Abstract
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.
Highlights
Short chain organic acids like formic acid (FA, HCOOH) and acetic acid (AA, CH3COOH) are ubiquitous in the main biogeochemical reservoirs on Earth including the biosphere, hydrosphere, atmosphere, and geosphere
In a recent study that focused on methanol and acetone (Ganzeveld et al, 2008), it was found that a commonly applied algorithm to simulate global acetone and methanol biogenic exchanges substantially overestimates ambient concentrations and emission strengths
We present results of in situ, high vertically resolved ambient concentration measurements of FA and AA within and above a primary rainforest canopy in the central Amazon during both the 2010 dry and 2011 wet seasons
Summary
Short chain organic acids like formic acid (FA, HCOOH) and acetic acid (AA, CH3COOH) are ubiquitous in the main biogeochemical reservoirs on Earth including the biosphere, hydrosphere, atmosphere, and geosphere. Active metabolic consumption of oxidized volatile organic compounds (VOCs) by plants can lead to efficient uptake thereby sustaining a large concentration gradient between the atmosphere and the intercellular air spaces; a process that is generally not considered in commonly applied dry deposition schemes. More recently in a study during the wet to dry season and the dry to wet season transitions in the southwestern Amazon, net ecosystem scale uptake of FA and AA was implied from the higher ambient concentrations above the canopy (Kuhn et al, 2002) This is consistent with results during the dry season in a tropical rainforest in Costa Rica where net deposition of both FA and AA was observed (Karl et al, 2004). We perform a similar analysis of ambient FA/AA ratios in both the central and southwestern Amazon to gain insights into the possible atmospheric sources of FA and AA in both dry and wet seasons
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
