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Quantification of chemical and physical processes influencing ozone during long-range transport using a trajectory ensemble

Author(s): M. Cain | J. Methven | E. J. Highwood

Journal: Atmospheric Chemistry and Physics Discussions
ISSN 1680-7367

Volume: 12;
Issue: 1;
Start page: 3019;
Date: 2012;
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During long-range transport, many distinct processes – including photochemistry, deposition, emissions and mixing – contribute to the transformation of air mass composition. Partitioning the effects of different processes can be useful when considering the sensitivity of chemical transformation to, for example, a changing environment or anthropogenic influence. However, transformation is not observed directly, since mixing ratios are measured, and models must be used to relate changes to processes. Here, four cases from the ITCT-Lagrangian 2004 experiment are studied. In each case, aircraft intercepted an air mass several times during transport over the North Atlantic, providing a unique dataset and quantifying the net changes in composition from all processes acting. A new framework is presented to deconstruct the change in O3 mixing ratio (ΔO3) into its component processes, which were not measured directly, taking into account the uncertainty in measurements, initial air mass variability and its time evolution. The results show that the net chemical processing (ΔO3chem) over the whole simulation is greater than net physical processing (ΔO3phys) in all cases. This is in part explained by cancellation effects associated with mixing. In contrast, each case is in a regime of either photochemical destruction (lower tropospheric transport) or production (an upper tropospheric biomass burning case). However, physical processes influence O3 indirectly through addition or removal of precursor gases, so that changes to physical parameters in a model can have a larger effect on ΔO3chem than ΔO3phys. Despite its smaller magnitude, the physical processing distinguishes the lower tropospheric export cases, since the photochemical O3 change is −5ppbv per day in all three cases. Processing is quantified using a Lagrangian photochemical model with a novel method for simulating mixing through an ensemble of trajectories and a background profile that evolves with them. The model is able to simulate the magnitude and variability of the observations (of O3, CO, NOy and some hydrocarbons) and is consistent with the time-average OH following air-masses inferred from hydrocarbon measurements alone (by Arnold et al., 2007). Therefore, it is a useful new method to simulate air mass evolution and variability, and its sensivity to process parameters.
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