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.