The Impact of Current CH4 and N2O Loss Process Uncertainties on Model Calculated Ozone and Global Lifetimes

Abstract:

The atmospheric loss processes of CH₄ and N₂O, 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). Uncertainties in CH₄ loss due to reaction with OH and O(¹D) have relatively small impacts on present day calculated global total ozone (±0.2-0.3%), with the OH+CH₄ uncertainty impacting tropospheric ozone by ±3-5%. 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 SH polar ozone decline and future recovery. The O(¹D)+N₂O reaction has uncertainty in both the total rate coefficient and branching ratio for the O₂+N₂ and 2*NO product channels. This uncertainty results in a substantial range in present day stratospheric odd nitrogen (±10-25%) and global total ozone (±1-2.5%). This uncertainty also impacts 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. The uncertainty ranges in calculated CH₄ and N₂O global lifetimes are also examined: these ranges are significantly reduced when using the updated SPARC estimated uncertainties compared with those from JPL-2010.