The production and loss mechanisms, and the magnitude of nitrous acid (HONO) fluxes to the atmosphere, will be investigated by constructing improved instrumentation capable of fast response measurements suitable for eddy-covariance flux, and by deploying the new sensor to field test it by making detailed measurements of HONO concentrations and fluxes and their covariance with other nitrogen oxides, oxidants, and environmental conditions. Photolysis of nitrous acid (HONO) is a major source of OH radicals in the boundary layer but poorly understood. The PI's will (1) construct and test a 2-channel Quantum-Cascade laser absorption spectrometer (QCLAS) capable of high frequency measurement of HONO and NO2, (2) measure HONO, NO, NO2, and NOy at a suburban site influenced by vehicle emissions, and above a forest canopy at a rural site, and (3) interpret the concentration and flux observations within a photochemical model framework, based on GEOS-Chem, to provide new insights into the processes controlling HONO in the boundary layer.
These research activities will enhance existing research infrastructure by providing instrument development and supporting research partnerships. The instrument addresses a critical societal need for a reliable fast-response instrument to measure HONO in continuous operation to assess pollution impacts. The HONO-NO2 QCLAS will enable participation in a variety of future field measurement campaigns to examine HONO production and loss. This work will directly support a graduate-student thesis project and will serve as a research topic for two undergraduate students and a high school teacher at the Harvard Forest LTER as part of Research Experience for Undergraduates (REU) program and Research Experience for Teachers (RET) summer programs.
This project's goal was to provide better understanding of how much nitrous acid (HONO) is present in the atmosphere and where it comes from. Because HONO splits apart in sunlight to yield a hydroxyl radical (OH), one of a handful of potent species that initiates and maintains chemical reactions in the atmosphere, understanding all of its sources is crucial. Previous studies have indicated higher levels of HONO in the atmosphere than can be explained by known chemical reactions. If these results are correct there is an additional source of HONO associated with sunlight and surfaces such as the forest canopy. However there is some uncertainty about the previous results because most of the measurements relied on extracting HONO from the air into a liquid, which could be affected by interferences. Our approach was to build a laser-based instrument (known as a differential absorption spectrometer) that could identify HONO based on the unique spectrum of light absorbed by an air sample and that minimized handling of the air so chances of interference were reduced. After building and extensively testing the new instrument we installed it at Harvard Forest, a research tower in a typical New England deciduous forest. Much to our surprise we did not observe excess HONO as has been reported for other locations. We observed HONO concentrations that were higher at night when it is not being destroyed by light and decreasing in the day as expected for the known balance between photodestruction and creation of HONO by combining NO and OH. The reason for the discrepancy between our results and other observations is still unclear, but sensitivity to interfering species in the other techniques is a strong possibility. Alternatively Harvard Forest is different and the reactions that produce HONO are not important there. As a next step direct spectrometer measurements need to be repeated at sites where excess HONO has been observed before. Moreover, the nighttime reaction involving nitrogen dioxide (NO2), which is presumed to be the main source of HONO formed at night, does not seem to explain the observed HONO concentrations. As part of the instrument testing before we made measurements over the forest we took it totwo other sites with very different conditions. The first field test was an experiment to quantify HONO in jet engine exhaust as a function of engine power, from idle to full-throttle conditions. Results from this study will allow modelers to compute the impact of emissions on air quality near the Earth's surface and aloft where emissions can have an effect on the stratospheric ozone levels. For the second field campaign we participated in a large air pollution study in Houston, TX. In the Houston study several other instruments participated that were also measuring HONO. An informal intercomparison among the methods showed quite good agreement for this polluted urban air, helping to confirm that our spectroscopic approach was valid. Using observations from this study we conclude that the frequent assumption that HONO chemistry is at "steady state" is not correct close to emission sources such as highways. Assuming that HONO is at steady state leads to the conclusion that there are additional reactions generating HONO, but accounting for HONO present in the exhaust initial production as fresh exhaust mixes with he surrounding air followed by slow decay shows that HONO is only being produced by known reactions. This distinction is important for understanding the chemistry of urban smog; the unknown reactions of HONO lead to formation of new OH when the HONO decomposes in the light, but the HONO produced in fresh exhaust just gives ack the original OH when it decomposes. Measuring HONO is difficult because it is a relatively "sticky" and eactive gas that is easy to lose on surfaces or generate from other gases in the air. It also has low concentrations that require the very best signal processing to accurately convert the intensity of light reaching the detector to a HONO oncentration. The advances in sample handling and signal processing that were required to develop this instrument ill be useful for improving instruments to measure other reactive gases that have very low atmospheric oncentrations. This work served as a thesis project for Harvard University graduate student, Ben Lee. The student learned valuable skills in instrument development through a collaborative partnership iwth Dr. Bill Munger at Harvard University. Development of the HONO spectrometer drove software updates and technology advances that will benefit instruments for many other trace gases. In addition, the trainee was able to make connections with established as well as young scientists through these field experiments that are so important in atmospheric science.
