Global Ozone - CO Correlations from OMI and AIRS : Constraints on Tropospheric 1 Ozone

32 33 We present a global data set of free tropospheric ozone-CO correlations with 2 o × 2.5 o 34 spatial resolution from the OMI and AIRS satellite instruments for each season of 2008. 35 OMI (ozone) and AIRS (CO) have near daily global coverage and observe coincident 36 scenes with similar vertical sensitivities. The resulting ozone-CO correlations are highly 37 statistically significant (positive or negative) in most regions of the world, and are less 38 noisy than previous satellite-based studies that used sparser data. We interpret the 39 observed ozone-CO correlations with the GEOS-Chem chemical transport model to infer 40 constraints on ozone sources. Driving GEOS-Chem with different meteorological fields 41 generally shows consistent ozone-CO correlation patterns, except in some tropical 42 regions where the correlations are strongly sensitive to model transport error associated 43 with deep convection. GEOS-Chem reproduces the general structure of the observed 44 ozone-CO correlations and regression slopes (dO 3 /dCO), although there are some large 45 regional discrepancies. We examine the model sensitivity of dO 3 /dCO to different ozone 46 sources (combustion, biosphere, stratosphere, and lightning) by correlating the ozone 47 change from that source to CO from the standard simulation. The model reproduces the 48 observed positive dO 3 /dCO in the extratropical northern hemisphere in spring-summer, 49 driven by combustion sources. Stratospheric influence there is also associated with a 50 positive dO 3 /dCO because of the interweaving of stratospheric downwelling with 51 continental outflow. The well-known ozone maximum over the tropical South Atlantic is 52 associated with negative dO 3 /dCO in the observations; this feature is reproduced in 53 GEOS-Chem and supports a dominant contribution from lightning to the ozone 54 maximum. A major model discrepancy is found over the Northeast Pacific in summer-fall 55 where dO 3 /dCO is positive in the observations but negative in the model, for all ozone 56 sources. We suggest that this reflects a model overestimate of lightning at northern mid-57