A national analysis of weekday/weekend ozone (O 3 ) differences demonstrates significant variation across the country. Weekend 1-hr or 8-hr maximum O 3 varies from 15% lower than weekday levels to 30% higher. The weekend O 3 increases are primarily found in and around large coastal cities in California and large cities in the Midwest and Northeast Corridor. Both the average and the 95th percentile of the daily 1-hr and 8-hr maxima exhibit the same general pattern. Many sites that have elevated O 3 also have higher O 3 on weekends even though traffic and O 3 precursor levels are substantially reduced on weekends. Detailed studies of this phenomenon indicate that the primary cause of the higher O 3 on weekends is the reduction in oxides of nitrogen (NO x ) emissions on weekends in a volatile organic compound (VOC)-limited chemical regime. In contrast, the lower O 3 on weekends in other locations is probably a result of NO x reductions in a NO x -limited regime. The NO x reduction explanation is supported by a wide range of ambient analyses and several photochemical modeling studies. Changes in the timing and location of emissions and meteorological factors play smaller roles in weekend O 3 behavior. Weekday/ weekend temperature differences do not explain the weekend effect but may modify it.

A national analysis of weekday/weekend ozone (O 3 ) differences conducted using 1997-1999 data found that many urban areas experienced at least 5% higher 8-hr maximum O 3 concentrations on weekends than on weekdays even though emissions of precursors were significantly lower on weekends. This phenomenon was observed mostly in urban areas in the Northeast, Midwest, and coastal California. A similar analysis using 2008-2010 O 3 data shows that this phenomenon has mostly vanished. From 1997From -1999From to 2008From -2010, the percentage of U.S. monitoring sites that experienced 95th percentile daily 8-hr maximum average O 3 concentration on weekends that were 5% or more higher than on weekdays declined from about 35% to less than 5%. At the same time the percentage of sites that experienced higher weekday concentrations increased from 3% to about 27%. The majority (68%) of the sites, however, exhibited little sensitivity to the weekday/weekend emission changes as they had similar (AE5%) O 3 on weekdays and weekends. Similar trends were observed for the three other O 3 metrics examined: the 95th percentile of the 1-hr maximum and the April-September means of the 1-hr and 8-hr daily maxima. Over this time period, U.S. emissions of O 3 precursors declined significantly. However, a greater decline in nitrogen oxides (NO x ) emissions has caused an increase in the volatile organic compounds (VOC)/NO x emission ratios and it appears that this is the reason for the shift away from higher weekend O 3 concentrations.Implications: In areas where weekend emissions of ozone precursors are lower than on weekdays because of mainly lower motor vehicle emissions, an inadvertent test of ozone control strategies occurs. Such a test provides information on how control strategies that produce emission changes similar to those that occur on weekends affect ozone concentrations. In the late 1990s, lower NO x emissions on weekends resulted in higher levels of ozone in many urban areas. Emission controls that have been enacted since then appear to have eliminated that phenomenon in most urban areas. However, most areas now indicate that weekend emission reductions now have little effect on ozone concentrations at most sites.

Because the U. S. Environmental Protection Agency (EPA) has changed the National Ambient Air Quality Standards (NAAQS) for ambient particulate matter (PM), there is a great deal of interest in determining recent PM trends. This paper examines trends in PM 10 (i.e., particulate matter less than 10 micrometers in diameter) for areas of the United States based on their attainment status-for PM 10 and ozone nonattainment and attainment areas. The analysis also focuses on urban, suburban, and rural areas, and eastern and western areas. The time period of evaluation is from 1988 through 1995. To shed further light on the ambient PM 10 trends, trends in ambient SO 2 , NO 2 , and volatile organic compounds (VOCs) are also analyzed. Finally, trends in emission inventories of SO 2 , NO x , VOCs, and PM 10 are evaluated. Results of the analysis show that widespread and similar reductions in PM 10 levels have occurred over the last seven years. Annual reductions range from 3.0% to 3.8%, with the greatest reductions coming in PM 10 nonattainment areas, but with very significant reductions also in PM 10 attainment areas, ozone attainment areas, and rural areas. The widespread reductions appear to be due to a set of controls or common factors that are having a fairly uniform effect in all of the areas. The consistency of the reductions in different areas

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