2.3 Chemical Measurements at the PROPHET Field Station in Michigan

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PROPHET Tower at the University of Michigan.

ACOM Scientists participated through the month of July 2016 in a field campaign in a forested area of Northern Michigan. The PROPHET-AMOS campaign – Program for Research on Oxidants; Photochemistry, Emissions and Transport - Atmospheric Measurements of Oxidants in Summer – took place at the PROPHET Lab and Tower at the University of Michigan Biological Station near Pellston, MI (shown in photo). The goal of the project was to study the cycling of chemicals between the biosphere and the atmosphere, and to understand the roles of various oxidants in the system. The project involved a comprehensive suite of measurements of ambient concentrations of biogenic organic compounds, their oxidation products, and various oxidants (O3, OH, HO2 and peroxy radicals RO2). Approximately 20 research groups participated; ACOM’s team (Geoff Tyndall, John Ortega, Deedee Montzka, and Andy Weinheimer) measured concentrations of NO and NO2 from a manifold running from the top of the 100-ft tall tower down to the lab. Other instruments either sampled from the manifold or, in the case of measurements of highly reactive (or low-volatility) compounds, were positioned directly on the tower to avoid delays between sampling and measurement. Rates of emission of a number of compounds were also measured by eddy correlation and gradient techniques, as well as by direct measurements employing enclosures placed over branches and over the soil.

The forest consists of a mixture of broad leaf and coniferous trees, most of a similar age and height. The chemistry was dominated by isoprene emissions, and up to 90% of the OH reactivity was due to isoprene. The ambient NOx concentrations were found to be very low, and were probably usually dominated by local soil emissions. A typical pattern showed NO2 building up during the night, then dropping after daybreak, as the boundary layer rose. NO, which was typically zero at night, was produced in the early morning from photolysis of NO2, but also decreased by late morning as the NO2 was vented. Two full days of NO2 and NO data are shown in the Figure.

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Figure 1. NO2 and NO data from PROPHET field campaign.

A number of the measurements were made more challenging by the low concentrations. One of the project organizers, Steve Bertman (Western Michigan University) noted that since such campaigns began in 1998, the levels of anthropogenic pollutants at the site have dropped noticeably. Under the low concentrations of NO encountered, peroxy radicals react at similar rates with NO and HO2, making for a rich suite of product molecules including nitrates and peroxides. As anthropogenic NOx levels continue to decline as a result of tighter emissions controls, more and more of the continental US will be in such a transition region between NOx-dominated and NOx-limited chemistry. The results from the PROPHET study will provide interesting insights into the fascinating and vigorous chemistry occurring in this setting.