Analysis of the CEPEX ozone data using a 3D chemistry-meteorology model

The ozone (O-3) data collected during the Central Equatorial Pacific Experiment (CEPEX) are analysed with the aid of the 3D global photochemistry model MATCH-MPIC (Model of Atmospheric Transport and Chemistry-Max-Planck-Institute for Chemistry version). This study focuses on using MATCH-MPIC to address three specific questions: (I) are the individual CEPEX O-3 soundings, in particular the extremely low O-3 levels occasionally observed in the upper troposphere (UT), reproducible by a state-of-the-art global photochemical model driven with analysed meteorological data from the same time period as the measurements (March 1993); (2) are the CEPEX O-3 data likely to be representative of the mean state of the regional atmosphere, or do they instead indicate the degree of variability in this region of the atmosphere; and (3) what causes the UT O-3 minima? It is found that MATCH-MPIC is not able to reproduce the soundings obtained during CEPEX on an individual basis; however, the model does reproduce some of the key features present in the observations, such as UT minima and mid-tropospheric maxima similar to those observed during CEPEX. The UT O-3 minima computed by the model are mainly due to convective pumping of low-O-3 marine boundary-layer air, as is demonstrated by comparison with the results of a run in which convective transport of O-3 is suppressed. The UT O-3 minima simulated by MATCH-MPIC (with O-3 between 5 and 10 nmol mol(-1)) are less intense than those observed (with O-3 < 5 nmol mol(-1)), even at relatively high model spatial and temporal resolution, and with a convection scheme that would be expected to readily produce UT O-3 minima via intense pumping of low-O-3 marine boundary-layer air directly into the UT convective outflow regions. This may indicate that additional photochemical loss processes are involved, either in situ in the UT or, in particular, near the surface in the convective inflow regions, where the model tends to overestimate the observed O-3 levels. In addition, although a significant temporal variability for O-3 in this region is computed, it is still less than indicated by the observations in the UT. This high degree of variability implies that individual O-3 soundings will often differ considerably from the mean over a longer period. For instance, 20-50% less O-3 in the UT during the CEPEX period is computed with MATCH-MPIC than the average for March. Thus, the CEPEX expedition was perhaps fortunate in encountering extreme conditions which produced extensive UT O-3 minima, which have helped to demonstrate one end of the large degree of variability of O-3 in the tropical troposphere.