Ecosystem-scale compensation points of formic and acetic acid in the central Amazon

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) con- trol precipitation acidity in remote regions and may repre- sent 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 interac- tions. 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 veg- etation 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 in- dividual branches (FA/AA = 3.0± 0.7) and mesocosm am- bient 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 de- position during the dry season to net emissions during the wet season. This switch was associated with reduced ambi- ent concentrations in the wet season (FA 1.0). Our observations provide the first ecosystem-scale evidence of bidirectional FA and AA exchange between a forest canopy and the atmosphere con- trolled by ambient concentrations and ecosystem scale com- pensation 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.