It was recently proposed (Franco et al., Nature 2021) that methanediol (MD, HOCH2OH ) formed by hydration of formaldehyde in liquid cloud droplets is outgassed to a larger extent than previously estimated, and reacts in the gas phase with the hydroxyl radical (OH), leading to formic acid (HCOOH). Whereas the resulting global production of formic acid is greatly dependent on poorly constrained parameters, such as the Henry’s law constant (HLC) of methanediol and the rate constant of its reaction with OH, Franco et al. suggest, based on global model calculations and on newly conducted chamber experiments (for the rate constant) and on statistical prediction methods (for the HLC), that this mechanism explains the large “missing source” of HCOOH in the atmosphere (e.g. Stavrakou et al. 2012). If true, this finding would be of tremendous importance for our understanding of the biogeochemical cycling of oxygenated organic compounds. For this reason, it is of utmost importance to double check the validity of thehypotheses and parameterizations behind this assessment. Here we examine two critical aspects of this determination: the HLC (taken equal to either 10^4 or 10^6 M atm^-1 in model simulations by Franco et al.) and the rate of the MD+OH reaction (taken equal to 7.5 × 10^12 cm^3 s^-1 ). The representation of chemical processing in liquid clouds in global models is also briefly discussed. Plausible ranges for those parameters are proposed , and causes of uncertainty are discussed . The potential consequences for the resulting production of formic acid are briefly explored.
Fire emissions are an important component of global models, which help to understand the influence of sources, transport and chemistry on atmospheric composition. Global fire emission inventories can vary substantially due to the assumptions made in the emission creation process, including the defined vegetation type, fire detection, fuel loading, fraction of vegetation burned and emissions factors. Here, we focus on the uncertainty in emission factors and the resulting impact on modeled composition. Our study uses the Community Atmosphere Model with chemistry (CAM-chem) to model atmospheric composition for 2014, a year chosen for the relatively quiet El Niño Southern Oscillation activity. We focus on carbon monoxide (CO), a trace gas emitted from incomplete combustion and also produced from secondary oxidation of volatile organic compounds (VOCs). Fire is a major source of atmospheric CO and VOCs. Modeled CO from four fire emission inventories (CMIP6/GFED4s, QFED2.5, GFAS1.2 and FINN1.5) are compared after being implemented in CAM-chem. Multiple sensitivity tests are performed based on CO and VOC emission factor uncertainties. We compare model output in the 14 basis regions defined by the Global Fire Emissions Database (GFED) team and evaluate against CO observations from the Measurements of Pollution in the Troposphere (MOPITT) satellite-based instrument. For some regions, emission factor uncertainty spans the results found by using different inventories. Finally, we use modeled ozone (O3) to briefly investigate how emission factor uncertainty influences the atmospheric oxidative environment. Overall, accounting for emission factor uncertainty when modeling atmospheric chemistry can lend a range of uncertainty to simulated results.