Sources and budgets for CO and O 3 in the northeastern Pacific during the spring of 2001: Results from the PHOBEA‐II Experiment

Ground and airborne measurements of CO, ozone, and aerosols were obtained in the northeastern (NE) Pacific troposphere during 9 March–31 May 2001 as part of the PHOBEA‐II project (Photochemical Ozone Budget of the Eastern North Pacific Atmosphere). The GEOS‐CHEM global three‐dimensional model of tropospheric chemistry was used for flight planning, as well as for data analysis after the field mission to examine the origin of CO and ozone over the NE Pacific. The model successfully reproduces the observed CO levels, their temporal variability, and vertical gradients, strongly suggesting a good understanding of the sources, transport and chemistry of CO in the NE Pacific. For ozone the model underestimates mean surface observations by 8 ppbv, overestimates aircraft profiles by 5 ppbv, and does not reproduce the temporal variability of the observations. Possible explanations for model error in the ozone simulation are discussed. We find a pervasive influence of Asian and European anthropogenic sources on the levels of CO in the NE Pacific troposphere. In the 0–6 km column over our surface site, Asian and European emissions account for 33% (42 ppbv) and 15% (21 ppbv) of CO, respectively. Asian and European emissions are responsible for smaller fractions of the 0–6 km ozone column, 12% (5 ppbv) and 5% (2 ppbv), respectively. The full influence of Asian emissions (including secondary ozone production by export of its precursors) approaches 16% of ozone. The model successfully captures three intercontinental transport events observed during the PHOBEA‐II campaign: one at the surface and two in the free troposphere. While all three events were characterized by large CO enhancements (by 20–40 ppbv), only one case showed significant ozone enhancement (by 20 ppbv), induced by high‐latitude transport of Asian pollution mixed in with stratospheric ozone.