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Thesis. 1975. M.S.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Bibliography: leaf 35. / M.S.

Quantitative information about halogenated hydrocarbons is important for understanding their impact on atmospheric ozone chemistry and climate change, their regulation, and the devising of improved substitutes. The Montreal Protocol aimed to regulate the utilization and manufacturing of hydrochlorofluorocarbon compounds (HCFCs), contributing to ozone layer depletion. The 2016 Kigali Amendment to the Montreal Protocol agreement, Annex C listed 274 HCFCs. Only 16 of them have been measured experimentally. The rest were set to zero by default. These reported global warming potentials (GWPs) play a crucial role in formulating policies for gradually reducing the usage and production of HCFCs to prevent atmospheric impact. Here we are studying 1-chloro-1-fluoro-ethane (CH3CHFCl) as a test of past theory. There are no prior experimental measurements of the reactivity of CH3CHFCl with hydroxyl (OH) radicals, which primarily determines its atmospheric lifetime, nor of its infrared (IR) spectrum. Saturated hydrofluorocarbons (HFCs) are non-ozone depleting substitutes for chlorofluorocarbons deprecated under the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer, but they exhibit high global warming potentials (GWPs) and the Kigali Amendment adopted in 2016 outlines their phase down. Unsaturated HFCs offer more reactive alternatives, whose likely short atmospheric lifetimes would imply small GWPs. Because their GWPs are smaller than those for saturated HFCs by several orders of magnitude, and especially for fully fluorinated examples, several halogenated olefins are under consideration for practical application. We studied HCF2CF2CF=CF2, cis-HCF=HCF, trans HCF=HCF, CF2=CH2 unsaturated HFCs.

Investigation of aerosol chemistry and growth under atmospheric conditions in a novel rotating aerosol suspension chamber with cavity ring-down spectroscopy provided key insight into the effect of pollutants and other vapors on the overall atmospheric lifetime of particulate matter. The Atmospheric Cloud Simulation Instrument (ACSI) creates a well-defined and controllable atmosphere of suspended particles, analyte gases, and background gas molecules, which remains stable up to several days. Preliminary studies have shown that monodisperse polystyrene latex (dp = 0.994 µm) and polydisperse ammonium sulfate (CMD dp = 100 nm) particles remain suspended for at least 22 hours while the chamber rotates at 2 RPM. Further investigation into the aerosol dynamics showed the coagulation efficiency of high concentration particle suspensions (>10^6 particles/cm3) depends on particle phase state and composition. The coagulation efficiency decreased with increased humidity in the model atmosphere and with increased ion concentrations in the aerosols. The decrease in efficiency is attributed to repulsive forces from like-charges on the particle surfaces. In addition to humidity, the spectroscopy integrated into the main chamber monitors the real-time response to a perturbation in the model atmosphere, such as the introduction of a gas-phase reactant. Cavity ring-down spectroscopy, performed in situ along the center axis, records mid-infrared spectra (1010 cm-1 to 860 cm-1) to identify gas species evolved from gas-particle heterogeneous chemistry. In accord with previous studies, my results show that a known reaction between monomethyl amine and ammonia occurs readily on suspended ammonium sulfate particles in >50% RH and the extent of the reaction depends on the humidity of the model atmosphere. Acidic ammonium bisulfate aerosols also produced a detectable amount of ammonia upon exposure to monomethyl amine in a model atmosphere with >50% RH. Overall, the new ACSI approach to atmospheric science provides the opportunity to study the influence of interfacial chemistry on particle growth, aging, and re-admission of gas-phase compounds. / Doctor of Philosophy / "Molecules don't have a passport." - Carl Sagan. Gas molecules and particles emitted into the atmosphere in one area can travel thousands of kilometers over the course of hours to days, even weeks for some compounds. The gas-solid interactions that occur over the lifetime of particulate matter are largely unknown. I focused my doctorate on bridging the knowledge gap between traditional environmental monitoring research and highly controlled laboratory experiments. To do so, I designed a new instrument capable of creating stable model atmospheres that more accurately simulate the gas-particle interactions in Earth's atmosphere than previous environmental chambers. The Atmospheric Cloud Simulation Instrument design included a rotating chamber to increase the duration of stable particle suspensions in a laboratory and a multi-pass infrared spectrometer to monitor gas-phase reactions in situ. I explored the effect of humidity and particle composition on particle-particle coagulation and gas-particle reactions. For example, liquid aerosols at humidities higher than 35% RH do no coagulate as fast as a solid particle with the same composition in <35% RH. Similarly, the same liquid aerosols produced more gaseous product during a heterogeneous reaction with a 'pollutant' gas than solid particles. Overall, the ACSI will be an important tool for future experiments exploring individual aspects of complex atmospheric processes.

<div>Halogen chemistry in the Arctic boundary layer catalytically destroys O3 and impacts the chemical lifetimes of hydrocarbons, the HOx-NOx cycle, and atmospheric mercury. While many advances have been made in the last several decades in understanding the sources, sinks, and recycling pathways of halogens in the Arctic there are still many unknowns. Previous studies have shown that Br2, BrCl and Cl2 are produced photochemically in the Arctic tundra snowpack, but the magnitude of this production is still poorly understood. Additionally, while there have been suggestions that the tundra snowpack should also produce I2, there have been no previous measurements of I2 in the Arctic. The lack of measurements of the halogen production capacity of Arctic snowpacks has left the community to rely on one-dimensional modeling to estimate the impact of snowpack-derived halogen chemistry on the Arctic atmosphere. Because modeling is inherently dependent on understanding recycling mechanisms, mixing processes, and sinks this leaves the effect of halogens on atmospheric chemistry in the Arctic highly uncertain.</div><div><br></div><div>This work describes efforts to address these uncertainties through measurements made during two field campaigns in Utqiaġvik (formerly Barrow), Alaska in January – February 2014, and February – May 2016. The first measurements of I2 in the Arctic, both in the snowpack interstitial air, and in the air above the snowpack demonstrate that iodine chemistry is active in the Arctic atmosphere, and that I2 is produced photochemically in the tundra snowpack. The effects of active iodine chemistry on both O3 and bromine chemistry is examined through zero- and one dimensional modeling. The first speciated measurements of snowpack phase iodine reveal that much like previous reports of iodine enriched aerosols, the Arctic snowpack is highly enriched in iodine. Vertical profiles of I- in the snowpack suggest that there is a consistent, non-radiation dependent source of iodine to the Arctic environment. It seems likely that this source is transport of iodine-enriched aerosols from the mid-latitudes. However, unlike the Antarctic, and previous</div><div>observations in the mid-latitudes, most Arctic snowpack phase iodine is inorganic, which may contradict transport from the mid-latitudes as a source. One-dimensional modeling was also utilized, in conjunction with the first vertical profile measurements of Br2 and Cl2 between 1 and</div><div>7 m above the snowpack surface to examine the community’s understanding of recycling mechanisms, mixing, sources, and sinks of halogens in the Arctic Atmosphere.</div>

The work presented in this thesis document reflects the results of a study carried out to better quantify the magnitude and fate of the anthropogenic air pollutants emitted from East Asia. Simulations of anthropogenic sulfur compounds by a regional on-line coupled chemistry-climate model suggest that large portions of East Asia have high SOx concentrations, and most subregions within East Asia are net exporters of SOx (SO2+SO4) (i.e. the anthropogenic S emissions from the region are greater than the deposition to the region). Among them, China is responsible for ~ 85% of the total emissions, and ~ 50 % of its total emitted SOx is exported to locations outside its borders. During the later winter to early spring when the continental outflow conditions predominate, about 20% of the total emitted SOx within the investigated area has been exported to North Pacific Ocean based on our model simulations. Those exported anthropogenic SOx from East Asia (mainly in the form of sulfate) is likely large enough to perturb the sulfate aerosol concentration over the North Pacific Ocean. Our investigation by integrating numerical simulations through a regional off-line full chemistry transport model, which is driven by the meteorological conditions calculated by a regional climate model, with field measurements of both gaseous and particulate species at a rural site adjacent to the largest industrialized area in China suggests that CO emissions from China, especially eastern China are likely underestimated by ~ 50 % in the current East Asia anthropogenic emission inventories. In addition, a 60-90 % underestimation of particulate carbonaceous emission in the inventories is suggested. Further statistical diagnoses, together with the back-trajectory analysis show that the missing CO sources are likely associated with SO2 sources that are already accounted for in the current inventories. This in turn suggests the emission factors of coal-combustors used in the current inventories are likely underestimated.

Global simulations of atmospheric methyl chloride (CH3Cl) are conducted using the GEOS-Chem model in order to understand better its sources and sinks. Though CH3Cl is one of the most abundant organic chlorine species in the stratosphere, not much is known about its sources and the budget remains unbalanced. In addition to the known sources (1.5 Tg yr-1) from ocean, biomass burning, incineration/industry, salt marshes, and wetlands, a hypothetical aseasonal biogenic source of 2.9 Tg yr-1 is added in order to match needed emissions. Observations from 7 surface sites and 8 aircraft field experiments are used to evaluate the model simulations. The model results with a priori emissions and sinks reproduce CH3Cl observations at northern mid and high latitudes reasonably well. However, the seasonal variation of CH3Cl at southern mid and high latitudes is severely overestimated. Simulated vertical profiles show disagreements in the vicinities of major sources, principally reflecting the uncertainties in the estimated distributions of our added pseudo-biogenic and the biomass burning sources. Inverse modeling is applied to obtain optimal source distributions of CH3Cl on the basis of surface and aircraft observations and model results. We resolve the seasonal dependence of the biogenic and biomass burning sources for each hemisphere. The aircraft in situ measurements are found to provide better constraints on the emission sources than surface measurements. The a posteriori emissions result in better agreement with the observations particularly at southern high latitudes. The a posteriori biogenic and biomass burning source decrease by 13 and 11% to 2500 and 545 Gg yr-1, respectively, while the a posteriori net ocean source increases by about a factor of 2 to 761 Gg yr-1. The decrease in biomass burning emissions is largely due to the reduction in the emissions in seasons other than spring in the northern hemisphere. The inversion results indicate that the biogenic source has a clear winter minimum in both hemispheres, likely reflecting the decrease of biogenic activity during that season.

Halogen-driven ozone and nonmethane hydrocarbon losses in springtime Arctic boundary layer are investigated using a regional chemical transport model (CTM). Surface observation of O3 at Alert and Barrow and aircraft observations of O3 and hydrocarbons during the TOPSE experiment from February to May in 2000 are analyzed. We prescribe halogen radical distributions based on GOME BrO observations and calculated or observed other halogen radical to BrO ratios. GOME BrO shows an apparent anti-correlation with surface temperature over high BrO regions. At its peak, area of simulated near-surface O3 depletions (O3 LT 20ppbv) covers GT 50% of the north high latitudes. Model simulated O3 losses are in agreement with surface and aircraft O3 observations. Simulation of halogen distributions are constrained using aircraft hydrocarbon measurements. We find the currently chemical mechanism overestimate the Cl/BrO ratios. The model can reproduce the observed halogen loss of NMHCs using the empirical Cl/BrO ratios. We find that the hydrocarbon loss is not as sensitive to the prescribed boundary layer height of halogen as that of O3, therefore producing a more robust measure for evaluating satellite column measurement. Tropospheric tracer transport and chemical oxidation processes are examined on the basis of the observations at northern mid-high latitudes and over the tropical Pacific and the corresponding global 3D CTM (GEOS-CHEM) simulations. The correlation between propane and ethane/propane ratio is employed using a finite mixing model to examine the mixing in addition to the OH oxidations. At northern mid-high latitudes the model agrees with the observations before March. The model appears to overestimate the transport from lower to middle latitudes and the horizontal transport and mixing at high latitudes in May. Over the tropical Pacific the model reproduces the observed two-branch slope values reflecting an underestimate of continental convective transport at northern mid-latitudes and an overestimate of latitudinal transport into the tropics. Inverse modeling using the subsets of observed and simulated data is more reliable by reducing (systematic) biases introduced by systematic model transport model transport errors. On the basis of this subset we find the model underestimates the emissions of ethane and propane by 14 5%.

<p>SO₂ and NO_x (NO+NO₂) are important trace gases in the atmosphere as they adversely affect air quality and are precursors to sulfate and nitrate aerosols in the atmosphere. However, there are significant uncertainties in the emission inventories and the atmospheric chemistry processes of both gases. Addressing these uncertainties will help us to 1) better regulate their emissions from anthropogenic activities, 2) understand the formation mechanism of aerosol pollution events, during which rapid accumulation of nitrate and sulfate aerosols are commonly observed, and 3) better constrain the impact of SO₂, NO_x, sulfate aerosols and nitrate aerosols to the global radiation balance. Stable isotopes of nitrogen and sulfur are useful tools in understanding both the origins and chemistry of SO₂ and NO_x since different emission sources usually display distinct sulfur and nitrogen isotopic compositions, and different SO₂ and NO_xoxidation pathways fractionate sulfur and nitrogen isotopes differently. In this dissertation, five studies are conducted to 1) use sulfur isotopes to investigate the sources and chemistry of atmospheric sulfur, and 2) improve our understanding of the isotopic fractionation processes associated with the atmospheric chemistry of reactive nitrogen. </p><p>Using stable sulfur isotopes, we first analyzed the sources of sulfate aerosols collected at Baring Head, New Zealand and atmospheric deposition at the Atacama Desert. At Baring Head, we found that the secondary sulfate, i.e., sulfate formed from atmospheric oxidation of SO₂, is mainly observed in fine aerosols (<1 µm) while the sulfate in coarse aerosols (>1 µm) is mostly sea salt sulfate. 73-77% of the secondary sulfate is sourced from biogenic emissions by ocean phytoplankton, and the rest is originated from anthropogenic activities. The sulfate deposition across the Atacama Desert, on the other hand, is a mixture of sea salt sulfate (only near the coast), anthropogenic SO₂ emissions, local soil, and lake salts. Then, sulfur isotopes were used to investigate the formation chemistry of sulfate aerosols collected during a strong winter haze episode in Nanjing, China, where the sources of SO₂ were well-understood. We found that, although the sources of sulfur remain unchanged during the haze episode, the sulfur isotopic compositions of sulfate vary significantly, suggesting isotopic fractionation occurred during the formation of sulfate aerosols. We interpreted the variation using a Rayleigh distillation model to evaluate the contribution of sulfate formation pathways. The model suggested that the Transition Metal Ion catalyzed O₂ oxidation pathway contributed 49±10% of the total sulfate production, while the O₃/H₂O₂ oxidations accounted for the rest. </p><p>Next, we conducted experiments in an atmospheric simulation chamber to determine the isotopic fractionations between NO and NO₂. This isotopic fractionation is controlled by a combination of two factors: 1) the equilibrium isotopic exchange between NO and NO₂ molecules, and 2) the kinetic isotope effects of the NO_x photochemical cycle, namely the Leighton Cycle Isotope Effect (LCIE). Our experiments showed that the fractionation factor during the isotopic exchange is 1.0289±0.0019, and the fractionation factor of LCIE is 0.990±0.005. A model was constructed to assess the relative importance of the two factors, showing the isotopic exchange should be the dominant factor when NO_x >20 ppb, while LCIE should be more important at low NO_x concentrations (<1 ppb) and high rates of NO₂photolysis. Last, we quantified the overall nitrogen isotopic fractionation during the formation of nitrate aerosols collected at Baring Head, New Zealand. Our results showed that significant and variable (0-15‰) isotopic fractionations occurred during the formation of nitrate aerosols. The isotopic fractionation factors are lower in the summer and higher in the winter, which is mainly caused by seasonal variations in nitrate formation pathways. </p><p>Overall, this dissertation first applied stable sulfur isotopes in aerosol samples collected in different environments, demonstrating that isotopes are excellent tools in identifying the origins and chemistry of atmospheric sulfur. Then, we investigated the isotopic fractionation processes during the atmospheric nitrogen chemistry, which can be useful for future studies aimed at understanding the origins and chemistry of atmospheric nitrogen using stable nitrogen isotopes.</p>

With rising atmospheric CO2 concentrations, a detailed understanding of processes that impact atmospheric CO2 fluxes is required. While a sink of atmospheric carbon from the continents to the ocean from carbonate mineral weathering is, to some degree, offset by carbonate mineral precipitation in the oceans, efforts are underway to make direct measurements of these fluxes. Measurement of the continental sink has two parts: 1) measurement of the dissolved inorganic carbon (DIC) flux leaving a river basin, and 2) partitioning the inorganic carbon flux between the amount removed from the atmosphere and the portion from the bedrock. This study attempted to improve methods to measure the DIC flux using existing data to estimate the DIC flux from carbonate weathering within the limestone karst region of south central Kentucky. The DIC flux from the Barren River drainage basin upstream from Bowling Green in southern Kentucky and northern Tennessee, and the upper Green River drainage basin, upstream from Greensburg, Kentucky, was measured, each for a year, using U.S.G.S. discharge data and water-chemistry data from municipal water plants. A value of the (DIC) flux, normalized by time and area of carbonate rock, of 4.29 g km-3 day-1 was obtained for the Barren River, and 4.95 kg km-3 for the Green. These compared favorably with data obtained by Osterhoudt (2014) from two nested basins in the upper Green River with values of 5.66 kg km-3 day-1 and 5.82 kg km-3 day-1 upstream from Greensburg and Munfordville, respectively. Additional normalization of the values obtained in this study by average precipitation minus evapotranspiration over the area of carbonate rock, or water available for carbonate dissolution, resulted in values of 5.61x107 g C (km3 H20)- 1 day-1 (grams of carbon per cubic kilometer of water, per day) for the Barren, and 7.43x107g C (km3 H20)-1 day-1 for the Green River. Furthermore, a statistical relationship between the total DIC flux and time-volume of water available for dissolution has been observed, yielding an r2 value of 0.9478. This relationship indicates that the primary variables affecting DIC flux for these drainage basins are time and the volume of water available for dissolution.