Ozone dry deposition, an important tropospheric ozone sink, is expected to evolve with climate and land use, but coarse representation of ozone dry deposition in most large-scale atmospheric chemistry models hinders understanding of the influence of this sink on ozone air quality. Ozone uptake by stomata, injurious to plants, has been emphasized in the atmospheric chemistry community as the predominant terrestrial ozone depositional sink. However, there is an abundance of observational evidence from monitoring sites around the world suggesting that nonstomatal deposition processes (e.g., surface-mediated aqueous reactions on leaf cuticles, reactions with soil organic matter) are important ozone sinks. With observations from one of the longest ozone eddy covariance datasets available and a model hierarchy, I find substantial variations in nonstomatal ozone dry deposition, which is a non-negligible amount of the total ozone dry deposition, and identify the individual deposition processes driving observed variability. I pinpoint the responses of ozone pollution to changes in precursor emissions, climate, and ozone dry deposition at the beginning and end of the 21st century using a new version of the NOAA GFDL chemistry-climate model that more explicitly resolves deposition processes by leveraging the interactive biophysics of the land component of the model. My work highlights that under strong precursor emission controls there is a shift in the high ozone pollution season over northern mid-latitudes to a wintertime peak, and that wintertime ozone is sensitive to ozone dry deposition due to the long ozone lifetime. For both summer and winter, I find that neglecting variations in nonstomatal deposition and dependencies on environmental conditions may hinder accurate identification of the processes driving observed trends and variability in ozone pollution. In light of the sensitivity of ozone to dry deposition during winter, I identify a need for developing observational constraints on the wintertime depositional sink.

Photolysis of nitrate ion in the natural environment produces NO, NO2, and O3, releasing these toxic gases into the atmosphere. Work done by other groups has shown ionic strength dependence of the ratio of products from photolysis of aqueous nitrate ion. To better understand the kinetic mechanisms of nitrate photolysis, the effects that ionic strength in solution have on nitrate ion symmetry breaking are needed. Different solvation environments induce nitrate bonding motifs that may be correlated to the product ratio. Fourier-transform infrared spectra of aqueous nitrate–ion solutions were obtained over a range of temperatures for several total electrolyte concentrations. The electric fields (arising from water molecules and ions in solution) in aqueous potassium nitrate solution distort the trigonal planar shape of the nitrate ion, which may favor a specific initial path of the decomposition of nitrate during photolysis. Van’t Hoff plots of the relative peak areas corresponding to the formally-degenerate asymmetric stretching mode reveal the relative energies of the two solvation geometries. The difference in energy between the two geometries is linearly proportional to the ionic strength of the solution. Electronic structure calculations suggest that the more symmetric geometry has an increased stability relative to the less-symmetric geometry in high ionic strength solutions. Thus, the relative amounts of the nitrate ion solvation geometries can be correlated to the amount of products produced during photolysis to help explain the ionic-strength dependence of the product yields. Nitrate geometries at the water—CCl4 interface and aqueous carbonate ion bonding motifs are being investigated to identify pure-water effects. Preliminary results suggest that the more symmetric geometry nitrate is stabilized at the water—CCl4 interface and the lesssymmetric carbonate solvation geometry has an increased stability relative to the more symmetric geometry in high ionic strength solutions.

This treatise is a recapitulation of the theoretical and experimental study of hysteresis in atmospheric kinetics. The original problem arose from a theoretical study of a series of reactions for clean air. Upon evaluation a bistable equilibrium was predicted. The steady-state analysis had delineated a metastable region for the set of reactions. This bounded region is the hysteresis that this research project evaluated.

Formaldehyde (HCHO) plays a primary role in tropospheric chemistry. Its photochemical activity is an important source of radical species such as HCO, H, and subsequently HO2 as well as molecular hydrogen and carbon monoxide. As a source of hydrogen radicals (HOx = OH + HO2), HCHO plays a significant role in the oxidative capacity of the atmosphere, and an important part in the interrelated chemistries of ozone and the HOx and NOx (NO + NO2) cycles. Accurate atmospheric photolysis rates of HCHO are thus required in order to properly model tropospheric chemistry. Despite extensive studies HCHO’s spectroscopy and photochemistry remains to be well characterized. Absolute room temperature absorption cross sections for the A1A2 – X1A1 electronic transition of formaldehyde have been measured over the spectral range 30285 – 32890 cm-1 (304 – 330 nm) using ultraviolet (UV) laser absorption spectroscopy. Absorption cross sections were obtained at an instrumental resolution better than 0.09 cm-1 which is slightly broader than the Doppler width of a rotational line of HCHO at 300K (~0.07 cm-1) and so we were able to resolve all but the most closely spaced lines. Qualitative comparisons with spectral simulations show varying agreement depending on vibronic band. Refined state origins and transition dipole moments for each vibronic band have been reported. There is evidence of areas of perturbation and the need to optimize higher order spectral constants. Pressure broadening parameters have been measured and increase with the strength of intermolecular interaction between formaldehyde and the collision partner. Comparisons between the available high-resolution studies and spectral simulations indicate that the HCHO absorption cross section is still not well characterized. The relative quantum yield for the production of radical products, H+HCO, from the UV photolysis of formaldehyde (HCHO) has been measured directly using a Pulsed Laser Photolysis – Pulsed Laser Induced Fluorescence (PLP – PLIF) technique across the same spectral region. Relative yields were normalized to a value of 0.69 at 31750 cm-1 based on the current NASA-JPL recommendation. The resulting absolute radical quantum yields agree well with previous experimental studies and show more wavelength dependent behavior than the recommendation. This provides support for the complicated competition among the various HCHO dissociation pathways.

The regulated control of pollutants is essential to maintaining good air quality in urban areas. A major concern is the formation of tropospheric ozone, which can be especially harmful to those with lung conditions and has been linked to the occurrence of asthma. Ozone is formed through reactions of oxidized volatile organic compounds with nitrogen oxides, and the accurate modeling of the process is necessary for smart and effective regulations. Ambient measurements are important to understanding the mechanisms involved in tropospheric chemistry. This dissertation describes the characterization of a novel instrument for the ambient measurement of dinitrogen pentoxide, N2O5, and the results of several field studies. This is an important intermediate in the major nighttime loss pathway of nitrogen oxides. The understanding of this process requires correct modeling formation, as any nitrogen oxides not removed at night will result in increased ozone formation at sunrise. Calibration studies have been performed in order to quantify the loss of reactive species within the instrument, and the sampling flow and N2O5 detection have been well characterized. The results of the laboratory measurements are presented. Results are presented from the SHARP Field Study in Houston, TX in the spring of 2009. N2O5 measurements are compared to measurements of other species, including nitric acid and nitryl chloride, which were performed by other research groups. Mixing ratios exceeding 300 ppt were observed following ozone exceedance days, and a dependence of the concentration on both wind speed and direction was noticed. There was a strong correlation determined between N2O5 with HNO3 and ClNO2 indicating both a fast heterogeneous hydrolysis and N2O5 as the primary source of the species. Observed atmospheric lifetimes for N2O5 were short, ranging from several seconds to several minutes. We have also investigated the presence of N2O5 in College Station, TX. Low mixing ratios peaking at approximately 20 ppt were observed, with longer atmospheric lifetimes of up to several hours. The role of biogenic emissions in the NO3-N2O5 equilibrium is discussed.

Soot aerosols are known to be an important atmospheric constituent. The physical and chemical properties of soot allows it to act as a precursor of gas-surface heterogeneous reactions, providing active sites for the reduction and oxidation of trace species in the atmosphere, potentially affecting atmospheric composition. In this work the heterogeneous reaction of NO2 on soot leading to nitrous acid (HONO) formation was studied through a series of kinetic uptake experiments and HONO yield measurements. The soot was collected from a diffusion flame using propane and kerosene fuels using two different methods. A low pressure fast-flow reactor coupled to a Chemical Ionization Mass Spectrometer (CIMS) was used to monitor NO2 and HONO signals evolution using atmospheric-level NO2 concentration. HONO yields up to 100 percent were measured and NO2 uptake coefficients varying from 5.6x10-6 to 1.6x10-4 were obtained. Heating soot samples before exposure to NO2 increased HONO yield and the NO2 uptake coefficient on soot due to the removal of the organic fraction from the soot backbone unblocking active sites, which become accessible for the heterogeneous reaction. From the kinetic uptake curves and the effect observed in the HONO yield and NO2 uptake coefficient measurements by heating the soot samples, our results support a complex oxidation-reduction mechanism of reaction. This heterogeneous reaction mechanism involves a combination of competitive adsorptive and reductive centers on soot surface where NO2 is converted into HONO, and the presence of processes on soot where HONO can be decomposed producing other products. Atmospheric soot "aging" effect on the reactivity of soot toward NO2 and HONO yield was studied by coating the soot surface with glutaric acid, succinic acid, sulfuric acid, and pyrene. Glutaric and succinic acid increased both HONO yield and the NO2 uptake coefficients, while sulfuric acid decreased both. However, pyrene did not show any particular trend.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 155-168). Available also in a digital version from Dissertation Abstracts.

This dissertation investigates the impact of bromine on tropospheric ozone, OH, and mercury in the preindustrial and present-day atmosphere through use of modeling and observations from satellite. We developed bromine simulation capabilities coupled to oxidant-aerosol chemistry in the GEOS-Chem chemical transport model (CTM). Standard gas-phase mechanisms for bromine chemistry were unable to reproduce recent estimates of tropospheric BrO from satellite. Agreement was improved significantly after imposing HBr+HOBr heterogeneous chemistry in the model. Under present-day conditions, we find that bromine decreases ozone by 6.5%, < 1 – 8 ppb, and global mean OH by 4%. Most ozone loss is due to HOBr production and photolysis, with additional contributions from \(NO_x\) and ozone loss through \(BrNO_3\) hydrolysis. Simulations of the pre-industrial atmosphere are important as baselines for ozone air quality and radiative forcing calculations. However, standard models for the pre-industrial overestimate ozone observations taken a century ago at Montsouris and cannot reproduce the observed aseasonality. We find that bromine chemistry significantly improves this agreement. However, bromine chemistry has negligible impact on the ozone radiative forcing, as concentrations of BrO remain similar. Despite the small change in BrO concentrations, lower ozone in the preindustrial leads to a 40% greater Br mixing ratios. We estimate that this change may have increased the lifetime of atmospheric Hg(0) against oxidation to Hg(II) by 70% since the pre-industrial, making atmospheric mercury a more global pollutant. Additionally, we develop a retrieval algorithm for stratospheric profiles of BrO number density from SCIAMACHY limb near-UV observations. Zonal means of our BrO profile retrievals throughout April 2008 show common features expected from stratospheric photochemistry and dynamics. We apply simulated \([BrO]/[Br_y]\) ratios to the BrO profile retrievals and estimate a stratospheric loading of \(23.5 \pm 6 ppt Br_y\). This supports the 23 ppt stratospheric \(Br_y\) assumed in the satellite-derived climatology of tropospheric BrO that we used to evaluate our GEOS-Chem simulation. Our results imply \(7 \pm 6 ppt\) Br from short-lived bromocarbons, at the higher end of the 3 – 8 ppt range suggested by observations. / Engineering and Applied Sciences

This thesis explores the factors controlling the distribution of black carbon (BC) in the atmosphere/troposphere and its implications for climate forcing. BC is of great climate interest because of its warming potential. Estimates of BC climate forcing have large uncertainty, in part due to poor knowledge of the distribution of BC in the atmosphere. This dissertation first examines the factors controlling the sources of BC in the Arctic in winter and spring using a global chemical transport model (GEOS-Chem). Emission inventories of BC and wet scavenging of aerosols in the model are updated to reproduce observed atmospheric concentrations of BC as well as observed snow BC content in the Arctic in winter-spring. The simulation shows a dominant contribution of fuel (fossil fuel and biofuel) combustion to BC in Arctic spring. Arctic snow BC content is dominated by fuel combustion sources in winter, but has equal contributions from open fires and fuel combustion in spring. The estimated decrease in Arctic snow albedo due to BC deposition in spring is 0.6%, resulting in a regional surface radiative forcing of 1.2 W m-2. The dissertation then extends the evaluation of the BC simulation to the global scale using aircraft observations over source regions, continental outflow and remote regions and ground-based measurements. The observed low BC concentrations over the remote oceans imply more efficient BC removal than is currently implemented in models. The simulation that has total BC emissions of 6.5 Tg C a-1 and a mean tropospheric lifetime of 4.2 days for 2009 (vs. 6.8 &plusmn 1.8 days for the AeroCom models) captures the principal features of observed BC. The simulation estimates a global mean BC absorbing aerosol optical depth of 0.0017 and a top-of-atmosphere direct radiative forcing (DRF) of 0.19 W m-2, with a range of 0.17-0.31 W m-2 based on uncertainties in the BC atmospheric distribution. The DRF is lower than previous estimates, which could be biased high because of excessive BC concentrations over the oceans and in the free troposphere. / Engineering and Applied Sciences

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