It is the future of the stratospheric ozone layer, which protects life at Earth’s surface from harmful ultraviolet (UV) radiation, that is the focus of the present work. Fundamental changes in the composition and structure of the stratosphere in response to anthropogenic climate forcing may lead to catastrophic ozone loss under current, and even reduced, stratospheric halogen loading. In particular, the evolution toward a colder, wetter stratosphere, threatens to enhance the heterogeneous conversion of inorganic halogen from its reservoir species to its catalytically active forms, and thus promote in situ ozone loss. Water vapor concentrations control the availability of reactive surface area, which facilitates heterogeneous chemistry. Furthermore, the rates of the key heterogeneous processes are tightly controlled by the ambient humidity. Thus, credible predictions of UV dosage require a quantitative understanding of both the sensitivity of these chemical mechanisms to water vapor concentrations, and an elucidation of the processes controlling stratospheric water vapor concentrations. Toward this end, we present a set of four case studies utilizing high resolution in situ data acquired aboard NASA aircraft during upper atmospheric research missions over the past two decades. 1) We examine the broad scale humidity structure of the upper troposphere and lower stratosphere from the midlatitudes to the tropics, focusing on cirrus formation and dehydration at the cold-point tropical tropopause. The data show evidence for frequent supersaturation in clear air, and sustained supersaturation in the presence of cirrus. These results challenge the strict thermal control of the tropical tropopause. 2) We investigate the likelihood of cirrus-initiated activation of chlorine in the midlatitude lower stratosphere. At midlatitudes the transition from conditions near saturation below the local tropopause to undersaturated air above greatly reduces the probability of heterogeneous activation and in situ ozone loss in this region. 3) We probe the details of heterogeneous processing in the wintertime Arctic vortex, and find that in situ measurements of OH provide incontrovertible evidence for the heterogeneous reaction of HOCl with HCl. This reaction is critical to sustaining catalytically active chlorine and prolonging ozone loss in the springtime vortex. 4) We revisit the topic of midlatitude ozone loss with an emphasis upon the response of ozone in this region to changes in the chemical composition and thermal structure of the lower stratosphere induced by anthropogenic climate change. Specifically, we show evidence for episodic moisture plumes in the overworld stratosphere generated by the rapid evaporation of ice injected into this region by deep convection, and find that these high water vapor plumes have the potential to alter the humidity of the lower stratosphere, and drastically increase the rate of heterogeneous chemistry and in situ ozone loss, given sufficient reactive surface. / Earth and Planetary Sciences

Stratospheric water vapor plays an important role in the Earth system, both through its role in stratospheric ozone destruction and as a greenhouse gas contributing to radiative forcing of the climate. Highly accurate water vapor measurements are critical to understanding how stratospheric water vapor concentrations will respond to a changing climate. However, the past disagreement among water vapor instruments on the order of 1 – 2 ppmv hinders understanding of the mechanisms which control stratospheric humidity, and the reliable detection of water vapor trends. In response to these issues, we present a new dual axis water vapor instrument that combines the heritage Harvard Lyman-\(\alpha\) hygrometer with the newly developed Harvard Herriott Hygrometer (HHH). The Lyman-\(\alpha\) instrument utilizes ultraviolet photo-fragment fluorescence detection, and its accuracy has been demonstrated though rigorous laboratory calibrations and in situ diagnostic procedures. HHH employs a tunable diode near-IR laser to measure water vapor via direct absorption in a Herriott cell; it demonstrated in-flight precision of 0.1 ppmv (1-sec) with accuracy of 5%±0.5 ppmv. We describe these two measurement techniques in detail along with our methodology for calibration and details of the measurement uncertainties. We also examine the recent flight comparison of the two instruments with several other in situ hygrometers during the 2011 MACPEX campaign, in which five independent instruments agreed to within 0.7 ppmv, a significant improvement over past comparisons. Water vapor measurements in combination with simultaneous in situ measurements of \(O_3\), CO, \(CO_2\), HDO, and HCl are also used to investigate transport in the Tropical Tropopause Layer (TTL). Data from the winter 2006 CR-AVE campaign and the summer 2007 TC4 campaign are analyzed in a one-dimensional mixing model to explore the seasonal importance of transport within the TTL via slow upward ascent, convective injection, and isentropic transport from the midlatitude stratosphere. The model shows transport from midlatitudes to be significant in summer and winter, affecting ozone concentrations and therefore the radiative balance of the TTL. It also shows significant convective influence up to 420 K potential temperature in both seasons, which appreciably increases the amount of water vapor above the tropopause. / Engineering and Applied Sciences

This dissertation investigates the impact of 2000-2050 climate change on fine particulate matter \((PM_{2.5})\) air quality. We first applied a multiple linear regression model to study the correlations of total \(PM_{2.5}\) and its components with meteorological variables using the past decadal \(PM_{2.5}\) observations over the contiguous US. We find that daily variation in meteorology can explain up to 50% of \(PM_{2.5}\) variability. Temperature is positively correlated with sulfate and organic carbon (OC) almost everywhere. The correlation of nitrate with temperature is negative in the Southeast but positive in California and the Great Plains. Relative humidity (RH) is positively correlated with sulfate and nitrate, but negatively with OC. Precipitation is strongly negatively correlated with all \(PM_{2.5}\) components. We then compared the observed correlations of \(PM_{2.5}\) with meteorological variables with results from the GEOS-Chem chemical transport model. The results indicate that most of the correlations of \(PM_{2.5}\) with temperature and RH do not arise from direct dependence but from covariation with synoptic transport. We applied principal component analysis and regression to identify the dominant meteorological modes controlling \(PM_{2.5}\) variability, and showed that 20-40% of the observed \(PM_{2.5}\) daily variability can be explained by a single dominant meteorological mode: cold frontal passages in the eastern US and maritime inflow in the West. From 1999-2010 observations we further showed that interannual variability of annual mean \(PM_{2.5}\) in most of the US is strongly correlated with the synoptic period T of the dominant meteorological mode as diagnosed from a spectral-autoregressive analysis. We then used the observed local \(PM_{2.5}\)-to-period sensitivity to project \(PM_{2.5}\) changes from the 2000-2050 changes in T simulated by fifteen IPCC AR4 GCMs following the SRES A1B scenario. We project a likely increase of \(\sim 0.1 \mu g m^{-3}\) in annual mean \(PM_{2.5}\) in the eastern US arising from less frequent frontal ventilation, and a likely decrease of \(\sim 0.3 \mu g m^{-3}\) in the northwestern US due to more frequent maritime inflows. These circulation-driven changes are relatively small, representing only a minor climate penalty or benefit for \(PM_{2.5}\) regulatory purpose. / Engineering and Applied Sciences

The atmospheric concentration of methane \((CH_4)\) - the most significant non-\(CO_2\) anthropogenic long-lived greenhouse gas - stabilized between 1999 and 2006 and then began to rise again. Explanations for this behavior differ but studies agree that more measurements and better modeling are needed to reliably explain the model-data discrepancies and predict future change. This dissertation focuses on measurements of \(CH_4\) and inverse modeling of atmospheric \(CH_4\) fluxes using field measurements at a variety of spatial scales. We first present a new fast-response instrument to measure the isotopic composition of \(CH_4\) in ambient air. The instrument was used to characterize mass fluxes and isofluxes (a isotopically-weighted mass flux) from a well-studied research fen in New Hampshire. Eddy-covariance and automatic chamber techniques produced consistent estimates of both the \(CH_4\) fluxes and their isotopic composition at sub-hourly resolution. We then characterize fluxes of \(CH_4\) from aircraft engines using measurements made with the same instrument during the Alternative Aviation Fuel Experiment (AAFEX), a study that aimed to determine the atmospheric impacts of alternative fuel use in the growing aviation industry. Emissions of \(CO_2\), \(CH_4\), and \(N_2O\) from different synthetic fuels were statistically indistinguishable from those of the widely used JP-8 jet fuel. We then present airborne observations of the long-lived greenhouse gas suite – \(CO_2\), \(CH_4\), \(N_2O\), and CO – during two aircraft campaigns, HIPPO and CalNex, made using a similar instrument built specifically for the NCAR HIAPER GV aircraft. These measurements are compared to data from other onboard sensors and show excellent agreement. We discuss the details of the end-to-end calibration procedures and the data quality-assurance and quality-control (QA/QC). Lastly, we quantify a top-down estimate of California’s \(CH_4\) emission inventory using the CalNex \(CH_4\) observations. Observed \(CH_4\) enhancements above background concentrations are simulated using a lagrangian transport model driven by validated meteorology. A priori source-specific emission inventories are optimized in a Bayesian inversion framework to show that California’s \(CH_4\) budget is 1.6 ± 0.34 times larger than the current estimate of California’s Air Resources Board (CARB), the body charged with enforcing the California Global Solutions Act and tracking emission changes over time. Findings highlight large underestimates of emissions from cattle and natural gas infrastructure. / Earth and Planetary Sciences

This thesis explores the natural forces controlling variability of the tropospheric oxidants on interannual to glacial-interglacial time scales. The oxidants (primarily OH and ozone) determine the lifetime of many trace gases of human interest, including air pollutants and long-lived greenhouse gases such as methane. The oxidants respond to meteorological conditions, precursor emissions (natural and anthropogenic), and surface and overhead stratospheric boundary conditions, all of which have changed since the Last Glacial Maximum (LGM; ~21ka). This dissertation first examines in mechanistic detail the effect of variability in the lightning source of nitrogen oxides \((NO_x)\) precursors on interannual variability (IAV) of the oxidants in the recent past. An optimized technique is presented to constrain the lightning \(NO_x\) source in the GEOS-Chem global chemical transport model (CTM) to time-varying satellite data from the Lightning Imaging Sensor. This constraint improves the ability of the CTM to reproduce observed IAV in 9-year (1998-2006) hindcasts of tropical ozone and OH. IAV in ozone and OH is more sensitive to lightning than to biomass burning, despite greater IAV in \(NO_x\) from the latter source. The sensitivity of OH to lightning reflects positive chemical feedbacks on ozone production, \(HO_x\) recycling, and loss frequencies. This dissertation next introduces an offline-coupled climate-biosphere-chemistry framework for determining oxidant levels at and since the LGM. Detailed simulations of tropospheric composition are performed by GEOS-Chem driven by meteorological fields from the GISS ModelE general circulation model, land cover from the BIOME4-TG global terrestrial equilibrium vegetation model, and fire emissions from the LMfire model. Time slice simulations are presented for the present day, preindustrial, and two different possible representations of the LGM climate. Sensitivity of the results to uncertainty in lightning and biomass burning emissions is tested. Though well-buffered, all simulations find net reduced oxidative capacities relative to the present day. The most important parameters for controlling tropospheric oxidants over glacial-interglacial periods are changes in overhead ozone, tropospheric \(H_2O\), and lightning. The results are discussed in the context of the ice-core record, particularly for methane. / Engineering and Applied Sciences

Ozone in surface air is the primary cause of polluted air in the United States. The current ozone observing network is insufficient either to assess air quality or to fully inform our understanding of the factors controlling tropospheric ozone. This thesis investigates the benefit of an instrument in geostationary orbit for observing near surface ozone using Observing System Simulation Experiments (OSSEs). / Earth and Planetary Sciences

Methane is the second most influential anthropogenic greenhouse gas. There are large uncertainties in the magnitudes and trends of methane emissions from different source types and source regions. Satellite observations of methane offer dense spatial coverage unachievable by suborbital observations. This thesis evaluates the capabilities of using satellite observations of atmospheric methane to provide high-resolution constraints on continental scale methane emissions. In doing so, I seek to evaluate the supporting role of suborbital observations, to inform the emission inventories on which policy decisions are based, and to enable inverse modeling of the next generation of satellite observations. / Earth and Planetary Sciences

Mercury (Hg) is a potent neurotoxin, has both natural and anthropogenic sources to the environment, and is globally dispersed. Humans have been using Hg since antiquity and continue its use in large quantities, mobilizing Hg from stable long-lived geologic reservoirs to actively cycling surface terrestrial and aquatic ecosystems. Human activities, such as mining and coal combustion, have perturbed the natural biogeochemical cycle of Hg. However, the distribution of natural versus anthropogenic Hg in the environment today and the extent of anthropogenic perturbation (i.e., enrichment) are uncertain. Previous model estimates of anthropogenic enrichment have been limited by a lack of information about historical emissions, examined only near-term effects, or have not accounted for the full coupling between biogeochemical reservoirs. Presented here is a framework that integrates recently available historical emission inventories and overcomes these barriers, providing an improved quantitative understanding of global Hg cycling. / Earth and Planetary Sciences

Isoprene emissions affect human health, air quality, and the oxidative capacity of the atmosphere. Globally anthropogenic non-methane volatile organic compounds (NMVOC) emissions are lower than that of isoprene, but local hotspots are hazardous to human health and air quality. In Africa the tropics are a large source of isoprene, while Nigeria appears as a large contributor to regional anthropogenic NMVOC emissions. I make extensive use of space-based formaldehyde (HCHO) observations from the Ozone Monitoring Instrument (OMI) and the chemical transport model (CTM) GEOS-Chem to estimate and examine seasonality of isoprene emissions across Africa, and identify sources and air quality consequences of anthropogenic NMVOC emissions in Nigeria. / Earth and Planetary Sciences

Knowledge of NOx (NO2 + NO) emissions is useful to understand processes affecting air quality and climate change. Emission inventories of surface NOx have high uncertainties. Satellite remote sensing has enabled measurements of trace gases in the atmosphere over a large regional and temporal scale. Inverse modeling of NO2 observations from satellites can be used to improve existing emissions inventories. This study seeks to understand the difference in two methods of inverse modeling: the mass balance approach and the adjoint approach using the GEOS-Chem chemical transport model and its adjoint. Using both synthetic satellite observations and those derived from the SCIAMACHY satellite instrument, this paper found that the performance of these two inversions was affected by pixel smearing and observational error. Smearing reduced the accuracy of the mass balance approach, while high observational error reduced the accuracy of the adjoint approach. However, both approaches improved the a priori emissions estimate.