The Role of Sulfur Dioxide in Stratospheric Aerosol Formation Evaluated Using In-Situ Measurements in the Tropical Lower Stratosphere.

Stratospheric aerosols (SAs) are a variable component of the Earth's albedo that may be intentionally enhanced in the future to offset greenhouse gases (geoengineering). The role of tropospheric-sourced sulfur dioxide (SO2) in maintaining background SAs has been debated for decades without in-situ measurements of SO2 at the tropical tropopause to inform this issue. Here we clarify the role of SO2 in maintaining SAs by using new in-situ SO2 measurements to evaluate climate models and satellite retrievals. We then use the observed tropical tropopause SO2 mixing ratios to estimate the global flux of SO2 across the tropical tropopause. These analyses show that the tropopause background SO2 is about 5 times smaller than reported by the average satellite observations that have been used recently to test atmospheric models. This shifts the view of SO2 as a dominant source of SAs to a near-negligible one, possibly revealing a significant gap in the SA budget.

Evaluation of UT/LS hygrometer accuracy by intercomparison during the NASA MACPEX mission.

Acquiring accurate measurements of water vapor at the low mixing ratios (< 10 ppm) encountered in the upper troposphere and lower stratosphere (UT/LS) has proven to be a significant analytical challenge evidenced by persistent disagreements between high-precision hygrometers. These disagreements have caused uncertainties in the description of the physical processes controlling dehydration of air in the tropical tropopause layer and entry of water into the stratosphere and have hindered validation of satellite water vapor retrievals. A 2011 airborne intercomparison of a large group of in situ hygrometers onboard the NASA WB-57F high-altitude research aircraft and balloons has provided an excellent opportunity to evaluate progress in the scientific community toward improved measurement agreement. In this work we intercompare the measurements from the Midlatitude Airborne Cirrus Properties Experiment (MACPEX) and discuss the quality of agreement. Differences between values reported by the instruments were reduced in comparison to some prior campaigns but were nonnegligible and on the order of 20% (0.8 ppm). Our analysis suggests that unrecognized errors in the quantification of instrumental background for some or all of the hygrometers are a likely cause. Until these errors are understood, differences at this level will continue to somewhat limit our understanding of cirrus microphysical processes and dehydration in the tropical tropopause layer.

Note: Compact, two-dimension translatable slit aperture.

A compact, light-weight, two-dimension translatable slit aperture is described. The slit dimensions are scalable, allowing for wide application. With all metal construction, the device would be suitable for high temperature degassing and vacuum compatible. Alternatively, the main structure may be printed using a 3D printer for rapid prototyping and/or lighter weight. The precision of the slit movement is 0.014 mm.

Black carbon aerosol size in snow.

The effect of anthropogenic black carbon (BC) aerosol on snow is of enduring interest due to its consequences for climate forcing. Until now, too little attention has been focused on BC's size in snow, an important parameter affecting BC light absorption in snow. Here we present first observations of this parameter, revealing that BC can be shifted to larger sizes in snow than are typically seen in the atmosphere, in part due to the processes associated with BC removal from the atmosphere. Mie theory analysis indicates a corresponding reduction in BC absorption in snow of 40%, making BC size in snow the dominant source of uncertainty in BC's absorption properties for calculations of BC's snow albedo climate forcing. The shift reduces estimated BC global mean snow forcing by 30%, and has scientific implications for our understanding of snow albedo and the processing of atmospheric BC aerosol in snowfall.

Global-scale seasonally resolved black carbon vertical profiles over the Pacific.

[1] Black carbon (BC) aerosol loadings were measured during the High-performance Instrumented Airborne Platform for Environmental Research Pole-to-Pole Observations (HIPPO) campaign above the remote Pacific from 85°N to 67°S. Over 700 vertical profiles extending from near the surface to max ∼14 km altitude were obtained with a single-particle soot photometer between early 2009 and mid-2011. The data provides a climatology of BC in the remote regions that reveals gradients of BC concentration reflecting global-scale transport and removal of pollution. BC is identified as a sensitive tracer of extratropical mixing into the lower tropical tropopause layer and trends toward surprisingly uniform loadings in the lower stratosphere of ∼1 ng/kg. The climatology is compared to predictions from the AeroCom global model intercomparison initiative. The AeroCom model suite overestimates loads in the upper troposphere/lower stratosphere (∼10×) more severely than at lower altitudes (∼3×), with bias roughly independent of season or geographic location; these results indicate that it overestimates BC lifetime.

Arc Discharge Synthesis and Photoluminescence of 3D Feather-like AlN Nanostructures.

A complex three-dimensional (3D) feather-like AlN nanostructure was synthesized by a direct reaction of high-purity Al granules with nitrogen using an arc discharge method. By adjusting the discharge time, a coral-like nanostructure, which evolved from the feather-like nanostructure, has also been observed. The novel 3D feather-like AlN nanostructure has a hierarchical dendritic structure, which means that the angle between the trunk stem and its branch is always about 30° in any part of the structure. The fine branches on the surface of the feather-like nanostructure have shown a uniform fish scale shape, which are about 100 nm long, 10 nm thick and several tens of nanometers in width. An alternate growth model has been proposed to explain the novel nanostructure. The spectrum of the feather-like products shows a strong blue emission band centered at 438 nm (2.84 eV), which indicates their potential application as blue light-emitting diodes.

Quantifying stratospheric ozone in the upper troposphere with in situ measurements of HCl.

We have developed a chemical ionization mass spectrometry technique for precise in situ measurements of hydrochloric acid (HCl) from a high-altitude aircraft. In measurements at subtropical latitudes, minimum HCl values found in the upper troposphere (UT) were often near or below the detection limit of the measurements (0.005 parts per billion by volume), indicating that background HCl values are much lower than a global mean estimate. However, significant abundances of HCl were observed in many UT air parcels, as a result of stratosphere-to-troposphere transport events. We developed a method for diagnosing the amount of stratospheric ozone in these UT parcels using the compact linear correlation of HCl with ozone found throughout the lower stratosphere (LS). Expanded use of this method will lead to improved quantification of cross-tropopause transport events and validation of global chemical transport models.

Evidence that nitric acid increases relative humidity in low-temperature cirrus clouds.

In situ measurements of the relative humidity with respect to ice (RHi) and of nitric acid (HNO3) were made in both natural and contrail cirrus clouds in the upper troposphere. At temperatures lower than 202 kelvin, RHi values show a sharp increase to average values of over 130% in both cloud types. These enhanced RHi values are attributed to the presence of a new class of HNO3-containing ice particles (Delta-ice). We propose that surface HNO3 molecules prevent the ice/vapor system from reaching equilibrium by a mechanism similar to that of freezing point depression by antifreeze proteins. Delta-ice represents a new link between global climate and natural and anthropogenic nitrogen oxide emissions. Including Delta-ice in climate models will alter simulated cirrus properties and the distribution of upper tropospheric water vapor.

The detection of large HNO3-containing particles in the winter Arctic stratosphere.

Large particles containing nitric acid (HNO3) were observed in the 1999/2000 Arctic winter stratosphere. These in situ observations were made over a large altitude range (16 to 21 kilometers) and horizontal extent (1800 kilometers) on several airborne sampling flights during a period of several weeks. With diameters of 10 to 20 micrometers, these sedimenting particles have significant potential to denitrify the lower stratosphere. A microphysical model of nitric acid trihydrate particles is able to simulate the growth and sedimentation of these large sizes in the lower stratosphere, but the nucleation process is not yet known. Accurate modeling of the formation of these large particles is essential for understanding Arctic denitrification and predicting future Arctic ozone abundances.

Removal of Stratospheric O3 by Radicals: In Situ Measurements of OH, HO2, NO, NO2, ClO, and BrO.

Simultaneous in situ measurements of the concentrations of OH, HO(2), ClO, BrO, NO, and NO(2) demonstrate the predominance of odd-hydrogen and halogen free-radical catalysis in determining the rate of removal of ozone in the lower stratosphere during May 1993. A single catalytic cycle, in which the rate-limiting step is the reaction of HO(2) with ozone, accounted for nearly one-half of the total O(3) removal in this region of the atmosphere. Halogen-radical chemistry was responsible for approximately one-third of the photochemical removal of O(3); reactions involving BrO account for one-half of this loss. Catalytic destruction by NO(2), which for two decades was considered to be the predominant loss process, accounted for less than 20 percent of the O(3) removal. The measurements demonstrate quantitatively the coupling that exists between the radical families. The concentrations of HO(2) and ClO are inversely correlated with those of NO and NO(2). The direct determination of the relative importance of the catalytic loss processes, combined with a demonstration of the reactions linking the hydrogen, halogen, and nitrogen radical concentrations, shows that in the air sampled the rate of O(3) removal was inversely correlated with total NOx, loading.