Ozone air quality over North America: Part I - Review of reported trends

Ozone and precursor trends can be used to measure the effectiveness of regu latory programs that have been implemented. In this paper, we review trends in the concentrations of O-3, NOx, and HCs over North America that have be en reported in the literature. Although most existing trend studies are con founded by meteorological variability, both the raw data trends and the tre nds adjusted for meteorology collectively indicate a general decreasing tre nd in O-3 concentrations in most areas of the United States during 1985-199 6. In Canada, mean daily maximum 1-hr O-3 concentrations at urban sites sho w mixed trends with a majority of sites showing an increase from 1980 to 19 93. Mean daily maximum 1-hr O-3 at most regionally representative Canadian sites appears to decrease from 1985 to 1993 or shows no significant change. There are far fewer data and analyses of NOx and HC trends. Available stud ies covering various ranges of years indicate decreases in ambient NOx and BC concentrations in Los Angeles, CA, decreases in HC concentrations in nor theastern U.S. cities, and decreases in NOx concentrations in Canadian citi es. Two key needs are long-term HC and NOx measurements, particularly at ru ral sites, and a systematic comparison of trend detection techniques on a r eference data set.