Figure 5. FLEXPART-WRF Biomass burning (BB) CO concentrations and age are simulated, binning trajectories over St. Louis within a 2.5˚ x 2.5˚ grid box and averaged vertically 500 m with daily temporal resolution. FLAMBE emissions of BB CO are converted into particle amounts and are released. (a) The base simulation, Boundary Layer Simulation, releases emissions within the PBL, (b) NRL’s adjusted pyro-convention scheme is implemented, PyroCb Simulation, particles are released from pyroCbs and high-altitude injection as specified in Table 1, and (c). The combined (a) and (b). With panels (d-f), corresponding to plume ages in simulations. The average BB CO g cm-3per grid cell (top) lighter shading indicated higher concentration and transported CO plume ages (bottom) lighter shading indicates older air.
The FLEXPART-WRF plume origin path (forward trajectories) has been used to calculate residence times of air parcels above St. Louis (Figure 5). Figure 5a-5f show the average residence times of the biomass burning plume origin paths expressed as plume age and CO tracer particle amounts as concentrations. Results are displayed for the BASELINE simulation (Figure 5a and 5d), PYRO simulation (Figure 5b and 5e), and combined (Figure 5c and 5f). The lighter shading represents a higher concentration of CO particle counts (Figure 5a-5c) and for plume age the lighter shading indicates older air (Figure 5d-5f). The major contributions below 3 km are from the central U.S. fires. The contributions from the intermountain west U.S. fires dominates above 3 km, in the range of 80 to 90%. Other regions contribute in the range of 5 to 10%. Biomass burning plumes are indicated to have impacted the surface ozone each day but with varying magnitudes. The surface impacts were connected to mechanisms causing air parcels to move downward (e.g. shortwave on 30 August). Likewise, strong vertical motion was evident in bringing simulated air within the boundary to the middle and upper portions of the troposphere. For example, on 24-26 August at the 8 to 12 km layer, both simulations indicate enhancements of 0.74 ppm of CO that has been aged ∼7 days. This enhancement corresponded to ∼40 ppb of excess O3 at the 11 to 12 km layer in the ozonesondes. Ozone impacts for August from biomass burning, sharply increase after 3 km and vary daily above; up to 12 km where the biomass burning contributions to ozone decrease to near negligible amounts. The high-altitude smoke injection simulation indicates that plumes that get injected above the 4 km height can impact the surface. Evidence of aged lofted air, ranging from 4 to 15 km, impacting the surface is seen 22 August to 1 September 2013, where a constant ∼2 ppm of CO enhancement at the surface is present. Additional evidence of biomass burning emissions enhancing ozone within the lower tropopause and PBL (0 to 6 km) is present on 21-24 August over the St. Louis area — a greater than 15 ppbv enhancement of ozone within a 6 day aged plume containing an additional 9 ppm of CO.
The average plume age for the August month ranged between 4 to 7 days. Plumes reaching the lower troposphere (below 3.5 km) were ~3 days fresher then plumes above. The simulated high-altitude smoke injection is typically ~2 days fresher than the boundary layer injected smoke when above 3 km but can be 2 days older when below. The average aged plume lead to 15 ppbv increase in ozone. As plumes aged, they became more ozone enriched. Fresher plumes have higher levels of CO but lower amounts of ozone. An example of this occurred 23 August, where and ozone exceedance at the surface was indicated in both simulations to be from a polluted air mass layer with mixed ages. From the lower and middle troposphere (below 1.5 km and above 5.5 km) the plumes ages averaged 2 days older, with about 7 ppm fewer CO, and 20 ppbv more ozone than the air in the layers in-between.
4.5. High-altitude smoke injection analysis
The impacts of high-altitude smoke injection are demonstrated using two examples from commonly encountered summertime synoptic meteorological situations in the central U.S. (i) cyclonic flow or a system of low pressure (e.g. cut-off low 17-21 August) and (ii) anti-cyclonic flow or a system of high pressure (e.g. blocking high 25-30 August). Table 3 lists each episode’s ozone enriched plume identified by an ozonesonde over St. Louis (Figure 2). For each identified ozone enhancement, the simulated source-contribution relationships from biomass burning (Table 1) and stratospheric intrusions (Table 2) are quantified. The corresponding travel time of the plume (or plume age) and concentrations are indicated by FLEXPART-WRF (Figure 5). St. Louis area for two selected days (21 and 30 August). Each plume was individually characterized with model simulations and ozonesonde observational data in addition to the meteorological conditions leading to the plume reaching St. Louis is discussed below.