1The University of Arizona-Biosphere 2, P.O. Box 8746, Tucson, AZ 85738, USA
2Department of Physical Geography and Ecosystems Analysis, Lund University, Sölvegatan 12, 223 62 Lund, Sweden
3Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research/Atmospheric Environmental Research, Kreuzeckbahnstr. 19, 82467 Garmisch-Partenkirchen, Germany
4Department of Chemistry & Biochemistry, University of Arizona, P.O. Box 210041, 1306 East University Blvd., Tucson, AZ 85721–0041, USA
5Instituto de Fisica, Universidade de Sao Paulo, Rua do Matao, Travessa R,187 Sao Paulo SP 05508–900, Brazil
6Large Biosphere-Atmosphere Experiment (LBA), Instituto Nacional de Pesquisas da Amazônia, Av. André Araújo, 2936, Aleixo, CEP 69060–001, Manaus, Brazil
7Biogeochemistry Department, Max Planck Institute for Chemistry, P.O. Box 3060, 55020 Mainz, Germany
8Department of Ecology and Evolutionary Biology, University of Arizona, P.O. Box 210088, BioSciences West 310, Tucson, AZ, 85721, USA
9Department of Soil, Water and Environmental Science, University of Arizona, P.O. Box 210038, Tucson, AZ 85721–0038, USA
Received: 31 Jul 2011 – Discussion started: 14 Sep 2011
Abstract. Organic acids, central to terrestrial carbon metabolism and atmospheric photochemistry, are ubiquitous in the troposphere in the gas, particle, and aqueous phases. As the dominant organic acids in the atmosphere, formic acid (FA, HCOOH) and acetic acid (AA, CH3COOH) control precipitation acidity in remote regions and may represent a critical link between the terrestrial carbon and water cycles by acting as key intermediates in plant carbon and energy metabolism and aerosol-cloud-precipitation interactions. However, our understanding of the exchange of these acids between terrestrial ecosystems and the atmosphere is limited by a lack of field observations, the existence of biogenic and anthropogenic primary and secondary sources whose relative importance is unclear, and the fact that vegetation can act as both a source and a sink. Here, we first present data obtained from the tropical rainforest mesocosm at Biosphere 2 which isolates primary vegetation sources. Strong light and temperature dependent emissions enriched in FA relative to AA were simultaneously observed from individual branches (FA/AA = 3.0 ± 0.7) and mesocosm ambient air (FA/AA = 1.4 ± 0.3). We also present long-term observations of vertical concentration gradients of FA and AA within and above a primary rainforest canopy in the central Amazon during the 2010 dry and 2011 wet seasons. We observed a seasonal switch from net ecosystem-scale deposition during the dry season to net emissions during the wet season. This switch was associated with reduced ambient concentrations in the wet season (FA < 1.3 nmol mol−1, AA < 2.0 nmol mol−1) relative to the dry season (FA up to 3.3 nmol mol−1, AA up to 6.0 nmol mol−1), and a simultaneous increase in the FA/AA ambient concentration ratios from 0.3–0.8 in the dry season to 1.0–2.1 in the wet season. These observations are consistent with a switch between a biomass burning dominated source in the dry season (FA/AA < 1.0) to a vegetation dominated source in the wet season (FA/AA > 1.0). Our observations provide the first ecosystem-scale evidence of bidirectional FA and AA exchange between a forest canopy and the atmosphere controlled by ambient concentrations and ecosystem scale compensation points (estimated to be 1.3 ± 0.3 nmol mol−1: FA, and 2.1 ± 0.4 nmol mol−1: AA). These results suggest the need for a fundamental change in how future biosphere-atmosphere exchange models should treat FA and AA with a focus on factors that influence net exchange rates (ambient concentrations and ecosystem compensation points) rather than treating emissions and deposition separately.
Revised: 02 Dec 2011 – Accepted: 05 Dec 2011 – Published: 16 Dec 2011
Jardine, K., Yañez Serrano, A., Arneth, A., Abrell, L., Jardine, A., Artaxo, P., Alves, E., Kesselmeier, J., Taylor, T., Saleska, S., and Huxman, T.: Ecosystem-scale compensation points of formic and acetic acid in the central Amazon, Biogeosciences, 8, 3709-3720, doi:10.5194/bg-8-3709-2011, 2011.