Ozone-Aerosol Relationships in the Northeast Pacific During ACE-Asia

As part of the PHOBEA project we made observations of CO, NMHCs, O3 and sub-micron aerosol scattering during the ACE-Asia timeframe, both at the Cheeka Peak Observatory (CPO) in Washington State and using a small plane to get vertical profiles to 6 km. We have used these observations to better understand the ozone-aerosol relationship in this region. In addition, we can compare the results from the spring of 2001 with our earlier measurements taken during the springs of 1997 (CPO), 1998 (CPO) and 1999 (aircraft). The overall pattern for the ozone/sub-micron aerosol scattering coefficient is for a positive correlation in the free tropospheric data. The relationship is statistically significant in both the 1999 and 2001 datasets. The relationship is very weak or non-existent for the surface data. We interpret the positive relationship in the free troposphere to mean two things: (1) that upstream photochemical sources of ozone must be an important, perhaps dominant, source of ozone in the Northeastern Pacific troposphere during spring and (2) that the sub-micron aerosol scattering must be dominated by anthropogenic sources or the sub-micron aerosol must be, on average, co-transported, with anthropogenic pollutants. Two types of episodes tend to obscure the ozone-aerosol relationship: intrusions of upper tropospheric/lower stratospheric (UT/LS) air and episodes of high mineral dust. The UT/LS episodes have very high O3 mixing ratios and low aerosol scatter and the mineral dust episodes tend to show lower O3 then expected, even if elevated CO is present. However given that the bulk of the data shows a positive aerosol-ozone relationship, we conclude that these episodes represent "perturbations" to the more usual positive aerosol-ozone situation described above.