Observations show that the ratio of peroxypropionyl nitrate (PPN) to peroxyacetyl nitrate (PAN) increases to a maximum in the afternoon period at Boulder and at Niwot Ridge. In Boulder air and in easterly flow reaching Niwot Ridge, maximum average PPN/PAN ratios were near 15%, while in westerly flow or more remote continental air, the ratio was much smaller and average 4%. Relative to the PAN-mixing ratios, methyl nitrate (MN) was less than 0.2% in the afternoon and less than 0.9% at night in Boulder air masses. The sum of ambient n-propyl nitrate (NPN) and 2-butyl nitrate (2BN) mixing ratios was also small, the diurnal average at Niwot Ridge being about 2% of the PAN-mixing ratio. The diurnal average contribution of MN+PPN+(NPN+2BN) to the total odd nitrogen budge at Niwot Ridge was at most 4%, leaving a 21--27% shortfall between the sum of individual measurements of odd nitrogen species and the total odd nitrogen (NOy) measurement. ¿American Geophysical Union 1990