This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
This award supports acquisition of a state-of-the-art high-resolution proton-transfer time-of-flight mass spectrometer (PTR-ToF-MS) that will be used to further understanding of the atmospheric chemistry of biogenic volatile organic compounds (BVOCs). This instrument has the important advantage over the more common quadrupole PTR-MS of a much better mass resolution, which allows isobaric ions of different elemental composition to be separated. Research will include measurements of BVOC emissions from forests and study of BVOC reactions with atmospheric oxidants in the context of their ozone-forming potential and production of secondary organic aerosols as well as the susceptibility of BVOC emissions to changing biological and climatological factors. The studies will utilize a combination of field measurements and laboratory reaction chamber experiments. While many previous studies have looked at the formation of BVOCs, significant gaps in understanding still exist. In particular, chemical models currently underpredict the organic loading of atmospheric aerosols by factors of 4 to 8. It is hypothesized that this 'missing organic mass' consists of as-yet unidentified BVOC precursors originating from plant emissions. The PTR-ToF-MS will be used to search for and identify these compounds and quantify their emissions. Measurements will be made at Harvard Forest, which was established as a Long Term Ecological Research (LTER) site more than 20 years ago, and the Harvard environmental chamber.
The results will be of great importance in the context of both air quality and climate forcing. In addition, the instrument will be used in the context of a new consortium for studies of biogenic emissions and production of aerosols. Four New England institutions are proposed as initial components of a regional center: Amherst College, Harvard University, Boston College, and the University of Vermont. A focus of the center will be enhanced research opportunities for undergraduates. Activities will include monthly group meetings, summer and academic-year research opportunities that will focus on undergraduate students, social activities to enhance mentoring and networking, and biennial research symposia.
The goal of this project was to acquire a new Proton Transfer Reaction Time-of-Flight Mass Spectrometer (PTR-TOF-MS) to separate and measure individual BVOCs in air samples in real time and to use these measurements to further our understanding of the role of BVOCs in atmospheric chemistry. Biogenic volatile organic compounds (BVOCs) play a critical role in the chemistry of the lower atmosphere. These compounds fuel the oxidation cycle which, through interaction with anthropogenic pollutants such as NOx, determines whether oxidants, such as ozone, are consumed or produced. The oxidation process also transforms volatile primary BVOCs into more highly oxidized and less volatile products, which, through processes that are not well understood, form atmospheric particles composed of secondary organic material (SOM). The ozone and secondary organic material produced in the VOC oxidation cycle have profound implications for air quality and climate. Hence, the steps in this process, from the factors controlling the emission of biogenic VOCs, to the gas phase chemical reactions, to the SOM formation processes, are critical to understanding the ways in which natural and man-made emissions interact and ultimately mitigating the negative impacts of anthropogenic emissions on the environment. The research performed under this award has directly contributed to elucidating these processes. The rates of BVOCs emission from and uptake by a forest canopy in western Massachusetts (the Harvard Forest) were measured over the course of a full growing season. These measurements are being used to identify previously unknown emitted compounds, to constrain the processes (such as temperature, light and moisture levels, season, etc.) influencing emissions and deposition, and to predict exchange patterns at other locations or under future climate scenarios. One of the major BVOCs emitted by vegetation is isoprene (C5H10). Despite its central importance, our understanding of the atmospheric chemistry of isoprene is incomplete, particularly in pristine regions where isoprene emissions are high. We have performed laboratory experiments using an environmental chamber to simulate the reactions of isoprene with the atmospheric oxidant OH and to investigate the nature and quantities of the reaction products. We were able to show that under conditions where NOx is low, the compounds methyl vinyl ketone (MVK) and methacrolein (MACR) are produced in small quantities ((3.8 ± 1.3)% and (2.5 ± 0.9)%, respectively) from isoprene oxidation. These yields were previously poorly quantified, but are important because they are accompanied by re-generation of the atmospheric oxidants OH and HO2. Hence, these results represent a significant step forward in the ability to accurately describe isoprene oxidation chemistry and oxidant cycling under low-NOx conditions. Among the products of low-NOx isoprene oxidation are the isoprene epoxydiols (IEPOXs). These compounds were recently identified as potential contributors to the particulate material (SOM) produced when isoprene is oxidized. As part of this project, we have been able to determine that IEPOXs are the dominant contributor to the SOM mass generated under the reaction conditions of these experiments and to quantify that contribution. Differentiation between the oxidation products of a BVOC such as isoprene using mass spectrometric techniques can be challenging, because these products tend to have similar molecular formulas. We have developed new separation methods that can be used in conjunction with PTR-TOF-MS to aid in the identification of BVOCs in air samples. Application of these separation methods to the analysis of ambient air in anthropogenically influenced isoprene source regions is being used to test new hypotheses about the oxidation mechanism and products, leading to further improvements in our ability to model these systems. Together, the improvements in our knowledge of the atmospheric role of BVOCs gained from research performed under this award will help scientists and regulators formulate effective emission control guidelines for protecting the environment and human health.
