Continuing development in the regime of decreased atmospheric column OH at Fritz Peak, Colorado

A new regime of reduced hydroxyl (OH) column abundances at Fritz Peak Observatory, first observed in the fall of 1991, continues to develop. Beginning in 1991, morning OH abundances in the fall season at Fritz Peak (40°N, 105°W) were found to be about 15% lower than the previous 1980–1990 average. Measurements for July–October 1997 indicate a continued regime of depressed (25–30%) OH abundances. The August‐October afternoon abundances were also low in 1997, yielding a PM‐AM asymmetry of about 6% which is near the annual average for Fritz Peak. This is the first occasion since 1980 that the fall PM‐AM difference of about 35% has not appeared. The winter season maximum of 15%, a persistent feature of the 1980–1990 measurement record, also failed to appear in 1996–97.