Photochemistry, Emissions, and Transport (PROPHET)

For over 20 years now, UMBS researchers have been studying atmospheric chemistry above the forest canopy to better understand the processes linked to ozone formation. Ozone is a key component of smog, and is both hazardous to human health and harmful to vegetation. Scientists have long known that forests both emit and absorb large quantities of various volatile organic compounds (VOC) smog precursors, (aka biogenic compounds, produced by living organisms) but they lack a full understanding of the “flux”, or dynamic flow of these chemicals in and out of the forest canopy, as well as an understanding of processes occurring between the forest floor and the canopy leading to these fluxes. Scientists also have an incomplete understanding of the way that these chemicals react with each other and other chemicals in the air to form smog.

Since 1997, UMBS has conducted its atmospheric chemistry research with its PROPHET program (Photochemistry, Emissions, and Transport) program, a one-of-a-kind consortium of individually-funded scientists whose mutual interests and varied experiences have created a series of vibrant collaborations. Scientists from all over the world are attracted to come together and do research here for a few key reasons: our forest is large and undisturbed and a typical northern temperate forest. It is in a pristine environment minimally impacted by industrial or vehicle pollution. Scientists have been working here for years and have developed useful historical databases containing long-term information about the chemical and physical composition of the atmosphere over the decades. Perhaps most significantly, the Station offers significant infrastructure for this research -- with a permanent scaffolding tower that stretches above the forest canopy that supports extensive vertical sampling capabilities from the top to the bottom of the forest flow. The infrastructure is flexible enough that researchers can design experiments to answer specific questions. UMBS is part of the AmeriFlux Network, the National Acid Deposition Program, and the USDA UVB monitoring network, which makes many additional continuous measurements of atmospheric parameters important to air chemistry work.

Work at UMBS has contributed a much deeper understanding of the dynamics of biogenic VOC emissions that allows for comparison of daytime and nocturnal emissions and decay, as well chemistry with nitrogen compounds and hydroxy radicals, of key importance to the dynamics of smog formation. One compound that has been a particular priority for air chemistry is isoprene – the most abundant non-methane hydrocarbon emitted from forests and other natural sources and one that dominates the photochemistry at the UMBS PROPHET site. Isoprene emissions are relatively high at UMBS, and NOx concentrations are relatively low, making UMBS an ideal site for studies of this important chemical in a clean environment. Like other volatile chemicals produced by the forest, isoprene concentrations vary spatially and diurnally in the forest. One of the longest records of isoprene flux in one location was measured at PROPHET. Biogenic VOC can also combine with other compounds in the atmosphere to form nanoscale particles that affect visibility and radiative forcing. Results from PROPHET can help us predict the impact of atmospheric processes on forest properties.

In the most recent intensive measurement campaign during the summer of 2016, one group of scientists studied the flux of a large suite of biogenic VOC compounds, including isoprene, moving between the forest canopy and the atmosphere to determine how many chemicals actually drive chemical reactions between the forest biosphere and surrounding atmosphere. They measured ecosystem-atmospheric fluxes and compared the results to predictions from a state of science chemical transport model developed to predict this exchange -- and determined the importance of chemicals not currently represented in this model. They found that a far smaller number of volatile organic chemicals contributed to the total flux of chemicals out of the forest than the number of VOCs responsible for the downward fluxes back in to the forest. What’s more, the largest errors in the model used to simulate overall forest-atmosphere flux of VOC were associated with known, rather than unknown chemical species. Although further research is needed to characterize the full forest-atmosphere VOC flux for other important ecosystems (as a function of environmental conditions, stress, etc.), this is a positive finding for current models in terms of their potential to adequately represent forest-atmosphere fluxes.