Photochemistry, ozone production, and dilution during long‐range transport episodes from Eurasia to the northwest United States

On the basis of observations from the 1997–2002 Photochemical Ozone Budget of the Northeast Pacific (PHOBEA) experiments, we have identified 11 transpacific long‐range transport (LRT) episodes, which contain significantly elevated levels of CO, O 3 , and aerosol scattering. The LRT episodes were determined from aircraft and ground‐based observations of CO, O 3 , aerosol scattering coefficient, and 281 whole air samples analyzed for nonmethane hydrocarbons (NMHC). The ratio of excess O 3 to excess CO (ΔO 3 /ΔCO) for the 11 LRT episodes ranged from −0.06 to 1.52. Lower ΔO 3 /ΔCO ratios (<0.10) are characteristic of LRT episodes transported in the boundary layer or in the presence of substantial mineral dust. These events lack O 3 enhancements, even though O 3 precursors (CO, NMHCs) are elevated. Ratios of ΔO 3 /ΔCO of 0.2–0.5 are characteristic of LRT episodes of industrial and/or biomass burning where excess CO is coincident with expected excesses in O 3 . High ΔO 3 /ΔCO ratios (>0.50) are found in LRT episodes transported higher in the free troposphere and are probably due to a mixing of LRT pollution plumes with ozone‐rich upper tropospheric air. Using PHOBEA observations, backward trajectories, and data from other experiments in the North Pacific (TRACE‐P, ACE‐Asia, PEM‐West B) we calculate OH concentrations using two different methods. For the LRT episodes we obtain mean OH concentrations between 1.9 × 10 5 and 1.3 × 10 6 molecules cm −3 . We also present a method using dispersion models and observations to calculate the rate of dilution, k dil , with background air during LRT. A low k dil indicates less mixing with background air during transport, while a high value represents more entrainment with background air. For the April 2001 LRT episode we calculate a mean k dil of 0.010 ± 0.004 hr −1 and an OH radical concentration of 2 × 10 5 molecules cm −3 . On the basis of these calculations we find that the large mineral dust transport episode, which took place in April 2001, was associated with the lowest OH concentration of the 11 episodes considered, implicating a strong role for heterogeneous chemistry during LRT.