Joshua Digangi

Active and widespread halogen chemistry in the tropical and subtropical free troposphere

Significance Our measurements show that tropospheric halogen chemistry has a larger capacity to destroy O3 and oxidize atmospheric mercury than previously recognized. Halogen chemistry is currently missing in most global and climate models, and is effective at removing O3 at altitudes where intercontinental O3 transport occurs. It further helps explain the low O3 levels in preindustrial times. Public health concerns arise from bioaccumulation of the neurotoxin mercury in fish. Our results emphasize that bromine chemistry in the free troposphere oxidizes mercury at a faster rate, and makes water-soluble mercury available for scavenging by thunderstorms. Naturally occurring bromine in air aloft illustrates global interconnectedness between energy choices affecting mercury emissions in developing nations and mercury deposition in, e.g., Nevada, or the southeastern United States. Halogens in the troposphere are increasingly recognized as playing an important role for atmospheric chemistry, and possibly climate. Bromine and iodine react catalytically to destroy ozone (O3), oxidize mercury, and modify oxidative capacity that is relevant for the lifetime of greenhouse gases. Most of the tropospheric O3 and methane (CH4) loss occurs at tropical latitudes. Here we report simultaneous measurements of vertical profiles of bromine oxide (BrO) and iodine oxide (IO) in the tropical and subtropical free troposphere (10°N to 40°S), and show that these halogens are responsible for 34% of the column-integrated loss of tropospheric O3. The observed BrO concentrations increase strongly with altitude (∼3.4 pptv at 13.5 km), and are 2–4 times higher than predicted in the tropical free troposphere. BrO resembles model predictions more closely in stratospheric air. The largest model low bias is observed in the lower tropical transition layer (TTL) over the tropical eastern Pacific Ocean, and may reflect a missing inorganic bromine source supplying an additional 2.5–6.4 pptv total inorganic bromine (Bry), or model overestimated Bry wet scavenging. Our results highlight the importance of heterogeneous chemistry on ice clouds, and imply an additional Bry source from the debromination of sea salt residue in the lower TTL. The observed levels of bromine oxidize mercury up to 3.5 times faster than models predict, possibly increasing mercury deposition to the ocean. The halogen-catalyzed loss of tropospheric O3 needs to be considered when estimating past and future ozone radiative effects.

Convective transport of water vapor into the lower stratosphere observed during double‐tropopause events

We present in situ observations of convectively injected water vapor in the lower stratosphere from instruments aboard two aircraft operated during the Deep Convective Clouds and Chemistry experiment. Water vapor mixing ratios in the injected air are observed to be 60–225 ppmv at altitudes 1–2 km above the tropopause (350–370 K potential temperature), well above observed background mixing ratios of 5–10 ppmv in the lower stratosphere. Radar observations of the responsible convective systems show deep overshooting at altitudes up to 4 km above the lapse rate tropopause and above the flight ceilings of the aircraft. Backward trajectories from the in situ observations show that convectively injected water vapor is observed from three distinct types of systems: isolated convection, a convective line, and a leading line-trailing stratiform mesoscale convective system. Significant transport of additional tropospheric or boundary layer trace gases is observed only for the leading line-trailing stratiform case. In addition, all observations of convective injection are found to occur within large-scale double-tropopause events from poleward Rossby wave breaking. Based on this relationship, we present a hypothesis on the role of the large-scale lower stratosphere during convective overshooting. In particular, the reduced lower stratosphere stability associated with double-tropopause environments may facilitate deeper levels of overshooting and convective injection.

On the Susceptibility of Cold Tropical Cirrus to Ice Nuclei Abundance

AbstractNumerical simulations of cirrus formation in the tropical tropopause layer (TTL) during boreal wintertime are used to evaluate the impact of heterogeneous ice nuclei (IN) abundance on cold cloud microphysical properties and occurrence frequencies. The cirrus model includes homogeneous and heterogeneous ice nucleation, deposition growth/sublimation, and sedimentation. Reanalysis temperature and wind fields with high-frequency waves superimposed are used to force the simulations. The model results are constrained by comparison with in situ and satellite observations of TTL cirrus and relative humidity. Temperature variability driven by high-frequency waves has a dominant influence on TTL cirrus microphysical properties and occurrence frequencies, and inclusion of these waves is required to produce agreement between the simulated and observed abundance of TTL cirrus. With homogeneous freezing only and small-scale gravity waves included in the temperature curtains, the model produces excessive ice con...

UV Fiber-Laser-Based Fluorescence and Blue Diode-Laser-Based Phosphorescence-Lifetime Methods

Combination of a novel, tunable pulsed UV-fiber laser with a fluorescence-detection design using carbon nanofiber coatings and blue diode lasers with a novel lifetime-phosphorescence method has provided new turn-key instrumentation for monitoring applications.