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The AquaVIT-1 Intercomparison of Atmospheric Water Vapor Measurement Techniques was conducted at the aerosol and cloud simulation chamber AIDA at the Karlsruhe Institute of Technology, Germany, in October 2007. The overall objective was to intercompare state-of-the-art and prototype atmospheric hygrometers with each other and with independent humidity standards under controlled conditions. This activity was conducted as a blind intercomparison with coordination by selected referees. The effort was motivated by persistent discrepancies found in atmospheric measurements involving multiple instruments operating on research aircraft and balloon platforms, particularly in the upper troposphere and lower stratosphere where water vapor reaches its lowest atmospheric values (less than 10 ppm). With the AIDA chamber volume of 84 m3, multiple instruments analyzed air with a common water vapor mixing ratio, either by extracting air into instrument flow systems, locating instruments inside the chamber, or sampling the chamber volume optically. The intercomparison was successfully conducted over 10 days during which pressure, temperature, and mixing ratio were systematically varied (50 to 500 hPa, 185 to 243 K, and 0.3 to 152 ppm). In the absence of an accepted reference instrument, the reference value was taken to be the ensemble mean of a core subset of the measurements. For these core instruments, the agreement between 10 and 150 ppm of water vapor is considered good with variation about the reference value of about ±10% (±1σ). In the region of most interest between 1 and 10 ppm, the core subset agreement is fair with variation about the reference value of ±20% (±1σ). The upper limit of precision was also derived for each instrument from the reported data. These results indicate that the core instruments, in general, have intrinsic skill to determine unknown water vapor mixing ratios with an accuracy of at least ±20%. The implication for atmospheric measurements is that the substantially larger differences observed during in-flight intercomparisons stem from other factors associated with the moving platforms or the non-laboratory environment. The success of AquaVIT-1 provides a template for future intercomparison efforts with water vapor or other species that are focused on improving the analytical quality of atmospheric measurements on moving platforms.

Acquiring accurate measurements of water vapor at the low mixing ratios (

International audience; Observations of CO2 were made in the upper troposphere and lower stratosphere in the deep tropics in order to determine the patterns of large-scale vertical transport and age of air in the Tropical Tropopause Layer (TTL). Flights aboard the NASA WB-57F aircraft over Central America and adjacent ocean areas took place in January and February, 2004 (Pre-AURA Validation Experiment, Pre-AVE) and 2006 (Costa Rice AVE, CR-AVE), and for the same flight dates of 2006, aboard the Proteus aircraft from the surface to 15 km over Darwin, Australia (Tropical Warm Pool International Cloud Experiment, TWP-ICE). The data demonstrate that the TTL is composed of two layers with distinctive features: (1) the lower TTL, 350?360 K (potential temperature(?); approximately 12?14 km), is subject to inputs of convective outflows, as indicated by layers of variable CO2 concentrations, with air parcels of zero age distributed throughout the layer; (2) the upper TTL, from ?=~360 K to ~390 K (14?18 km), ascends slowly and ages uniformly, as shown by a linear decline in CO2 mixing ratio tightly correlated with altitude, associated with increasing age. This division is confirmed by ensemble trajectory analysis. The CO2 concentration at the level of 360 K was 380.0(±0.2) ppmv, indistinguishable from surface site values in the Intertropical Convergence Zone (ITCZ) for the flight dates. Values declined with altitude to 379.2(±0.2) ppmv at 390 K, implying that air in the upper TTL monotonically ages while ascending. In combination with the winter slope of the CO2 seasonal cycle (+10.8±0.4 ppmv/yr), the vertical gradient of ?0.78 (±0.09) ppmv gives a mean age of 26(±3) days for the air at 390 K and a mean ascent rate of 1.5(±0.3) mm s?1. The TTL near 360 K in the Southern Hemisphere over Australia is very close in CO2 composition to the TTL in the Northern Hemisphere over Costa Rica, with strong contrasts emerging at lower altitudes (2 observed unexpected input from deep convection over Amazônia deep into the TTL. The CO2 data confirm the operation of a highly accurate tracer clock in the TTL that provides a direct measure of the ascent rate of the TTL and of the age of air entering the stratosphere.

Large (>2 μm diameter) HNO_3-containing polar stratospheric cloud (PSC) particles were measured in situ by the NOAA NO_y instrument on board the NASA ER-2 aircraft during seven flights in the 1999/2000 Arctic winter vortex. Here we discuss the detection of these large PSC particles, their spatial distribution, the ambient conditions under which they were detected, and our methods for interpreting NO_y time series with respect to particle sizes and number concentrations. The particles were observed through the use of two NO_y inlets on a particle separator extending below the ER-2 aircraft. The particle phase is assumed to be nitric acid trihydrate (NAT) or nitric acid dihydrate (NAD). Over a 48-day period, particles were sampled in the Arctic vortex over a broad range of latitudes (60–85°N) and altitudes (15–21 km). Typically, regions of the atmosphere up to 4 km above the observed large particle clouds were saturated with respect to NAT. Occasionally, large particles were measured in air subsaturated with respect to NAT, suggesting ongoing particle evaporation. Vortex minimum temperatures in the observation period suggest that synoptic-scale ice saturation conditions are not required for the formation of this type of particle. Three analytical methods are used to estimate size and number concentrations from the NO_y time series. Results indicate particle sizes between 5 and 20 μm diameter and concentrations from 10^(−5) to 10^(−3) cm^(−3). These low number concentrations imply a selective nucleation mechanism. Particle sizes and number concentrations were greater during the midwinter flights than the late winter flights. Knowledge of the geographical extent of large particles, actual sampling conditions, and particle size distributions offers multiple constraints for atmospheric models of PSC formation, which will lead to a better understanding of the process of denitrification and improvements in modeling future ozone loss.

Measurements of the concentrations of OH and HO_(2) (HO_(x)) in the high-latitude lower stratosphere imply the existence of unknown photolytic sources of HO_(x). The strength of the additional HO_(x) source required to match the observations depends only weakly on solar zenith angle (SZA) for 80° < SZA < 93°. The wavelengths responsible for producing this HO_(x) must be longer than 650 nm because the flux at shorter wavelengths is significantly attenuated at high SZA by scattering and absorption. Provided that the sources involve only a single photon, the strength of the bonds being broken must be < 45 kcal mole^(−1). We speculate that peroxynitric acid (HNO_4) dissociates after excitation to an unknown excited state with an integrated band cross section of 2-3 × 10^(−20) cm^(2) molecule^(−1) nm (650 < λ < 1250 nm).

The generation of sulfuric acid aerosols in aircraft exhaust has emerged as a critical issue in determining the impact of supersonic aircraft on stratospheric ozone. It has long been held that the first step in the mechanism of aerosol formation is the oxidation of SO_(2) emitted from the engine by OH in the exhaust plume. We report in situ measurements of OH and HO_(2) in the exhaust plumes of a supersonic (Air France Concorde) and a subsonic (NASA ER-2) aircraft in the lower stratosphere. These measurements imply that reactions with OH are responsible for oxidizing only a small fraction of SO_(2) (2%), and thus cannot explain the large number of particles observed in the exhaust wake of the Concorde.

During the 1993 NASA Stratospheric Photochemistry, Aerosols and Dynamics Expedition (SPADE), anomalously low nitric oxide (NO) was found in a distinct sunlit layer located above the mid-latitude tropopause. The presence of a significant amount of reactive nitrogen (NO_y) in the layer implies the systematic removal of NO, which is without precedent in stratospheric in situ observations. Large increases in measured chlorine monoxide (ClO) and the hydroperoxyl radical (HO_2) also were observed in the layer. Heterogeneous reaction rate constants of chlorine nitrate (ClONO_2) with hydrogen chloride (HCl) and H_2O to form nitric acid (HNO_3) on sulfate aerosol are enhanced in the NO removal layer by local increases in H_2O and aerosol surface area. The associated conversion of NO_x (= NO + NO_2) to HNO_3 is the most likely cause of the observed low NO and NO_x/NO_y values and high ClO values.

Using results from a time-dependent photochemical model to calculate the diurnal variation of NO and NO_2, we have corrected Atmospheric Trace MOlecule Spectroscopy (ATMOS) solar-occultation retrievals of the NO and NO_2 abundances at 90° solar zenith angle. Neglecting to adjust for the rapid variation of these gases across the terminator results in potential errors in retrieved profiles of ∼20% for NO_2 and greater than 100% for NO at altitudes below 25 km. Sensitivity analysis indicates that knowledge of the local O_3 and temperature profiles, rather than zonal mean or climatological conditions of these quantities, is required to obtain reliable retrievals of NO and NO_2 in the lower stratosphere. Extremely inaccurate O_3 or temperature values at 20 km can result in 50% errors in retrieved NO or NO_2. Mixing ratios of NO in the mid-latitude, lower stratosphere measured by ATMOS during the November 1994 ATLAS-3 mission compare favorably with in situ ER-2 observations, providing strong corroboration of the reliability of the adjusted space-borne measurements.

In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained through sunrise and sunset in the lower stratosphere during SPADE are compared to results from a photochemical model constrained by observed concentrations of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N_(2)O_(5) on sulfate aerosols agree with measured concentrations of NO, NO_(2), and ClO throughout the day, but fail to account for high concentrations of OH and HO_(2) observed near sunrise and sunset. The morning burst of [OH] and [HO_(2)] coincides with the rise of [NO] from photolysis of NO_(2), suggesting a new source of HO_(x) that photolyzes in the near UV (350 to 400 nm) spectral region. A model that allows for the heterogeneous production of HNO_(2) results in an excellent simulation of the diurnal variations of [OH] and [HO_(2)].

In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained in the lower stratosphere during SPADE are compared to results from a photochemical model that assimilates measurements of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N_2O_5 agree well with measured concentrations of NO and ClO, but concentrations of HO_2 and OH are underestimated by 10 to 25%, concentrations of NO_2 are overestimated by 10 to 30%, and concentrations of HCl are overestimated by a factor of 2. Discrepancies for [OH] and [HO_2] are reduced if we allow for higher yields of O(^1D) from O_3 photolysis and for heterogeneous production of HNO_2. The data suggest more efficient catalytic removal of O_3 by hydrogen and halogen radicals relative to nitrogen oxide radicals than predicted by models using recommended rates and cross sections. Increases in [O_3] in the lower stratosphere may be larger in response to inputs of NO_y from supersonic aircraft than estimated by current assessment models.