[1] Satellite observations of carbon monoxide (CO) from the Measurements of Pollution in the Troposphere (MOPITT) instrument are combined with measurements from the Transport and Chemical Evolution Over the Pacific (TRACE-P) aircraft mission over the northwest Pacific and with a global three-dimensional chemical transport model (GEOS-CHEM) to quantify Asian pollution outflow and its trans-Pacific transport during spring 2001. Global CO column distributions in MOPITT and GEOS-CHEM are highly correlated (R 2 = 0.87), with no significant model bias. The largest regional bias is over Southeast Asia, where the model is 18% too high. A 60% decrease of regional biomass burning emissions in the model (to 39 Tg yr À1 ) would correct the discrepancy; this result is consistent with TRACE-P observations. MOPITT and TRACE-P also give consistent constraints on the Chinese source of CO from fuel combustion (181 Tg CO yr À1 ). Four major events of trans-Pacific transport of Asian pollution in spring 2001 were seen by MOPITT, in situ platforms, and GEOS-CHEM. One of them was sampled by TRACE-P (26-27 February) as a succession of pollution layers over the northeast Pacific. These layers all originated from one single event of Asian outflow that split into northern and southern plumes over the central Pacific. The northern plume (sampled at 6-8 km off California) had no ozone enhancement. The southern subsiding plume (sampled at 2-4 km west of Hawaii) contained a 8-17 ppbv ozone enhancement, driven by decomposition of peroxyacetylnitrate (PAN) to nitrogen oxides (NO x ). This result suggests that PAN decomposition in trans-Pacific pollution plumes subsiding over the United States could lead to significant enhancements of surface ozone.

The importance of Arctic outflow events to the budgets of nitrogen oxides and hydrocarbons in the North Atlantic region is estimated using a climatology of isentropic airflow trajectories, in combination with current understanding of the levels of these compounds in the Arctic troposphere. We first review available measurements of nonmethane hydrocarbons (NMHCs), total reactive oxidized nitrogen (NOy), and major NOy species in the Arctic troposphere to develop best estimate average vertical profiles during January‐May outflow events. Measurements of these compounds in the winter‐spring Arctic are generally consistent. Average levels during March are ≥500 parts per 1012 by volume (NOy) and ∼20 parts per billion carbon (NMHC). Current evidence for a significant vertical gradient above the boundary layer is weak, although additional measurements are needed. Secondly, the flow patterns and frequency of Arctic outflow events which reach the North Atlantic region south of 50°–55°N are investigated using an 11‐year climatology of isentropic forward trajectories originating at 70°N in the months of January‐May. The dominant route of trajectories reaching the temperate North Atlantic originates north of Canada at 2–6 km altitude and continues southward along a semipermanent trough located near the East Coast of North America. Trajectories reaching the temperate North Atlantic originated in this region on ∼70% of the days analyzed. Significant subsidence occurs during the southward flow, resulting in warming conducive to photochemical processing of the Arctic pollutants. Based on these analyses, the southward fluxes of NOy and NMHCs out of the Arctic in events which reach the North Atlantic south of 50°N total 7.3 GgN/month NOy and 250 GgC/month NMHC during March. These values are biased low as they include only those trajectories originating below 6 km and exclude trajectories which pass over the United States or southeastern Canada. The calculated NOy flux during May is lower but may be underestimated due to uncertainty in conditions in the Arctic free troposphere in that month. The May flux of NMHCs is larger than that in March as a result of a more frequent occurrence of outflow events. These fluxes impact air parcels which are not affected by direct transport from source regions and appear to be seasonally significant relative to other sources of ozone precursors to the North Atlantic troposphere. If a significant fraction of the peroxyacetyl nitrate and alkyl nitrates which comprise most of the advected NOy decomposes over the North Atlantic, the transport of anthropogenic pollutants through the Arctic may play a significant role in the ozone budget of the North Atlantic troposphere.

[1] During Transport and Chemical Evolution over the Pacific (TRACE-P), there were several opportunities to perform in situ sampling coincident with overpasses of the Measurements of Pollution in the Troposphere (MOPITT) instrument on board the EOS Terra satellite. This sampling consisted of in situ vertical profiles of CO by NASA's DC-8 aircraft intended to provide data useful for validating MOPITT observations of CO column. One particular profile conducted over the central North Pacific revealed a layer of pollution characterized by CO mixing ratios more than double background values. Sampling of the surrounding region by both the NASA DC-8 and P-3B aircraft showed this layer to have a considerable geographic extent, at least 25°longitude ($2500 km) and 4°latitude ($400 km). Using back trajectory analysis, this polluted layer is followed back in time and compared with four consecutive MOPITT overpasses. MOPITT observations during these four overpasses agree well with the location of the layer as inferred by the trajectories; however, the detected CO column amount increases backward in time by just over 20%. Further analysis shows that the majority of this change in detected column abundance is consistent with two factors: (1) changes in the thickness of the polluted layer over time (9 ± 3%) and (2) changes in retrieved column abundance due to the altitude of the layer (7 ± 3%). This demonstrates that there are both real and artificial sources of variability that must be understood before MOPITT observations can be quantitatively useful. An unexpected finding was the difference in the variance of MOPITT observations depending on whether observations were taken under daylight or nighttime conditions. The variance in daytime observations of the polluted layer was approximately double that for nighttime data. The results of this analysis indicate that targeted in situ sampling of large-scale pollution events can provide insight leading to more realistic interpretation of MOPITT observations. Strategies for sampling such events repeatedly during their evolution could also provide more interesting opportunities for validation.