Summertime Characteristics of the Atmospheric Boundary Layer and Relationships to Ozone Levels over the Eastern United States

Abstract

This paper examines the spatial and temporal distributions of the mixing height, ventilation coefficient (defined as the product of mixing height and surface wind speed), and cloud cover over the eastern United States during the summer of 1995, using the high-resolution meteorological data generated by MM5 (Version 1), a mesoscale model widely used in air quality studies. The ability of MM5 to simulate the key temporal and spatial features embedded in the time series of observations of temperature, wind speed, and moisture is assessed using spectral decomposition methods. Also, mixing heights estimated from the MM5 outputs are compared with those derived from observations at a few locations where data with high temporal resolution are available in the Northeast. In addition, the uncertainties associated with the estimation of the evolution of the boundary layer during the morning time are examined. The results indicate that nighttime mixing heights averaged <200 m, rising to 1 km by 10 EST, and to about 2.5 km in the afternoon. Ventilation coefficients followed a similar diurnal pattern, increasing from 500 m 2 /s at night to 15,000 m 2 /s in the afternoon; the increase due to the growing mixing height and increasing surface wind speeds. Spatial variability of these parameters was relatively small (coefficient of variation ¼ 0.25) at night and in the afternoon when conditions were quasi-stationary, but increased (to 0.5) during morning and evening hours when mixing heights and wind speeds were changing rapidly.Analyses of surface ozone observations from about 400 sites throughout the eastern United States indicate that days with numerous stations reporting surface ozone concentrations in excess of 80 ppb (i.e., ''high ozone'' days) generally had less daytime cloud cover, lower surface wind speeds, higher mixing heights, and lower ventilation coefficients than did comparable ''low ozone'' days. Such meteorological features are consistent with a synoptic anticyclone centered over the mid-south region (Kentucky, Tennessee). Low ozone days were characterized by more disturbed weather conditions (low pressure systems, fronts, greater cloud cover, and precipitation events). Ozone observations at two elevated platforms ($400 m agl) in Garner, NC, and Chicago, IL, indicated that ozone concentrations aloft were about 40% larger on ''high ozone'' days than on ''low ozone'' days. On average, high levels of ozone persist aloft for about 2 to 3 days. Strong vertical mixing in the daytime can bring this pool of upper-level ozone downward to augment surface ozone production. Since ozone can be transported downwind several hundred kilometers from its source region over this time scale, depending on upper-level winds, effective ozone control strategies must take into consideration spatial scales ranging from local to regional, and time scales of the order of several days.

Climate controls on air quality in the Northeastern U.S.: An examination of summertime ozone statistics during 1993–2012

“…Daily maximum air temperatures correlate well with ozone concentrations in the Eastern (Rao et al, 2003;Rasmussen et al, 2012) and Northeastern U.S. (Bloomer et al, 2009). Of all the individual meteorological variables, it is generally believed that temperature has the strongest relationship with ozone (Jacob and Winner, 2009).…”

“…Other meteorological variables related to ozone include wind speed (Vukovich, 1994;Rao et al, 2003;Chan, 2009), mixing layer height (Rao et al, 2003), boundary layer ventilation (Rao et al, 2003), surface pressure (Vukovich, 1994) and surface radiation flux (Vukovich, 1994;Rao et al, 2003). Atmospheric moisture content is another meteorological variable linked to surface-level ozone (Chan, 2009;Rao et al, 2003), as are frontal passages (Leibensperger et al, 2008) and stagnation events (Vukovich, 1995).…”

Associations between ozone and morbidity using the Spatial Synoptic Classification system

“…The chemical processes and meteorological conditions involved in ozone formation typically extend over spatial scales of hundreds of thousands of square kilometres [19,20], which suggests that ozone formation is best understood in the context of large-scale atmospheric patterns and not just in terms of local conditions.…”

Influences of boundary layer phenomena and meteorology on ambient air quality status of an urban area in eastern India

“…The vertical mixing coefficient is the measure of turbulent mixing within the boundary layer and gives an idea of vertical movement of the pollutants. It is actually mixing length times the velocity of wind within the mixing length, which can be defined as the depth, measured upward from the earth's surface through which pollutants are vigorously mixed (Rao et al, 2003). The vertical mixing coefficient determines the air pollution potential of an area.…”