Pervasive cirrus clouds in the tropical tropopause layer (TTL) play an important role in determining the composition of stratospheric air through dehydration of tropospheric air entering the stratosphere. This dehydration affects Earth's energy budget and climate, yet uncertainties remain regarding the microphysical processes that govern TTL cirrus. TTL cirrus were sampled with the NASA Global Hawk UAV for over 30 hr in the Western Pacific in 2014 during the Airborne Tropical TRopopause EXperiment. In situ measurements by a Fast Cloud Droplet Probe and Hawkeye probe (combination Fast Cloud Droplet Probe, Two‐Dimensional Stereo optical array probe, and Cloud Particle Imager) provided particle concentrations and sizing between 1‐ and 1,280‐μm diameter and high resolution images for habit identification. We present the variability in ice concentrations, size distributions, and habits as functions of temperature, altitude, and time since convective influence. Observed ice particles were predominantly small and quasi‐spheroidal in shape, with the percentage of quasi‐spheroids increasing with decreasing temperature. In comparison to the large fraction of the population consisting of quasi‐spheroids, faceted habits (columns, plates, rosettes, and budding rosettes) constituted a smaller percentage of the overall population and exhibited the opposite correlation with temperature. The trend of higher percentages of faceted crystals occurring at warmer temperatures may be due to diffusional growth or aggregation as particles descend through cloud, and/or the more rapid diffusional growth rate at warmer temperatures. Sampling was typically well away from deep convection, however, and very few aggregates were observed, so the trend of higher percentages of faceted habits is likely attributable to diffusional growth.

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.

The oxidation of carbonyl sulfide (OCS) is the primary, continuous source of stratospheric sulfate aerosol particles, which can scatter shortwave radiation and catalyze heterogeneous reactions in the stratosphere. While it has been estimated that the oxidation of dimethyl sulfide (DMS), emitted from the surface ocean accounts for 8%–20% of the global OCS source, there is no existing DMS oxidation mechanism relevant to the marine atmosphere that is consistent with an OCS source of this magnitude. We describe new laboratory measurements and theoretical analyses of DMS oxidation that provide a mechanistic description for OCS production from hydroperoxymethyl thioformate, a ubiquitous, soluble DMS oxidation product. We incorporate this chemical mechanism into a global chemical transport model, showing that OCS production from DMS is a factor of 3 smaller than current estimates, displays a maximum in the tropics consistent with field observations and is sensitive to multiphase cloud chemistry.

Fires emit sufficient sulfur to affect local and regional air quality and climate. This study analyzes SO₂ emission factors and variability in smoke plumes from US wildfires and agricultural fires, as well as their relationship to sulfate and hydroxymethanesulfonate (HMS) formation. Observed SO₂ emission factors for various fuel types show good agreement with the latest reviews of biomass burning emission factors, producing an emission factor range of 0.47–1.2 g SO₂ kg⁻¹ C. These emission factors vary with geographic location in a way that suggests that deposition of coal burning emissions and application of sulfur-containing fertilizers likely play a role in the larger observed values, which are primarily associated with agricultural burning. A 0-D box model generally reproduces the observed trends of SO₂ and total sulfate (inorganic + organic) in aging wildfire plumes. In many cases, modeled HMS is consistent with the observed organosulfur concentrations. However, a comparison of observed organosulfur and modeled HMS suggests that multiple organosulfur compounds are likely responsible for the observations but that the chemistry of these compounds yields similar production and loss rates as that of HMS, resulting in good agreement with the modeled results. We provide suggestions for constraining the organosulfur compounds observed during these flights, and we show that the chemistry of HMS can allow organosulfur to act as an S(IV) reservoir under conditions of pH > 6 and liquid water content >10⁻⁷ g sm⁻³. This can facilitate long-range transport of sulfur emissions, resulting in increased SO₂ and eventually sulfate in transported smoke.

Although the tropopause is a well‐established concept, its definition and physical properties remain an active research topic. In the tropics, both the World Meteorological Organization established lapse rate tropopause definition and the minimum in the temperature profile (the cold point) are used to determine the tropopause height. We examine the differences produced by these two definitions using high‐resolution airborne in situ measurements of temperature, water vapor, and ozone in the tropical tropopause layer from a recent experiment over the western Pacific using the National Aeronautics and Space Administration (NASA) Global Hawk unmanned aircraft system. When the two definitions do not produce the same tropopause height, which is in about half of the cases, the combined temperature and trace gas analysis shows that the lapse rate definition better identifies the transition from the troposphere to the stratosphere.

We describe a newly developed single-photon laser-induced fluorescence sensor for measurements of nitric oxide (NO) in the atmosphere. Rapid tuning of a narrow-band laser on and off of a rotationally resolved NO spectral feature near 215 nm and detection of the red-shifted fluorescence provides for interference-free direct measurements of NO with a detection limit of 1 part per trillion by volume (pptv) for 1 s of integration, or 0.3 pptv for 10 s of integration. Uncertainty in the sensitivity of the instrument is typically ±6–9 %, with no known interferences. Uncertainty in the zero of the detector is shown to be <0.2 pptv. The instrument was deployed on the NASA DC-8 aircraft during the NASA/NOAA FIREX-AQ experiment (Fire Influence on Regional to Global Environments Experiment – Air Quality) during July–September 2019 and provided more than 140 h of NO measurements over 22 flights, demonstrating the ability of this instrument to operate routinely and autonomously. Comparisons with a seasoned chemiluminescence sensor during FIREX-AQ in a variety of chemical environments provides validation and confidence in the accuracy of this technique.