This project is centered around 1) the development of Negative Ion Proton Transfer Mass Spectrometry (NI-PTRMS) for the detection of carboxylic acids in the atmosphere and 2) an improved understanding of the gas phase ion chemistry on which the method relies. NI-PTRMS involves selective ionization of acidic species via transfer of a proton to an acetate ion. Since acetic acid is the weakest of the common carboxylic acids, acetate ions will react with all gas-phase carboxylic acids, and with strong inorganic acids as well. Several schemes, such as the high-pressure radioactive ionization of selected starting materials (acetic esters or anhydrides, for example) will be explored and evaluated for their potential to produce an intense, cluster-free source of the desired ion. The relative gas-phase acidities of simple carboxylic acids (propionic, butyric, and others) and some isotopomers of acetate will be measured. Thermochemistry and reaction rates of the first hydrate of acetate ion with carboxylic acids will also be determined.
Broader scientific impacts include applications to air quality and climate change because carboxylic acids are key intermediates in the formation of both tropospheric ozone and aerosols. Education plans include participation by both undergraduate and graduate students from communities that are underrepresented in the atmospheric sciences.
Recent information suggests that on the global scale, biomass burning is much more extensive and widespread than previously thought. Biomass burning refers to the burning of the world's forests and grasslands and agricultural lands following the harvest for land clearing and land conversion. Combustion products of biomass burning include carbon dioxide, carbon monoxide, methane, nonmethane hydrocarbons, nitric oxide, nitrous oxide, organic acids, inorganic acids and atmospheric particulates and aerosols. One poorly understood, but significant class of Volatile organic compounds(VOC) present in biomass burning is gas-phase organic acids and inorganic. Understanding the chemistry of volatile organic compounds (VOC’s); which are key ingredients in the formation of ozone and aerosols play a significant role in determining regional air quality, and possibly the global carbon cycle, and will contribute to the understanding of a major social problem: Climate change. These acids are extremely difficult to measure because of their adsorptive nature. It is thought that as much as 90% of global biomass burning is human-initiated and that such burning is increasing with time. Hence, biomass burning may be an important driver for global atmospheric and climatic change. To get a complete chemical picture of the atmosphere it is necessary to study and quantify the impact of constituents that are present in dilute concentrations or undergo very weak chemical reactions. Work in our laboratory uses two methods: 1. Mass Spectrometric methods to characterize some of the properties of the organic acids: Our initial goal was to develop an instrument, method, and protocol for the sensitive and rapid measurement of gas-phase acidities of key carboxylic acids in the troposphere. Our improvement lies in coupling a novel flow reactor for the chemical reaction orthogonally to a reflectron time-of-flight (TOF) mass spectrometric technique for fast temporal resolution and superior mass spectral resolution. The system was eventually determined to not be sensitive enough to transport soft ions from the flowtube to the time-of-flight source but at extreme pressure conditions, ambient ions were being produced. We collaborated with NOAA-ESRL scientists to measure the Henry’s Law coefficient of isocynic acid HNCO, using an acetate anion CI-quadrupole MS instrument. Later efforts at NCA&T sought to duplicate this work and expand the measurements to aqueous phases of varying pH. The effort was unsuccessful; the S/N would have been too poor for HNCO+• (m/z 43) or its fragments to be detected. Part of the issue is that there was plenty of ion current, but it was taken up by the major constituents of air which, effectively, consume the majority of the EI electrons and leave little for HNCO to ionize. The interest in HCNO was a result of a recent study of biomass fires and some field measurements that revealed that isocyanic acid (HNCO) can reach levels as high as 600 ppbv near fires and up to 200 pptv in ambient air. After the solubility was measured for the first time, potential human health effects of HNCO were identified. HNCO is highly soluble in the human body at a physiological pH of 7.4, and there exists an increased risk for atherosclerosis, cataracts, and rheumatoid arthritis via protein carbamylation. Subsequent measurements of HNCO solubility and pH-dependant hydrolysis rates made it possible for HNCO atmospheric sensitivity and in-cloud lifetimes to be estimated using a numerical cloud box model. This was the basis of one of the PhD dessertations. 2. Cavity Ring Down Spectroscopy to measure absorption cross sections for overtone induced photochemistry. This vibrational overtone excitation of an O-H bond in organic acid molecules present in the atmosphere has been shown to cause dissociation of molecules leading to OH radical production for several species. OH radicals in the atmosphere are considered as detergents and clean up VOC’s. An experimental setup consisting of a Cavity Ring-Down Spectroscopy (CRDS) instrument and a Ultra-Violet Absorption Spectroscopy (UV-Abs) instrument was built, calibrated, tested and used to measure the fourth O-H overtone absorption cross sections of acetic acid and peracetic acid. The acetic acid work is the basis of MS thesis for a student. Two publications and 12 conference presentations (local and national), and a third publication under preparation resulted from this grant. The grant has contributed to the increase in the nation’s underrepresented STEM workforce, and the outreach efforts contributed to promoting careers in geosciences among high school students. Five graduate students, seven undergraduates and 4 female students were provided support and participated in the research funded thorough this grant. 1 PhD, 1 MS and 3 BS students completed their studies and produced a thesis, or senior project based on the results of this research. 60 high school students attended a weeklong summer camp supported through this grant. A high school teacher was provided a two year research experience through a supplemental (Research Experience for Teachers) RET funding in collaboration with the Kenan Fellows Program run by NCSU.
