The impact of current CH4 and N2O atmospheric loss process uncertainties on calculated ozone abundances and trends

The atmospheric loss processes of N2O and CH4, their estimated uncertainties, lifetimes, and impacts on ozone abundance and long-term trends are examined using atmospheric model calculations and updated kinetic and photochemical parameters and uncertainty factors from SPARC [2013]. The uncertainty ranges in calculated N2O and CH4 global lifetimes computed using the SPARC estimated uncertainties are reduced by nearly a factor of two compared with uncertainties from Sander et al. [2011]. Uncertainties in CH4 loss due to reaction with OH and O(1D) have relatively small impacts on present day global total ozone (±0.2-0.3%). Uncertainty in the Cl + CH4 reaction affects the amount of chlorine in radical vs. reservoir forms and has a modest impact on present day SH polar ozone (~±6%), and on the rate of past ozone decline and future recovery. Uncertainty in the total rate coefficient for the O(1D) + N2O reaction results in a substantial range in present day stratospheric odd nitrogen (±20-25%) and global total ozone (±1.5-2.5%). Uncertainty in the O(1D) + N2O reaction branching ratio for the O2 + N2 and 2*NO product channels results in moderate impacts on odd nitrogen (±10%) and global ozone (±1%),with uncertainty in N2O photolysis resulting in relatively small impacts (±5% in odd nitrogen, ±0.5% in global ozone). Uncertainties in the O(1D) + N2O reaction and its branching ratio also affect the rate of past global total ozone decline and future recovery, with a range in future ozone projections of ±1-1.5% by 2100, relative to present day.