Michael P. Vermeuel

and 15 more

Dry deposition is the second-largest tropospheric ozone (O3) sink and occurs through stomatal and nonstomatal pathways. Current O3 uptake predictions are limited by the simplistic big-leaf schemes commonly used in chemical transport models (CTMs) to parameterize deposition. Such schemes fail to reproduce observed O3 fluxes over terrestrial ecosystems, highlighting the need for more realistic treatment of surface-atmosphere exchange in CTMs. We address this need by linking a resolved canopy model (1D Multi-Layer Canopy CHemistry and Exchange Model, MLC-CHEM) to the GEOS-Chem CTM, and use this new framework to simulate O3 fluxes over three north temperate forests. We compare results with in-situ measurements from four field studies and with standalone, observationally-constrained MLC-CHEM runs to test current knowledge of O3 deposition and its drivers. We show that GEOS-Chem overpredicts observed O3 fluxes across all four studies by up to 2×, whereas the resolved-canopy models capture observed diel profiles of O3 deposition and in-canopy concentrations to within 10%. Relative humidity and solar irradiance are strong O3 flux drivers over these forests, and uncertainties in those fields provide the largest remaining source of model deposition biases. Flux partitioning analysis shows that: 1) nonstomatal loss accounts for 60% of O3 deposition on average; 2) in-canopy chemistry makes only a small contribution to total O3 fluxes; and 3) the CTM big-leaf treatment overestimates O3-driven stomatal loss and plant phytotoxicity in these temperate forests by up to 7×. Results motivate the application of fully-online, vertically explicit canopy schemes in CTMs for improved O3 predictions.

Shaddy Ahmed

and 15 more

Reactive chlorine and bromine species emitted from snow and aerosols can significantly alter the oxidative capacity of the polar boundary layer. However, halogen production mechanisms from snow remain highly uncertain, making it difficult for most models to include descriptions of halogen snow emissions and to understand the impact on atmospheric chemistry. We investigate the influence of Arctic halogen emissions from snow on boundary layer oxidation processes using a one-dimensional atmospheric chemistry and transport model (PACT-1D). To understand the combined impact of snow emissions and boundary layer dynamics on atmospheric chemistry, we model \ch{Cl2} and \ch{Br2} primary emissions from snow and include heterogeneous recycling of halogens on both snow and aerosols. We focus on a two-day case study from the 2009 Ocean-Atmosphere-Sea Ice-Snowpack (OASIS) campaign at Utqia\.gvik, Alaska. The model reproduces both the diurnal cycle and high quantity of \ch{Cl2} observed, along with the measured concentrations of \ch{Br2}, \ch{BrO}, and \ch{HOBr}. Due to the combined effects of emissions, recycling, vertical mixing, and atmospheric chemistry, reactive chlorine is confined to the lowest 15 m of the atmosphere, while bromine impacts chemistry up to the boundary layer height. Upon including halogen emissions and recycling, the concentration of \ch{HO_x} (\ch{HO_x} = \ch{OH}+\ch{HO2}) at the surface increases by as much as a factor of 30 at mid-day. The change in \ch{HO_x} due to halogen chemistry, as well as chlorine atoms derived from snow emissions, significantly reduce volatile organic compound (VOC) lifetimes within a shallow layer near the surface.