Organic acids have primary and secondary sources in the atmosphere, impact ecosystem health, and are useful metrics for identifying gaps in organic oxidation chemistry through model-measurement comparisons. We photooxidized (OH oxidation) primary emissions from diesel and biodiesel fuel types under two engine loads in an oxidative flow reactor. formic, butyric, and propanoic acids, but not methacrylic acid, have primary and secondary sources. Emission factors for these gas-phase acids varied from 0.3-8.4 mg kg fuel. Secondary chemistry enhanced these emissions by 1.1 (load) to 4.4 (idle) × after two OH-equivalent days. The relative enhancement in secondary organic acids in idle versus loaded conditions was due to increased precursor emissions, not faster reaction rates. Increased hydrocarbon emissions in idle conditions due to less complete combustion (associated with less oxidized gas-phase molecules) correlated to higher primary organic acid emissions. The lack of correlation between organic aerosol and organic acid concentrations downstream of the flow reactor indicates that the secondary products formed on different oxidation time scales and that despite being photochemical products, organic acids are poor tracers for secondary organic aerosol formation from diesel exhaust. Ignoring secondary chemistry from diesel exhaust would lead to underestimates of both organic aerosol and gas-phase organic acids.

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.

Soil emissions formic acid from a forest and managed lawn increase with temperature, while acetic acid emissions increase with both temperature and moisture.

We report bidirectional exchange of volatile acids, including isocyanic and alkanoic acids, over a pine forest across multiple seasons. The exchange velocity of these acids is well correlated with dew point depression, suggesting an equilibrium‐driven continuum of flux. Wetness on forest surfaces impacts the vertical exchange of gases, and we suggest that water films and droplets drive equilibrium partitioning, with acids being solvated in surface wetness and released through evaporation. Despite their volatility, these acids partition into neutral‐to‐alkaline aqueous films, consistent with reported dew pH. This relationship between exchange velocity and dew point depression holds for a wetter mixed forest, but not a very dry orchard. Dew point depression is an excellent indicator of acid fluxes so long as the canopy is occasionally wetted.