Figure 2:

Inferred changes in seasonal entrainment rates of aloft O₃ to the surface over the past two decades. Trends in observed average 8–9 am ground-level O₃ tendency in units of ppb hr⁻¹ yr⁻¹ during 1990–2010 at CASTNET monitoring locations are shown for (a) Spring (March-May) and (b) Summer (June-August). (c and d) same as (a and b) but based on model simulations. The hourly tendency at each monitor is estimated as the difference in measured O₃ at an hour and at the previous hour. Seasonal-averages of the estimated tendencies at 8:00am and 9:00 am (times when the net O₃ tendency peak) are computed for each year and the trends for the 21-year period are shown. Squares indicate high-elevation sites (altitude > 1000m ASL), while circles represent sites with altitude < 1000m ASL. Filled symbols represent locations where the estimated trends are significant at the 95% level, while open symbols are locations where they are not. These trends are only computed at CASTNET monitors that reported data for at least 11 of the 21-years. It should be noted that both the magnitude and directionality of the trend at individual locations can change if a different subset of years within the 21-year period or a different data completeness-criteria is chosen.