Nitrogen oxides (NOx = NO+NO2) play a pivotal role in ground-level ozone formation, stratospheric ozone depletion, and acid deposition. Understanding atmospheric chemistry of reactive nitrogen species (NOY, where NOY is defined as the sum of NOx and the atmospheric oxidation products of NOx) is critical to pollution prevention and control efforts. While the homogeneous gas phase photochemistry of a large number of NOY has been studied, much less is known about the heterogeneous photochemistry of NOY species such as nitric acid (HNO3) and peroxynitric acid (HO2NO2). The photolysis of adsorbed HNO3 on ground surfaces has been proposed as a major daytime source of HONO in low-NOx environments. Little is known in particular about the UV absorption cross sections and the photolysis quantum yields of HNO3/H2O co-deposited on surface. The project seeks to characterize heterogeneous photolysis of surface-adsorbed HNO3 both in the absence and in the presence of water vapor, of HNO3/H2O co-deposited on surface, and of surface-adsorbed HO2NO2. Wavelength-dependent UV absorption cross sections of surface-adsorbed HNO3, HNO3/H2O, and HO2NO2 will be determined using Brewster-angle cavity ring-down spectroscopy. The photodissociation pathways and product yields from the heterogeneous photolysis of these species will be measured by combining laser photolysis either with Brewster angle cavity ring-down spectroscopy (fused silica surface) or with cavity ring-down spectroscopy (Al or ice film surface). The project will provide information essential to the assessment of the role of heterogeneous photolysis in converting NOx reservoirs into active forms and in producing tropospheric odd hydrogen radicals.
The project will directly lead to the research training of a graduate student and a postdoctoral scholar. In addition to presenting research findings at national and international conferences by the project director, postdoctoral scholar and graduate student, and publishing results in professional journals, the project director, Dr. Lei Zhu, plans to periodically give lectures in the local colleges and universities in the region to share the research results with scientists in the area, excite students' interests in science, and promote intellectual exchanges between different institutions. The Principal Investigator (PI) also plants to increase outreach efforts and to contribute to the goal of diversity and inclusiveness in university education. SUNY-Albany has existing outreach programs such as the Shepherd Project. The Shepherd Project has the goal of establishing long-term partnerships with faculty at historically minority colleges. The PI plans to become an active member in the Shepherd Program at SUNY-Albany.
Nitrogen oxides (NOx = NO+NO2) play a pivotal role in ground-level ozone and oxidant formation, stratospheric ozone depletion, and acid deposition. Understanding atmospheric chemistry of reactive nitrogen species (NOY, where NOY is defined as the sum of NOx and the atmospheric oxidation products of NOx) is critical to pollution prevention and control efforts. Field studies suggest that the photolysis of nitric acid (HNO3) on ground surfaces can convert this NOx reservoir into NOx and possibly HONO in the presence of free or adsorbed H2O molecules. To understand HNO3 surface photolysis and its potential role in photochemical HONO formation, my group has measured absorption cross sections of surface-adsorbed HNO3 in the 335-365 nm region by using Brewster angle cavity ring-down spectroscopy. The absorption cross sections for surface-adsorbed HNO3 in the 335-350 nm region are at least 3 orders of magnitude larger than those for HNO3 in the gas phase. We have also investigated the photolysis of surface-adsorbed HNO3 by combining laser photolysis with Brewster angle cavity ring-down spectroscopy. We have demonstrated that water vapor exhibits near UV absorption and determined water vapor near UV absorption cross sections using cavity ring-down spectroscopy. Through model simulation incorporating our H2O cross section data, we have shown that water vapor absorption in the 290-350 nm region can cause significant differences (up to 22% for standard US atmosphere) in direct beam and in diffuse radiation at the ground. Since both water vapor and ozone absorb near UV radiation while water vapor near UV absorption has not been considered in satellite ozone retrieval, water vapor near UV absorption will likely impact the accuracy of satellite tropospheric ozone measurement results. We also determined absorption cross sections of H2O adsorbed on fused silica surfaces as a function of wavelength in the near UV region and the heterogeneous nucleation of H2O on this surface, by exploring the application of Brewster angle cavity ring-down spectroscopy. The near UV absorption cross sections of adsorbed water are 4-5 orders of magnitude larger than those of liquid water. Thus, it is necessary to use near UV absorption cross section data obtained for adsorbed water to model the near UV optical properties of surfaces coated with water layers. In addition to single component spectroscopy and photolysis study in the gas phase and adsorbed on surfaces, we investigated the 308 nm photolysis of HNO3/H2O mixtures in the gas phase and adsorbed on surfaces. We have measured absorption of a laser probe beam by HNO3/H2O co-adsorbed on fused silica surfaces as a function of the mixture pressure in the 295-345 nm region. We have developed a method for calculating absorption by HNO3 and H2O co-deposited on the surface as a function of the HNO3/H2O mixture pressure using multi-component Langmuir adsorption isotherm, and absorption cross sections at a given wavelength for surface-adsorbed HNO3 and H2O. We have measured gas phase HNO3 photolysis product and temporal absorption profile in the 552-560 nm region, both in the absence and presence of H2O. Our study has provided information as to whether or not HNO3 photolysis in the gas phase or adsorbed on surfaces in the presence of H2O is a photochemical source of HONO. Besides intellectual merits outlined above, the completed NSF project has broader impacts. The project has directly lead to the research training of two postdoctoral scholars and three summer REU students, and has broadened the research participation of female post-docs and a female summer REU student. The NSF project has enhanced infrastructure for research through the establishment of collaborative partnerships to address important issues in atmospheric chemistry, and through the development of new, state-of-the-science techniques to study the photolysis of a monolayer of surface-adsorbed molecules in a time-resolved fashion, to determine mixture surface absorption and competitive adsorption, and to study molecular nucleation on surfaces. The findings from the NSF project were presented in conferences, and published in journals and on institutional website. The project has resulted in findings that will likely have transformative effect on atmospheric radiation, atmospheric circulation and climate change research. A quantitative understanding of the homogeneous and heterogeneous photolysis of HNO3 in the absence and presence of free or adsorbed H2O will yield information vital to the development of effective pollution prevention and control strategies to protect public health and the environment.
