Isoprene Photochemistry over the Amazon Rainforest

Martin, S. T., Harvard University

Aerosol Properties

Aerosol Processes

Liu Y, J Brito, MR Dorris, JC Rivera-Rios, R Seco, KH Bates, P Artaxo, S Duvoisin JR, FN Keutsch, S Kim, AH Goldstein, AB Guenther, AO Manzi, RA Souza, SR Springston, TB Watson, KA McKinney, and ST Martin. 2016. "Isoprene photochemistry over the Amazon rainforest." Proceedings of the National Academy of Sciences, 113(22), 10.1073/pnas.1524136113.


An image from the G-1 aircraft shows Manaus in the foreground and the Amazon River in the background. The prevailing trade winds carry the plume of urban pollution across the river toward the ARM Mobile Facility site. Courtesy Jason Tomlinson, ARM Aerial Facility.


An image from the G-1 aircraft shows Manaus in the foreground and the Amazon River in the background. The prevailing trade winds carry the plume of urban pollution across the river toward the ARM Mobile Facility site. Courtesy Jason Tomlinson, ARM Aerial Facility.

Science

Isoprene is a biogenic volatile organic compound that, when oxidized by sunlight, heavily influences atmospheric chemistry over forested areas. Using field measurements from the ground and the air, this paper tracked the fate of isoprene over the central Amazon for eight weeks during the wet season. The main research site, 70 kilometers downwind of the Manaus, Brazil, urban region, intercepted both background and polluted air masses during the GoAmazon2014/15 experiment. The paper points out how extremely susceptible the natural functioning of Amazonia is to increased emissions of pollutants, especially as related to climate.

Impact

In response to sunlight, isoprene reacts with hydroxyl radicals (OH) and molecular oxygen to produce isoprene peroxy radicals. For isolated regions of the planet, where human influences are scant, these radicals have been thought to follow hydroperoxyl (HO2) pathways typical of unpolluted regions rather than nitric oxide (NO) pathways typical of polluted regions. But until now, observational evidence has been scant in the tropics, and largely limited to Earth’s more temperate regions. The paper’s findings contribute to a more accurate understanding of the relative roles of the two reaction pathways, which is needed for better quantitative predictions of the concentrations of particulate matter, oxidation capacity, and consequent environmental and climate impacts. This paper begins to clear up uncertainties about these isoprene reaction pathways. That’s important for improving our understanding of how chemicals react in the atmosphere and affect climate simulations.

Summary

Isoprene accounts for approximately half of the global flux of non-methane biogenic volatile organic compounds to the atmosphere. The reactive chemistry of isoprene influences the oxidative capacity of the troposphere and the associated chemical cycles of atmospheric trace gases. Isoprene photooxidation products are also important sources of atmospheric organic particulate matter. All this means that accurate ambient measurements of the molecular identities and concentrations of isoprene oxidation products are a first-order requirement for testing models used to assess how different amounts of pollution regulate the relative importance of isoprene photooxidation pathways in the tropical forest of central Amazonia. This paper finds that in Amazonia both HO2 and NO reactive pathways for isoprene are equally important, despite NO levels that are historically regarded as low in this region. But the measurements show that the relative importance of the NO pathway is significantly greater than simulated at present by global chemical transport models of this important, nominally low-NO, forested region of Earth. This shift in isoprene photooxidation, sparked by human activities, suggests ongoing and possible future changes in the photochemistry active over the Amazon rainforest.