Seasonal Flux Measurements over a Colorado Pine Forest Demonstrate a Persistent Source of Organic Acids

Abstract

Forests can be both sources and sinks of volatile organic compounds to the atmosphere. The role that forests play in controlling organic acid concentrations remains poorly understood with multiple model-measurement comparisons reporting missing sources of formic acid. We conducted seasonal measurements of concentrations and eddy covariance fluxes of oxidized volatile organic compounds over a ponderosa pine forest in Colorado in 2016. Diel concentration profiles show mid-day maxima, consistent with previous studies. We observed persistent but variable upward fluxes of formic, propionic, methacrylic, and butyric acids from the pine forest during all seasons. Formic acid concentrations and fluxes were ∼10 times higher than the other organic acids with daytime exchange velocities on the order of 4–6 cm s–1. The other organic acids had similar exchange velocities as formic acid in the warmer seasons and much smaller exchange velocities in the colder seasons. The upward fluxes for all organic acids increased exponentially with temperature. The observed net upward flux demonstrated that dry deposition was smaller than ecosystem sources of the organic acids. Primary emissions from soil and pine trees were small, in contrast to estimates of in-canopy chemistry. Our study points to an underestimated ecosystem source of organic acids (e.g., in-canopy chemistry of large or multifunctional terpenoids), an overestimated dry deposition sink (potentially due to the arid environment), and/or an unresolved sink of organic acids in the upper boundary layer. Forests are potentially large sources of atmospheric organic acids in warmer seasons but further investigation into dry deposition mechanisms and in-canopy chemistry is warranted.

Revisiting particle dry deposition and its role in radiative effect estimates

“…This study uses data from two field campaigns: 1) the five distinct measurement periods of the SPiFFY study at the Manitou Experimental Forest and 2) the Black Carbon Aerosol Deposition Study (BCADS 2017) campaign at the southern Great Plains ( 2 ). Both sites were described previously ( 26 – 30 ).…”

Surface Wetness as an Unexpected Control on Forest Exchange of Volatile Organic Acids

“…Most annual precipitation is snowfall during winter and spring, although transient afternoon summer rainstorms occur. The site is well characterized (Fulgham et al, 2019; Karl et al, 2014; Kaser et al, 2013; Ortega et al, 2014; Pryor et al, 2013; Rhew et al, 2017). Our work was part of the Seasonal Particles in Forests Flux studY (SPiFFY), which spanned four seasonally representative campaigns in 2016: winter (1 February to 1 March), spring (15 April to 15 May), summer (15 July to 15 August), and fall (1 October to 1 November).…”

“…We measured the vertical exchange of acids from the 30 m MEFO tower using the eddy covariance technique (Fulgham et al, 2019) (section S1).…”

“…Due to their water solubility and high volatility, volatile organic acids are well suited to probe the influence of leaf wetness on bidirectional gas exchange. Fluxes of organic acids are poorly understood, with multiple studies observing unexplained upward fluxes of formic acid from forests (Alwe et al, 2019; Fulgham et al, 2019; Nguyen et al, 2015; Schobesberger et al, 2016). We previously reported persistent upward fluxes of formic, butyric, propionic, methacrylic, valeric, and heptanoic acids at Manitou Experimental Forest (Fulgham et al, 2019).…”

Simultaneous leaf-level measurement of trace gas emissions and photosynthesis with a portable photosynthesis system

“…Despite their ubiquity, models typically underestimate ambient concentrations of formic acid, even the structurally simplest of organic acids, implying a missing source (Paulot et al, 2011;Alwe et al, 2019). This missing source of formic acid is not soils (Mielnik et al, 2018), but flux studies (Fulgham et al, 2019) and vertical gradient measurements (Mattila et al, 2018) suggest a direct ecosystem source. Here we demonstrate the capacity of the PPS coupled to a CIMS system to investigate leaf-level organic acid sources.…”