1] A ship emission plume experiment was conducted about 100 km off the California coast during the NOAA Intercontinental Transport and Chemical Transformation (ITCT) 2K2 airborne field campaign. Measurements of chemical species were made from the NOAA WP-3D aircraft in eight consecutive transects of a ship plume around midday during 2.5 hours of flight. The measured species include NO x , HNO 3 , peroxyacetylnitrate (PAN), SO 2 , H 2 SO 4 , O 3 , CO, CO 2 , nonmethane hydrocarbons (NMHC), and particle number and size distributions. Observations demonstrate a NO x lifetime of $1.8 hours inside the ship plume compared to $6.5 hours (at noontime) in the moderately polluted background marine boundary layer of the experiment. This confirms the earlier hypothesis of highly enhanced in-plume NO x destruction. Consequently, one would expect the impact of ship emissions is much less severe than those predicted by global models that do not include rapid NO x destruction. Photochemical model calculations suggest that more than 80% of the NO x loss was due to the NO 2 + OH reaction; the remainder was by PAN formation. The model underestimated in-plume NO x loss rate by about 30%. In addition, a comparison of measured to predicted H 2 SO 4 in the plumes suggests that the photochemical model predicts OH variability reasonably well but may underestimate actual values. Predictions of in-plume O 3 production agree well with the observations, suggesting that model-predicted peroxy radical (HO 2 + RO 2) levels are reasonable. The model estimated ozone production efficiency ranges from 6 to 30. The largest model bias was seen in the comparison with measured HNO 3 . The model overestimated in-plume HNO 3 by about a factor of 6. This is most likely caused by underestimated HNO 3 sinks possibly involving particle scavenging. However, limited data availability precluded a conclusive test of this possible loss process.