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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 SO2at the tropical tropopause to inform this issue. Here we clarify the role of SO2in maintaining SAs by using new in situ SO2measurements to evaluate climate models and satellite retrievals. We then use the observed tropical tropopause SO2mixing ratios to estimate the global flux of SO2across the tropical tropopause. These analyses show that the tropopause background SO2is 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 SO2as a dominant source of SAs to a near-negligible one, possibly revealing a significant gap in the SA budget. First in situ measurements of SO2in the tropical UT/LSTypical SO2at the tropical tropopause is near 5-10?pptvFlux of SO2across the tropopause is a minor source of stratospheric aerosol

The efficiency of dehydration in the tropical tropopause layer (TTL) determines how closely water vapor will be reduced to the lowest saturation mixing ratio encountered along a trajectory to the stratosphere, thereby strongly influencing stratospheric humidity. The NASA Airborne Tropical Tropopause Experiment (ATTREX) provided an unprecedented number and quality of in situ observations to constrain the key mechanisms controlling this dehydration. Statistical analyses of the ATTREX data show that nucleation, growth, and sedimentation each result in TTL dehydration becoming increasingly inefficient at temperatures below 200 K. Because of these inefficiencies, models that ignore these mechanisms likely underestimate water vapor at the stratospheric entry point by ~10–20% at the lowest temperatures. New data set of TTL water and cirrus microphysical observationsIce nucleation, growth, and sedimentation are important limiting factors in dehydrationDehydration efficiency decreases with temperature below 200 K

During the 1997 Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission, simultaneous in situ observations of NOx and HOx radicals, their precursors, and the radiation field were obtained in the lower stratosphere. We use these observations to evaluate the primary mechanisms that control NOx−HNO3 exchange and to understand their control over the partitioning between NO2 and HNO3 in regions of continuous sunlight. We calculate NOx production (PNOx) and loss (LNOx) in a manner directly constrained by the in situ measurements and current rate constant recommendations, using approaches for representing albedo, overhead O3 and [OH] that reduce model uncertainty. We find a consistent discrepancy of 18% between modeled rates of NOx production and loss (LNOx = 1.18PNOx) which is within the measurement uncertainty of ±27%. The partitioning between NOx production processes is [HNO3 + OH (41 ± 2)%; HNO3 + hν (59 ± 2)%] and between NOx loss processes is [NO2 + OH, 90% to >97%; BrONO2 + H2O, 10% to <3%]. The steady-state description of NOx−HNO3 exchange reveals the significant influence of the tight correlation between the photolysis rate of HNO3 and [OH] established by in situ measurements throughout the lower stratosphere. Parametrizing this relationship, we find (1) the steady-state value of [NO2]24h-avg/[HNO3] in the continuously sunlit, lower stratosphere is a function only of temperature and number density, and (2) the partitioning of NOx production between HNO3 + OH and HNO3 + hν is nearly constant throughout most of the lower stratosphere. We describe a methodology (functions of latitude, day, temperature, and pressure) for accurately predicting the steady-state value of [NO2]24h-avg/[HNO3] and the partitioning of NOx production within these regions. The results establish a metric to compare observations of [NO2]24h-avg/[HNO3] within the continuously sunlit region and provide a simple diagnostic for evaluating the accuracy of models that attempt to describe the coupled NOx−HOx photochemistry in the lower stratosphere.

In situ observations of OH and HO2 from the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA), Stratospheric TRacers of Atmospheric Transport (STRAT), and Polar Ozone Loss in the Arctic Region in Summer (POLARIS) NASA ER-2 field campaigns are used to examine the partitioning of HOx in the lower stratosphere (tropopause to ~21 km) and upper troposphere (~10 km to tropopause). These measurements span a latitude range from 70°S to 90°N and a variety of atmospheric conditions as a result of seasonal changes and altitude. The response of the observed [HO2]/[OH] to changes in temperature, [O3], [CO], [NO], [ClO], and [BrO] is investigated. The measured ratio is accurately described (~±10%) by a steady-state model constrained by the measured mixing ratios of O3, CO, NO, ClO, and BrO, where the model is valid for conditions of HOx cycling much faster than HOx production and loss. The concentration of HO2 depends on [OH], which, to first order, has been observed to be a simple function of the solar zenith angle in the lower stratosphere.¹ The partitioning between OH and HO2 is controlled by the local chemistry between the HOx radicals and O3, CO, NO, ClO, and BrO. The response of [HOx] to changes in [NOx] and [O3] is demonstrated. Further observations are necessary to illustrate the response of HOx to changes in halogen concentrations. A quantitative understanding of [HO2]/[OH] is important, since many of the reactions that control this ratio are directly involved in catalytic removal of O3 in the lower stratosphere and production of O3 in the upper troposphere.

Extensive measurement campaigns by the NASA ER-2 research aircraft have obtained a nearly pole-to-pole database of the species that control HOx (OH + HO2) chemistry. The wide dynamic range of these in situ measurements provides an opportunity to demonstrate empirically the mechanisms that control the HOx system. Measurements in the lower stratosphere show a remarkably tight correlation of OH concentration with the solar zenith angle (SZA). This correlation is nearly invariant over latitudes ranging from 70° S to 90° N and all seasons. An analysis of the production and loss of HOx in terms of the rate determining steps of reaction sequences developed by Johnston and Podolske and Johnston and Kinnison is used to clarify the behavior of the system and to directly test our understanding of the system with observations. Calculations using in situ measurements show that the production rate of HOx is proportional to O3 and ultraviolet radiation flux. The loss rate is proportional to the concentration and the partitioning of NOy (reactive nitrogen) and the concentration of HO2. In the absence of heterogeneous reactions, the partitioning of NOy is controlled by O3 and HOx and the concentration of HO2 is controlled by NOy and O3, so that the removal rate of OH is buffered against changes in the correlation of O3 and NOy. The heterogeneous conversion of NO2 to HNO3 is not an important net source of HOx because production and removal sequences are nearly balanced. Changes in NOy partitioning resulting from heterogeneous chemistry have a large effect on the loss rates of HOx, but little or no impact on the measured abundance of OH. The enhanced loss rates at high NO2/HNO3 are offset in the data set examined here by enhanced production rates resulting from increased photolysis rates resulting from the decreased O3 column above the ER-2.

Earthquakes were induced in the Fort Worth Basin from 2008 through 2020 by increase in pore pressure from injection of oilfield wastewater (SWD). In this region and elsewhere, a missing link in understanding the mechanics of causation has been a lack of comprehensive models of pore pressure evolution (ΔPp) from SWD. We integrate detailed earthquake catalogs, ΔPp, and probabilistic fault slip potential (FSP) and find that faults near large‐scale SWD operations became unstable early, when ΔPp reached ∼0.31 MPa and FSP reached 0.24. Faults farther from SWD became unstable later, when FSP reached 0.17 and at much smaller ΔPp. Earthquake sequences reactivated with mean ΔPp of ∼0.05 MPa. The response of faults shows strong variability, with many remaining stable at higher ΔPp and few that became seismogenic at smaller changes. As ΔPp spread regionally, an ever‐increasing number of faults were impacted and the most sensitive became unstable. Increases in subsurface fluid pressure following years of deep disposal of oilfield wastewater are widely accepted to have caused earthquakes from 2008 through 2020 in the Fort Worth Basin (FWB) of north‐central Texas, underlying the population centers of the Dallas‐Fort Worth metropolitan area, were caused by increases in subsurface fluid pressure following years of deep disposal of oilfield wastewater. No earthquakes were reported in the area prior to 2008. Increasing the fluid pressure in subsurface reservoirs containing faults can cause the faults to become unstable and slip, producing earthquakes. A missing link to understanding the cause of these earthquakes is not knowing how much the fluid pressures have increased, and which geographic areas have been affected. This research combines several lines of evidence to show how the fluid pressure from deep injection increased over time and space leading to the increase in earthquakes. The changes in fluid pressure that made the faults unstable is surprisingly low, the subsurface region affected by the pressure change is large, and the natural sensitivity of the faults has significant variability. This work provides a critical component to understanding the specific conditions leading to induced seismicity in the FWB, which can be applied to similar cases elsewhere. Onset of induced earthquakes in the Fort Worth Basin occurred with pore pressure increases of ∼0.01–0.36 MPa from wastewater injectionPore pressure increase in the slip potential of the seismogenic faults indicates that the instability threshold is 17%–24% for this systemSlip potential of faults that became seismogenic is similar to those that have not ruptured, indicating inherent differences in sensitivity Onset of induced earthquakes in the Fort Worth Basin occurred with pore pressure increases of ∼0.01–0.36 MPa from wastewater injection Pore pressure increase in the slip potential of the seismogenic faults indicates that the instability threshold is 17%–24% for this system Slip potential of faults that became seismogenic is similar to those that have not ruptured, indicating inherent differences in sensitivity

While most black carbon (BC)‐containing particles are removed in the lower troposphere in the tropics, some are lofted to higher altitudes via convection where they may be distributed globally throughout the tropical tropopause layer (TTL). Single‐particle measurements of BC aerosol were made from the NASA WB‐57F aircraft during both the dry (February 2006) and wet (August 2007) seasons in Central America. BC mass loadings declined sharply with increasing altitude from the ground to 5 km. In the TTL, they were up to six times higher in the wet relative to the dry season. The variability in BC mass was examined using convective‐influence back trajectories to determine the source regions. The seasonal differences in the vertical profiles are explained by long‐range transport of (1) low‐BC air from the southern hemisphere in the dry season and (2) high‐BC air from biomass‐burning or pollution sources in Africa and Asia advected by the Asian monsoon circulation in the wet season. Tropical BC profiles from the ground to the lower stratosphere for two seasonsBC as a sensitive tracer of transportSeasonal variability of BC attributed to influence from the Asian monsoon

Observations of three stratospheric species (N2O, CH4, and NOy) are examined for their suitability as mixing tracers in the inner Arctic vortex region between late February and mid‐March 2000. NOyis highly inhomogeneous on isentropic surfaces and has little systematic vertical gradient between the 420 and 470 K potential temperature surfaces due to severe and extensive denitrification. Probability distribution functions of NOycalculated from in situ measurements do not show systematic and significant change as a function of time, indicating that mixing was too slow to substantially homogenize NOydistribution and therefore to strongly affect photochemical O3destruction rates during the period. We propose that the NOyinhomogeneity is useful for diagnosing small‐scale, irreversible mixing rates during the measurement period. A simple kinematic model is used to show that the NOystandard deviation on an isentropic level in the vortex has desirable properties for quantifying these rates. A practical method for deriving mixing rates for chemistry and transport models has been proposed for future studies.