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 measur... Show moreForests 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 similar to 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. Show less