Photochemical Ozone Budget of the Eastern North Pacific Atmosphere

PI: Dr. Dan Jaffe, University of Alaska

Three year project funded by the National Science Foundation, May 1996

NSF: ATM9529604


Observations of surface and free tropospheric ozone in the northern hemisphere show a spring maximum. Our previous measurements in Alaska and model simulations indicates that some of this peak can be explained by photochemical production from winter accumulated, anthropogenic, ozone precursors. In this regard, the most important ozone precursor is peroxyacetyl nitrate (PAN). Our measurements also show an important role for vertical transport in warming an airmass and causing thermal decomposition of PAN in the boundary layer. This anthropogenically caused ozone production may contribute to the long term trends in ozone which have been observed at several northern hemispheric locations during spring and summer. The largest ozone increases have been observed at Japanese stations, which is consistent with the rapid growth in industrial emissions from the Asian Pacific region.

To continue this work, we propose to make measurements of NOx, NOy, PAN, CO, O3, J_NO2, ROx radicals , aerosol number density, absorption (Bap), and light scattering (Bsp), meteorology and spectrally resolved UV radiation at a mountain top site at the tip of Washington state, Cheeka Peak during the spring of 1997 and 1998. In addition we will measure vertical distributions of NO, PAN, CO, O3 and aerosols from the University of Wyoming King Aircraft in the same region during the spring of 1998. During spring this region receives generally on-shore winds 60-70% of the time and samples air coming from the remote North Pacific atmosphere about 30-40% of the time. The work is to be conducted in collaboration with scientists from the University of Washington, NOAA's Pacific Marine Environmental Laboratory (PMEL) and NCAR. The goals of the project are:

1)To quantify the mixing ratio of these compounds in the North Pacific atmosphere;

2)To quantify the relationship between NOx, NOy, PAN, temperature, etc. in this region;

3)To quantify the ozone photochemical tendency and its influence on the ozone budget in the 1-D atmospheric column;

4)To identify synoptic conditions which give rise to airmass warming and therefore PAN decomposition, including both vertical transport and meridional advection.

5)To identify and quantify the influence of Asian emissions on airmasses which arrive to the west coast of the U.S.; and,

6)To quantify the influence of cloud types and heights on the O3-NOx photochemistry.


1. Measure PAN, NOx, NOy, CO, O3, J-NO2, Rn, NMHCs, aerosol number density, absorption (Bap) and light scattering (Bsp), and spectrally resolved UV during spring at the Cheeka Peak Observatory. If possible, we will also have measurements of peroxy radicals and OH from collaborative efforts by scientists at NCAR (Chris Cantrell) and Washington State University. During spring 1997 a preliminary campaign will be conducted which will include at least CO, O3, PAN, Rn and aerosol number density, Bsp and Bap. During the spring of 1998, all of the above measurements will be made as well as measurements of the vertical distribution of NO, O3, PAN, CO, and aerosols from the University of Wyoming King Aircraft.

2. Calculate the photochemical ozone budget using a 0-D and 1-D photochemical model constrained by observations of measured NO, NO2, O3, J-NO2, and J-O3 (derived from the UV measurements).

3. Compare the ROx radicals calculated from a photochemical model with concentrations derived from measured NO, NO2, O3, and J-NO2, and observations from direct measurements of ROx.

4. Identify specific synoptic situations which bring PAN rich air masses to the surface, including vertical transport and advection from the north. By using measured vertical profiles of temperature, winds and turbulence we will estimate the importance of vertical transport to the chemical composition measured at the surface. Using the surface measurements and the measured vertical distributions we will determine the importance of vertical transport to the ozone budget in this region, and the contribution of PAN from the free troposphere on the NOx budget at the surface. To do this we will parameterize the vertical transport in terms of an eddy diffusion coefficient, which is calculated from the Richardson number using Monin-Obukhov similarity theory. The Richardson number is derived from measurements of temperature and winds with height [Arya 1988] The eddy diffusion coefficient, along with measured vertical profiles of the species in question, can be used to calculate the vertical transport of those species. The eddy diffusion coefficients will also be used in a 1-D model to evaluate how the vertical transport affects the photochemistry and how the photochemical tendency for ozone varies with height.

5. Look for evidence of long range Asian anthropogenic sources in the springtime data. Evidence for this might include elevated concentrations of PAN, NOy, CO, O3, CN, Bsp- light scattering by particles and/or Bap-light absorption by particles, aerosol chemistry, isentropic back-trajectories and/or Rn.

6. Develop a climatology of cloud types and heights for the Cheeka Peak site for the time periods of the intensive campaigns. For each category of cloud type and conditions, quantify the influence on J-NO2 (from the direct measurements) and J-O3 (from the spectrally resolved UV data). Separating these by NOx concentration and modeled O3 production rates it should be possible to develop an overall evaluation of the cloud effects. In-cloud conditions are continuously monitored using forward scattering of infrared radiation. Cloud top and heights will be assessed from regional weather observations, satellite photos and our own observations.