Impacts of C1-C3 alkyl nitrates on tropospheric ozone chemistry

Alkyl nitrates (RONO2) are important reservoirs of tropospheric reactive nitrogen. They are produced from the oxidation of their parent alkanes (RH) in the presence of NOx and emitted from oceanic and biomass burning sources. Due to their relatively long lifetime of a few days to a few months, alkyl nitrates can be destroyed far away from their sources by photolysis or OH oxidation and alter regional tropospheric ozone concentrations. While C1-C3 RONO2 chemistry is well understood, information about their oceanic and biomass burning sources is limited. We derived a new estimate of C1-C3 RONO2 biomass burning emissions from the Global Fire Emissions Database and implemented these emissions into a global 3D chemistry-climate model UM-UKCA, along with C1-C3 RONO2 chemistry from the Master Chemical Mechanism, dry deposition and oceanic emissions. We performed six perpetual year UM-UKCA simulations designed to explore the statistical significance of the global and localised impacts of C1-C3 RONO2 on tropospheric ozone chemistry. We also compared the regional mean vertical profiles of C1-C3 RH and RONO2, NOx and O3 observed during the Atmospheric Tomography mission and simulated by UM-UKCA in 8 remote regions in February and August. We found that C1-C3 RONO2 oceanic emissions have the largest global impact on tropospheric ozone chemistry among all alkyl nitrate sources considered in this study, while their biomass burning emissions have the smallest impact. The combination of C1-C3 RONO2 chemistry and emissions increases tropospheric ozone burden by 2.96±0.69 Tg (1.09±0.25%) and decreases methane lifetime by 0.151±0.036 yr (1.56±0.37%). Statistically significant increases in the seasonal mean ozone concentrations of up to 2 ppbv (_5%) are located within 0-5 km over the Southern Ocean during boreal winter and autumn and within 0-10 km near the equator during boreal winter, summer and autumn