Long range chemical transport over North America at mid-tropospheric levels

We examine long-range transport of chemical species over North America that stems from the combination of extremely deep well-mixed layers over the Western U. S. and a persistent mid-tropospheric anticyclonic circulation. The mixed layers extend to the 500 mb level. The combined action of these large-scale features is one of dilution of the chemicals and their subsequent mid-level transport. A cursory examination of the historical record indicates that this synoptic regime occurs once every few years. In our investigation, observations from the National Weather Service's upper-air network are used to construct the trajectories. Mid-tropospheric air that begins its journey over the southwestern U. S. (near Albuquerque), tracks westward along the Mexico-U. S. Border, then continues on a track through the San Joaquin Valley of California before turning northeastward and ending up over the Reno-Lake Tahoe area two days later. From this point above the High Sierra, the air continues northeastward until it resides over Hudson Bay on day 5 — a trajectory that covers 5000 km in five days. Over a significant portion of this path, lower-tropospheric chemical species rise through the mixed layer and become entrained in the large-scale transport. Upon examination of products from the operational MM5 model (NCAR/Penn State Mesoscale Model - Version 5), we find that 24- and 48-h forecasts of the 500 mb wind field are faithful representations of the large-scale circulation. However, the forecast of thermodynamic structure in the lower troposphere exhibits some deficiency — namely, the deep adiabatic layers that are present in the initial conditions (in late afternoon, 00 UTC) are not retained in the 24- and 48-h forecasts. The model produces slightly stable layers throughout the troposphere. Although the root-mean-square error in the structure is relatively small (order of 1-1.5 C), the failure to identify these deep mixed layers is problematical. Further numerical experiments will be required to uncover the reasons for this deficiency. During our period of investigation, July 14-20, 2005, much of the Western U. S. experienced record-breaking high temperatures at the surface reporting stations. Yet, the synoptic situation we study is not typical of the pattern generally associated with extremely high surface temperatures — namely, that situation where the Pacific anticyclone intrudes into the northwestern U. S. with its limited cloud cover and low-level subsidence/ low-level inversion. We pay particular attention to the differences in the concentration of low-level ozone and other chemical species in our situation and that of the more-typical hot spell situation where the Pacific anticyclone intrudes onto the continent. The episode we study exhibited seasonal maxima of incident solar radiation at the surface, and as stated above, associated high temperatures. Yet ozone concentrations remained relatively low and the Federal ozone standards were not exceeded. The deep mixed layer provided an unusually large volume to dilute the emissions of ozone precursors. In general, lower peak ozone concentrations are associated with deeper mixed layers, but the decrease in peak concentration is not a linear function of depth. The absence of linearity stems from the fact that ozone production is more efficient at lower nitrogen oxide concentrations. Although it is probable that a greater mass of ozone is produced (due to the efficiency of its production at lower NOx concentrations), the surface concentrations remain low in the presence of dilution. The deep mixed layers lead to a situation where air pollution extends into the free troposphere. From there it is transported over synoptic and global scales. In such cases, there is potential to affect regional and global air pollution and potentially climate. Despite the relative low frequency of this event, air pollution managers should be cognizant of its characteristics. Knowledge of these characteristics should lead to a reduction in the number of false ozone warnings.