The Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) Experiment
The GoAmazon2014/5 Experiment was carried out in the environs of Manaus, Brazil, in the central region of the Amazon basin from January 2014 to December 2015.1 The aircraft and ground station data sets show that the concentrations of isoprene and other biogenic volatile organic compounds (BVOCs) dropped by more than two-thirds, NOx concentrations increased more than two orders of magnitude, and organic PM concentrations more than tripled in the Manaus pollution plume compared to the surrounding background air. The implication is that the oxidant cycle in the pollution plume was greatly accelerated, i.e., higher NOx leading to greater OH, thereby causing BVOC depletion and PM enhancement. Major new data sets pertaining to the production and properties of secondary organic aerosol (SOA) became available as a result of GoAmazon activities.
The atmospheric chemistry of isoprene contributes a substantial mass fraction of SOA over tropical forests. Isoprene reacts with hydroxyl radicals (OH) and molecular oxygen to produce isoprene peroxy radicals (ISOPOO). These radicals can react with hydroperoxyl radicals (HO2) to dominantly produce hydroxyhydroperoxides (ISOPOOH). They can alternatively react with nitric oxide (NO) to largely produce methyl vinyl ketone (MVK) and methacrolein (MACR). The ISOPOOH pathway is the major for SOA production. The GoAmazon2014/5 data set includes the first speciated measurements of ISOPOOH compared to MVK + MACR concentrations.2 Under polluted conditions, when the concentrations of reactive nitrogen compounds were high (>1 ppb), ISOPOOH concentrations dropped below the instrumental detection limit (<60 ppt). Once produced, ISOPOOH react with a second OH radical to produce isoprene epoxydiols (IEPOX). The IEPOX species in the gas phase can subsequently be taken up by particles. Much of the atmospheric PM mass concentration may derive from this single SOA pathway under background conditions over tropical forests. The GoAmazon2014/5 data sets show that IEPOX-derived SOA concentrations decreased under polluted compared to background conditions.
In this light, increased atmospheric pollution, as anticipated for some scenarios of Amazonian economic development, are of concern: pathways of PM production and associated tie-ins to climate that presently prevail over the tropical forest could be disrupted. An important climate impact could be the effects on CCN properties, both the size distribution of the aerosols as well as their intrinsic hygroscopic properties. The CCN data sets of GoAmazon2014/5 shows significant effects of the Manaus pollution plume. Model advances can be made by using these data sets to constrain the links among CCN variability, anthropogenic impacts, and parameterization in global models, especially in a tropical context.
These results highlight the GoAmazon2014/5 theme of anthropogenic-biogenic interactions in the tropical context of high emissions of BVOCs, high water vapor, and high sunlight.
References Cited
1. Martin, S. T., Artaxo, P., Machado, L. A. T., Manzi, A. O., Souza, R. A. F., Schumacher, C., Wang, J., Andreae, M. O., Barbosa, H. M. J., Fan, J., Fisch, G., Goldstein, A. H., Guenther, A., Jimenez, J. L., Pöschl, U., Silva Dias, M. A., Smith, J. N., Wendisch, M, "Introduction: Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5)," Atmospheric Chemistry and Physics, 2016, 16, 4785-4797.
2. Liu, Y., Brito, J., Dorris, M.R., Rivera-Rios, J.C., Seco, R., Bates, K.H., Artaxo, P., Duvoisin Jr., S., Keutsch, F.N., Kim, S., Goldstein, A.H., Guenther, A.B., Manzi, A.O., Souza, R.A.F., Springston, S.R., Watson, T.B., McKinney, K.A., Martin, S.T., "Isoprene photochemistry over the Amazon rainforest," Proceedings of the National Academy of Sciences, 2016, 113, 6125-6130.
