail formation and large ice crystal number densities in contrails. The observed decrease of nitric acid to water molar ratios in ice with increasing mean ice particle diameter suggests that ice-bound HNO3 concentrations are controlled by uptake of exhaust HNO3 in the freezing plume aerosol in young contrails and subsequent trapping of ambient HNO3 in growing ice particles in older (age > 1 h) contrails

Airborne measurements of the nitric acid partitioning in persistent contrails

This study reports the first systematic measurements of nitric acid (HNO&lt;sub&gt;3&lt;/sub&gt;) uptake in contrail ice particles at typical aircraft cruise altitudes. During the CIRRUS-III campaign cirrus clouds and almost 40 persistent contrails were probed with in situ instruments over Germany and Northern Europe in November 2006. Besides reactive nitrogen, water vapor, cloud ice water content, ice particle size distributions, and condensation nuclei were measured during 6 flights. Contrails with ages up to 12 h were detected at altitudes 10–11.5 km and temperatures 211–220 K. These contrails had a larger ice phase fraction of total nitric acid (HNO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;ice&lt;/sup&gt;/HNO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;tot&lt;/sup&gt; = 6%) than the ambient cirrus layers (3%). On average, the contrails contained twice as much HNO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;ice&lt;/sup&gt; as the cirrus clouds, 14 pmol/mol and 6 pmol/mol, respectively. Young contrails with ages below 1 h had a mean HNO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;ice&lt;/sup&gt; of 21 pmol/mol. The contrails had higher nitric acid to water molar ratios in ice and slightly higher ice water contents than the cirrus clouds under similar meteorological conditions. The differences in ice phase fractions and molar ratios between developing contrails and cirrus are likely caused by high plume concentrations of HNO&lt;sub&gt;3&lt;/sub&gt; prior to contrail formation. The location of the measurements in the upper region of frontal cirrus layers might account for slight differences in the ice water content between contrails and adjacent cirrus clouds. The observed dependence of molar ratios as a function of the mean ice particle diameter suggests that ice-bound HNO&lt;sub&gt;3&lt;/sub&gt; concentrations are controlled by uptake of exhaust HNO&lt;sub&gt;3&lt;/sub&gt; in the freezing plume aerosols in young contrails and subsequent trapping of ambient HNO&lt;sub&gt;3&lt;/sub&gt; in growing ice particles in older (age &gt; 1 h) contrails