Ambient PAN and PPN in southern California from 1960 to the SCOS97-NARSTO

We examine long-term trends in ambient levels of peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) in southern California from the first measurements made in 1960 to those made in 1997 as part of SCOS97-NARSTO. We analyze peak concentrations, 24-h averages and monthly averages. Although the data record is incomplete with respect to frequency and duration of measurements and is limited with respect to seasonal and spatial variations, peak concentrations have decreased from ca. 60–70 ppb to ca. 5–10 ppb for PAN and from ca. 5–6 to 1 ppb or less for PPN. Twenty-four-hour averages and monthly averages also show downward trends in ambient PAN and PPN. The PPN/PAN concentration ratios range from 0.10 to 0.28 and the limited data record give no indication of a long-term temporal trend. The downward trend in peak PAN concentrations is similar to that of peak ozone concentrations during the same 37-year period and is likely to result from increasingly stringent controls on emissions of oxides of nitrogen and of volatile organic compounds that are precursors to ozone, PAN, PPN and other photochemical oxidants.