Ozone photochemistry and the role of peroxyacetyl nitrate in the springtime northeastern Pacific troposphere: Results from the Photochemical Ozone Budget of the Eastern North Pacific Atmosphere (PHOBEA) campaign

In the spring of 1999 airborne measurements of NO, O-3, peroxyacetyl nitrat e (PAN), CO, CH4, H2O, volatile organic compounds, aerosols (particle count , light scattering, and light absorption), NO2 photolysis frequency, and st andard meteorological variables were made off the coast of Washington State as part of the Photochemical Ozone Budget of the Eastern North Pacific Atm osphere (PHOBEA) experiment. These measurements were used to constrain a ph otochemical box model to calculate the ozone photochemical tendency, T(O-3) , in this region. T(O-3) in marine flow from the remote Pacific was found t o be weakly ozone destroying from the surface up to 8 km. Values of T(O-3) increased from -0.83 ppbv d(-1) in the 0-2 km layer to -0.11 ppbv d(-1) in the 6-8 km layer. These results are compared to T(O-3) from other photochem istry experiments in the springtime Pacific. We also used the model to inve stigate the impacts of PAN decomposition on the mixing ratio of NOx (define d here as NO+NO2+NO3+2N(2)O(5)+ HNO2 +HNO4) and on T(O-3). PAN decompositio n was found to contribute from 11 to 30% toward NOx production and to enhan ce T(O-3) by 0.13 to 0.41 ppbv d(-1). The impacts of PAN decomposition were further investigated in a case study where measurements were made in a str ongly subsiding, air mass. In this air mass. PAN induced perturbations to N Ox and T(O-3) reached 20.1 pptv and 1.45 ppbv d(-1), respectively, more tha n three times that found in marine background average. Finally, we estimate how T(O-3) in the northeast Pacific atmosphere may change as a result of i ncreasing anthropogenic NOx emissions from Asia. The calculations suggest t hat while O-3 mixing ratios in the northeast Pacific are likely to increase , T(O-3) will remain close to its current value as a result of offsetting f actors.