Measurements of polluted air masses from Eurasia at a surface site in the Pacific Northwest: the importance of transport mechanisms on the chemical composition

Springtime measurements of carbon monoxide (CO), ozone (O₃), nonmethane hydrocarbons (NMHCs), and aerosols were collected at Cheeka Peak Observatory (CPO), located at 48.3 N and 124.6 W, during the Photochemical Ozone Budget of the Eastern North Pacific (PHOBEA-I and II) experiments, 1997-98 and 2001-02. The purpose of these experiments was to quantify the long-range transport (LRT) of pollutants from the Eurasian continent to the Pacific Northwest (PNW) United States and examine the impact on the photochemical O₃ budget and local air quality in this region. During this study, 8 distinct LRT events were documented at CPO, when the transport of Eurasian industrial pollution, dust, or haze from Asian biomass burning was measured. These events were defined by enhanced CO (up to 190 ppbv), O₃ (up to 60 ppbv), NMHCs, and aerosol scattering coefficients (reaching 40 Mm^-}{1). However, aircraft measurements over CPO (0-6 km) during the spring of 2001 and 2002 indicate that LRT events were often observed at altitudes above 2 km. For example, several aircraft vertical profiles collected during this experiment measured elevated CO and O₃ above 2 km due to LRT that did not correlate with enhanced measurements at CPO. In addition, not all LRT events contained elevated O₃, possibly due to different sources or the result of loss due to HOx chemistry in the boundary layer, or precipitation and heterogeneous chemistry along the transport path. These observations are employed in our subsequent analysis of the transport mechanisms bringing Eurasian pollution to the CPO surface site and how these transport pathways influence the chemical composition of the air masses observed at CPO.