Introduction
Wildfires in both forested and agricultural regions serve as a steady source
of pollutants into the atmosphere. Gas phase constituents such as methane
(CH4), carbon monoxide (CO), carbon dioxide (CO2), sulphur dioxide
(SO2), and nitrogen oxides (NOx; NO + NO2) can be produced
from burning of biofuels (Gadi et al., 2003; Radojevic, 2003), in addition
to precursors that induce ozone production (Jaffe and Wigder, 2012).
Additionally, wildfires produce large concentrations of aerosols which are
injected into the atmosphere or formed in the smoke plume via secondary
processes and include carbonaceous species (elemental and organic carbon)
(Park et al., 2003; Spracklen et al., 2007) and biogenic heavy metals
(including but not limited to Fe, Mn, Cd, Cu, Pb, Cr, and Ni) (Nriagu, 1989;
Radojevic, 2003). Soluble inorganic species such as sulphate, nitrate,
ammonium, and chloride are found in fire emissions and partitioned to the
particle phase through heterogeneous reactions with the gas phase species
released during the combustion process (Pio et al., 2008). Strong, turbulent
winds inside combustion zones from controlled and wild vegetation fires can
introduce considerable amounts of dust particles into the free troposphere,
which can subsequently be transported over thousands of kilometers with the
smoke (Clements et al., 2008; Ansmann et al., 2009; Baars et al., 2011).
Forest fires tend to be much larger than agricultural fires and enable
injection of smoke high into the free troposphere (Colarco et al., 2004;
Peterson et al., 2014). Yet smoke from agricultural and shrub- and
grassland fires can still be transported long distances. However, few
studies have documented how wildfires from any of the aforementioned biofuel
sources inject mineral dust into the atmosphere (Gaudichet et al., 1995;
Chalbot et al., 2013; Yang et al., 2013; Nisantzi et al., 2014),
particularly in heavily forested or agricultural regions such as the Pacific
Northwest of the US where dust sources are limited relative to arid
regions in Africa, the Middle East, and Asia. Prescribed burning (i.e.,
slash-and-burn techniques) and wildfires are common in these arid “dust
belt” regions, inducing the simultaneous emission of dust and smoke
(Streets et al., 2003; Pinker et al., 2010).
Aerosols produced directly from wildfires (i.e., carbonaceous and soluble
inorganic particulates) or injected into the free troposphere from smoke
plume dynamics (i.e., mineral dust) have diverse effects on climate and air
quality. For instance, absorbing aerosols such as soot from fires enhance
the semi-direct effect that affect cloud and atmospheric lapse rate,
particularly when the absorbing aerosols are above cloud (Ge et al., 2014).
Further, hygroscopic organic aerosol, sulphate, and nitrate can enable
aerosols to serve as cloud condensation nuclei (CCN) (Cruz and Pandis,
1997), whereas mineral dust and black carbon are effective ice-nucleating
particles (INPs) at sub-freezing temperatures (DeMott et al., 1999, 2003;
Vali et al., 2015). Both of these aerosol nuclei modify cloud
radiative properties and lifetime, impact precipitation formation, and have
been shown to originate from prescribed burns and wildfires (Eagan et al.,
1974; McCluskey et al., 2014). Enhanced pollutants from fires also severely
influence air quality and can prompt adverse health effects (Bravo et al.,
2002; Phuleria et al., 2005; Wiedinmyer et al., 2006). For instance, smoke
plumes from wildfires have been linked to childhood mortality (Jayachandran,
2008), asthma (Bowman and Johnston, 2005), and various respiratory illness
and diseases (Mott et al., 2002; Moore et al., 2006). These effects are
additionally complicated by aging from biogenic gases in the smoke plume
during transport. Further, previous air quality studies on the east coast of
the US have shown that enhanced aerosol optical depths (AODs) associated with
both wildfires and anthropogenic sources can cause large errors in
meteorological models used to forecast poor-air-quality events (Zamora et
al., 2005). Overall, the aerosol species emitted or formed from wildfire
plumes are complex in nature and possess several diverse climate and health
effects, thus demonstrating the need to better understand the various types,
sources, and transport pathways of these emissions.
Air quality is strongly dependent not only on emission sources such as
wildfires but also on weather and climate change (Jacob and Winner, 2009).
Regions with complex topography such as the Front Range of Colorado, US
(see Fig. 1), have unique meteorological phenomena
such as upslope/downslope flows that serve as agents for focusing or
cleaning out local air pollution from the Denver metropolitan area
(Haagenson, 1979). Typically, this region is characterized by good air
quality in terms of particulate matter (PM) relative to other larger urban
and industrial areas, although it experiences occasional pollution episodes
due to modulation of the mountain slope dynamics, oil and natural gas
production, and wildfires (Watson et al., 1998; Sibold and Veblen, 2006;
Brown et al., 2013). Here, we show that the Front Range air quality was
severely impacted by long-range-transported wildfire emissions from the
Pacific Northwest during August 2015. A reoccurring influx of smoke aerosols
infiltrated the Front Range region due to shifts in regional- and
synoptic-scale meteorology. Interestingly, mineral dust was also transported with the
smoke plume to the Front Range from the wildfires. This complex mixture of
aerosols can have numerous climate and health effects in the region and
should be evaluated to develop a better understanding of future influences
from wildfire emissions, especially considering a warmer and drier climate
will potentially lead to more frequent wildfires (Westerling et al., 2006;
Liu et al., 2010).
Map of monitoring locations, including NOAA DSRC in Boulder, which
housed the PX-375 and TOPAZ lidar instruments; the BAO, where the 449 MHz
wind profiler was deployed; downtown Denver; the CDPHE DESCI site, where
atmospheric extinction/visibility is measured; and the CDPHE sites where
PM2.5 and PM10 are monitored (see Table 1 for site
descriptions). The approximate area encompassing the Colorado Front Range is
highlighted by the dashed line. The Cheyenne Ridge in Wyoming is also
notated.
CDPHE sites used for particulate data within the Colorado Front
Range. Each site has an “×” for each measurement it maintained
throughout the current work. Elevation is provided in meters above mean sea
level (m a.m.s.l.).
City/site name
Site ID
Latitude
Longitude
Elevation
PM2.5
PM10
(∘N)
(∘W)
(m a.m.s.l.)
Boulder – CU/Athens
BOU
40.01
105.27
1621
×
Chatfield Park
CHAT
39.53
105.05
1685
×
Colorado College
CCOL
38.85
104.83
1833
×
Commerce City/Alsup Elementary
COMM
39.83
104.94
1565
×
Denver – continuous air monitoring site
CAMP
39.68
104.99
1610
×
×
Denver – National Jewish Health
NJH
39.74
104.94
1615
×
Fort Collins – CSU facilities
FTCF
40.57
105.08
1525
×
×
Greeley – hospital
GREH
40.42
104.71
1439
×
I-25 – Denver
I-25
39.73
105.02
1586
×
×
La Casa
CASA
39.78
105.01
1601
×
×
Longmont – municipal
LNGM
40.16
105.10
1517
×
Welby
WBY
39.84
104.95
1554
×
PM2.5: particulate matter with diameters ≤ 2.5 µm.
PM10: particulate matter with diameters ≤ 10 µm.
Methods
Satellite observations
The source of aerosols from the fires was determined using imagery from the
Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Terra
satellite. MODIS is a multi-spectral sensor with 36 spectral bands, ranging
in wavelength from 0.4 to 14.2 µm. AOD data at 550 nm from MODIS
were acquired from the Giovanni data server
(http://giovanni.gsfc.nasa.gov/giovanni/) for daily AOD at a 1∘
spatial resolution using a domain of 82 to 163∘ W and 26 to
59∘ N (MOD08_D3_051). MODIS AOD is retrieved from three spectral
channels (0.47 µm, 0.66, and 2.1 µm) using the algorithm
described by Kaufman et al. (1997) in
cloud-free pixels (10 km × 10 km grid box) (Ackerman et al.,
1998). Fire and surface thermal anomaly data were also acquired from the
MODIS Terra satellite using brightness temperature measurements in the 4 and
11 µm channels (https://earthdata.nasa.gov/labs/worldview/)
(Giglio, 2010). The fire detection strategy is based on absolute detection of
a fire (when the fire strength is sufficient to detect) and on detection
relative to its background (to account for variability of the surface
temperature and reflection by sunlight) (Giglio et al., 2003). The algorithms
include masking of clouds, bright surfaces, glint, and other potential false
alarms (Giglio et al., 2003). Swaths from overpasses over the Pacific
Northwest were used to determine the locations of fires on a daily basis.
In order to evaluate the types of aerosols present in enhanced AOD plumes
over the western US, aerosol subtype data were retrieved from Cloud-Aerosol
Lidar with Orthogonal Polarization (CALIOP) on board Cloud-Aerosol Lidar and
Infrared Pathfinder Satellite Observations (CALIPSO). Level-2 ValStage1 V.30
Vertical Feature Mask data obtained from NASA's Earth Observing System Data
and Information System (EOSDIS; https://search.earthdata.nasa.gov/)
contain vertically resolved data of aerosol layer subtype, including but not
limited to smoke, dust, and polluted dust (i.e., dust mixed with smoke)
(Vaughan et al., 2004; Omar et al., 2009; Winker et al., 2009). CALIPSO was
launched on 28 April 2006 and flies at an orbital altitude of 705 km as part
of the sun-synchronous “A-train” satellite constellation. CALIOP is an
elastic backscatter lidar operating at 532 and 1064 nm, completed with a
depolarization channel at 532 nm to enable detection of aerosols and clouds.
Granule data were acquired from orbital swaths that passed over the
northwestern US (domain includes Washington, Oregon, northern California,
Idaho, Nevada, Montana, Wyoming, Utah, and Colorado) from 15 August to
2 September 2015 and processed using modified Python code developed by the
Hierarchical Data Format (HDF) group at the University of Illinois,
Urbana-Champaign (http://hdfeos.org/). Aerosol subtypes were also
examined off the US west coast across the central North Pacific Ocean, in the
context of air mass trajectory analysis, to ensure mineral dust and smoke
were transported to Colorado from the Pacific Northwest fires rather than
from deserts or fires overseas.
Colorado air quality data
All air quality data were acquired from the Colorado Department of Public
Health and Environment (CDPHE;
http://www.colorado.gov/airquality/report.aspx) from 15 August to 2 September 2015 at
various sites throughout the Colorado Front Range (see
Fig. 1). The DESCI site (Denver Visibility
Station; 39.73∘ N, 104.96∘ W; 1633 m a.m.s.l.) is
highlighted in blue, near downtown Denver, where horizontal atmospheric
extinction (km-1) data measured with a transmissometer are available
through CDPHE. These data provide a quantitative measure of “haziness”
indicated throughout the text. Table 1 provides the
site latitudes, longitudes, and elevations, as well as which PM measurements were
available at each site. Hourly measurements included mass concentrations
(µg m-3) of particulate matter for particles with diameters ≤ 2.5 µm (PM2.5) and ≤ 10 µm (PM10). All times
shown are coordinated universal time (UTC; local time or mountain daylight
time (MDT) +6).
In situ aerosol observations at Boulder, Colorado
Real-time, hourly ambient aerosol samples were analyzed for PM2.5 mass
concentrations (µg m-3) and concentrations of various metals (ng m-3)
using the HORIBA, Ltd. PX-375 continuous particle mass and
elemental speciation monitor
(http://www.horiba.com/process-environmental/products/ambient/details)
from 26 August to 2 September 2015 at the National Oceanic and Atmospheric
Administration (NOAA) David Skaggs Research Center (DSRC) located in Boulder,
Colorado (39.99∘ N, 105.26∘ W, and 1672 m a.m.s.l.; see
Fig. 1). The PX-375 draws in air at 16.7 L min-1 through a US
Environmental Protection Agency (EPA) louvered PM10 inlet and
subsequently passes through a BGI Very Sharp Cut Cyclone
(VSCC™) to filter for particles smaller than
2.5 µm in diameter. Air is pulled through a nozzle for 60 min per
hourly sample, where particles are subsequently deposited in a 100 mm
diameter spot on Teflon™ polytetrafluoroethylene (PTFE) fabric filter
tape for analysis. Once the sample is collected for 60 min, beta-ray
attenuation and energy-dispersive X-ray fluorescence spectroscopy (EDXRF)
analyses are conducted for 60 min and 1000 s, respectively, per hourly
sample, simultaneous to the collection of the subsequent sample. Beta-ray
attenuation analysis is used to measure total PM2.5 mass concentrations,
and EDXRF is used to analyze concentrations of Ti, V, Cr, Mn, Fe, Ni, Cu, Zn,
As, Pb, Al, Si, S, K, and Ca. The EDXRF unit contains a complementary metal–oxide–semiconductor (CMOS) camera for
sample images. The calibration material used for X-ray intensity is National Institute of
Standards and Technology (NIST) Standard Reference Materials (SRM)
2783. Lower detection limits (LDLs) are shown in Table 2, and error was
calculated to be ±2 % for hourly metal concentrations. Hourly total
PM2.5 mass concentrations had an LDL of 2.00 µg m-3.
Lower detection limits (LDLs, ng m-3) for metals measured by
the PX-375 during 26 August–2 September 2015. Concentrations less than the LDLs were
excluded from analysis.
Species
LDL
Ti
2.29
V
0.23
Cr
0.61
Mn
0.93
Fe
1.51
Ni
0.33
Cu
0.78
Zn
1.21
As
0.02
Pb
0.80
Al
32.2
Si
5.17
S
1.11
K
4.37
Ca
1.18
Aerosol and ozone remote-sensing observations at Boulder,
Colorado
The Tunable Optical Profiler for Aerosol and oZone (TOPAZ) lidar was operated
at the DSRC on 9 days from 14 August through 2 September 2015, and it
collected about 62 h of ozone and aerosol profile data, primarily between
mid-morning and early evening local time. TOPAZ is a state-of-the-art,
tunable ozone differential absorption lidar. It emits pulsed laser light at
three ultraviolet wavelengths between 285 and 295 nm and measures ozone as
well as aerosol backscatter and extinction profiles with high temporal and
spatial resolutions (Alvarez et al., 2011). The TOPAZ lidar is mounted in a
truck with a rooftop two-axis scanner. This scanner permits pointing the
lidar beam at elevation angles between -5 and 30∘ at a fixed but
changeable azimuth angle. To achieve zenith operation, the scanner mirror is
moved out of the beam path. Typical TOPAZ operation consists of a scan
sequence at 2, 6, 20, and 90∘ elevation, repeated approximately every
5 min. The range-resolved ozone and aerosol observations at the shallow
elevations angles are projected onto the vertical and spliced together with
the zenith observations, resulting in composite vertical ozone and aerosol
profiles from about 15 m to 2–3 km above ground level
(a.g.l.) at 5 min
time resolution (Alvarez et al., 2012). In this study, we only used the lidar
aerosol extinction profiles measured at a wavelength of 294 nm. The aerosol
profile retrieval requires assumptions about the lidar calibration constant
and the aerosol extinction-to-backscatter or lidar ratio. For this study we
used an altitude-constant lidar ratio of 40 sr, which is a good
approximation for continental and urban aerosols. The lidar signal at the
aerosol wavelength of 294 nm is also affected by ozone absorption.
Therefore, uncertainties in the ozone observations can cause biases in the
aerosol retrieval. This, combined with uncertainties in the calibration
constant and lidar ratio, can lead to errors in the aerosol extinction
coefficient profiles of up to about 30 %. The precision of the 5 min
aerosol extinction measurements is typically better than 10 %.
Images of downtown Denver facing west taken at 14:00 UTC (08:00 MDT).
Images acquired from the CDPHE Visibility Station (DESCI; 39.73∘ N,
104.96∘ W; 1633 m a.m.s.l.). Only days of significant meteorological and
visibility transitions in August 2015 are shown. Days in red are those which
correspond to the haziest days during the study time period. In panels (a),
(d), and (f), the visibility of the foothills (and background high terrain)
is highlighted.
Meteorological data and analysis
A gridded perspective of synoptic-scale conditions across North America was
provided using the NOAA/National Centers for Environmental Prediction (NCEP)
Rapid Refresh numerical data package (RAP;
http://rapidrefresh.noaa.gov/; Benjamin et al., 2016). The RAP is an operational assimilation/modeling
system updated hourly, with 13 km horizontal resolution and 50 vertical
levels.
Air mass backward trajectory analyses were conducted using HYSPLIT 4 (HYbrid Single-Particle Lagrangian
Integrated Trajectory; Draxler and Rolph, 2011) and data from the NOAA/NCEP Global Data
Assimilation System (GDAS) (Kalnay et al., 1996). HYSPLIT trajectories do
not include processes that may affect particle concentrations such as
convective transport, wet removal, or dry removal, and they are only intended to
highlight the possible transport pathways. To study the potential for
transport from the Pacific Northwest fires region, and to eliminate
potential contribution from aerosol sources overseas, we used an ensemble of
backward trajectories initiated at multiple altitudes and times ending above
the NOAA building in Boulder. Ten-day back trajectories were initiated every
6 h (at 00:00, 06:00, 12:00, and 18:00 UTC) during 15 August–2 September 2015 at
500, 1000, and 2000 m a.g.l. (corresponding to 2172, 2672, and 3672 m a.m.s.l.).
A 449 MHz wind profiler (White et al., 2013) – deployed near the Boulder
Atmospheric Observatory in Erie, Colorado (BAO; 40.05∘ N, 105.01∘ W,
and 1577 m a.m.s.l.; location shown in Fig. 1) –
provided hourly-averaged profiles of horizontal wind. The high (low) mode
extended from 145 m (195 m) to 10 074 m (5059 m) a.g.l. with a vertical
resolution of 200 m (100 m). The wind-profiler data were edited objectively
using the vertical–temporal continuity method of Weber et al. (1993) and
then subjected to additional manual editing as needed. For the purpose of
this study, we utilized only the low-mode observations.
Results and discussion
Haze events induced poor air quality along Colorado's Front
Range
The shift in air quality was evident during three August haze events in the
Denver metro area. Figure 2 shows photos of notable
air quality transitions in Denver looking westward towards the foothills of
the Rocky Mountains, and Fig. 3 shows the
atmospheric extinction measurements from DESCI. Higher values of extinction
indicate hazier conditions. The image on 15 August shows typical clean
conditions, where the foothills were visible west of Denver. Extinction was
also relatively low on 15 August. On 17 August, a haze settled in the region,
creating a low-level pollution plume that masked the view of the foothills.
This haze continued to infiltrate the Denver metro area, reaching the
poorest visibility (i.e., highest extinction) on 23 August. This haze persisted
in the Denver metro area until 27 August, when clear conditions were
re-established and the foothills were once again visible. However, the air
quality deteriorated again by 29 August, with hazy conditions obscuring the
foothills. This haze event was shorter lived, clearing out on 31 August. The
cleaner conditions persisted until the end of the measurement period on 2 September.
The qualitative observations of the three separate haze events were
corroborated by in situ air quality measurements along the Front Range.
Figure 3 also shows hourly and daily averaged
PM2.5 mass concentrations (herein simply called “PM2.5”) at the
sites provided in Table 1. Overall, three separate
haze events occurred along the Front Range with the worst days visually
observed (Fig. 2) on 17, 23, and 29 August
(events 1, 2, and 3, respectively), when extinction was highest, PM2.5
reached maximum concentrations, and a cold front passed through (discussed
in Sect. 3.3). Prior to each of these events, PM2.5 was suppressed and
then slowly increased to each event's maximum concentrations on 17, 23, and
29 August. PM2.5 slowly decreased following each of these haze events.
PM10 (not shown) did not follow similar increases and decreases to the
PM2.5, suggesting the smaller particles contributing to PM2.5
originated from different, likely more distant sources as compared to
coarser particles contributing to the PM10, which are likely from more
local sources (VanCuren, 2003; Neff et al., 2008).
Biomass burning plume propagates towards Colorado
During the 15 August–2 September time period, fires in high-elevation (> 3000 ft a.m.s.l.)
forested areas and to some extent in shrub- and grasslands in
the Pacific Northwest were prominent, while few fire hot spots were located
in low-elevation agricultural land (see Fig. S1 in the Supplement). Figures 4–6
show MODIS retrievals of
fire hot spots and AOD during the first, second, and
third haze events in Colorado, when numerous fires were detected in
Washington, Oregon, northern California, northern Idaho, and northwestern
Montana. Three cases are defined as the time periods surrounding and
including the haze event days: case 1 (15–18 August), case 2 (20–23 August), and
case 3 (26–29 August).
Top panel shows atmospheric extinction measured at the CDPHE DESCI
site (see Fig. 1). Bottom panel shows hourly and daily averaged
PM2.5 mass concentrations at CDPHE sites. The pairs
of red dashed lines shows the times before (“B”) and after (“A”) cold-frontal
passages at BAO during or prior to each haze event. The daily averaged
PM2.5 in red represents the haziest days during or
following cold-frontal passages (i.e., events 1, 2, and 3 on 17, 23, and 29
August 2015, respectively).
Daily averaged aerosol optical depth (AOD; color bar, lower right)
at 550 nm and fire hot spots (black markers) detected by MODIS during the
first major haze case study between 15 and 18 August 2015. The haziest day from
the CDPHE data is labeled in red (i.e., event 1).
Same as Fig. 4 but for the second major haze event between 20 and
23 August 2015. The haziest day from the CDPHE data is labeled in red (i.e.,
event 2).
Same as Fig. 4 but for the third major haze event
between 26 and 29 August 2015. The haziest day from the CDPHE data is labeled
in red (i.e., event 3).
On 15 August, prior to the onset of the first haze event in Colorado, the plume
of enhanced AOD propagating from the fires in the Pacific Northwest remained
north of Colorado in Montana and southern Canada
(Fig. 4). The air above the Denver–Boulder area
contained relatively diminished AOD (0.12, averaged from the domain of
39.5∘ N, 104.5∘ W, 40.5∘ N, and
105.5∘ W). Although the core of the plume remained north of
Colorado, its more diffuse southern region drifted southeastward on 16 August.
By 17 August, enhanced AOD was observed along the Front Range in north-central
Colorado near Denver–Boulder (0.37). The AOD decreased slightly on 18 August
over Denver–Boulder (0.25), which is supported by the decrease of PM2.5
starting on 18 August from the CDPHE data (Fig. 3).
AOD increased in value and spatial extent on 20 August during the second haze
event, when more fires were detected in the Pacific Northwest (see increase
in number of MODIS hot spots in Fig. 5). This
plume contained a high density of aerosols that traveled over the
north-central US. The southern periphery of this plume impacted Colorado
east of the Continental Divide starting on 20 August, as corroborated by the
CDPHE air quality measurements in Fig. 3.
Although the AOD values were not as enhanced over Colorado as compared to
the core of the AOD plume, AOD values over the Front Range were enhanced as
compared to before the long-range transport of this plume. Enhanced AOD was
observed around Denver–Boulder and the Front Range the following 3 days
(0.26–0.35), with the largest values in this 4-day period observed on 23 August.
The third haze event (Fig. 6) followed a
similar evolution to the first two. The AOD plume remained north of Colorado
on 26–27 August and then infiltrated the northern and eastern part of the state
on 28–29 August. The AOD values over Denver–Boulder during this event
(0.26–0.45) were considerably larger than the two previous events. It is
important to note that AOD is a column measurement; thus the largest aerosol
concentrations may be elevated in the atmosphere as compared to what is
observed on the ground. However, the AOD observations still provide
information regarding the spatial extent of the plume of aerosols emitted
from the fires and show that Colorado was indeed impacted by air transported from
the Pacific Northwest fires.
Meteorological analysis for event 1 (17 August 2015). Top row shows
13 km resolution RAP gridded dataset of 500 hPa geopotential heights (black
contours) with 500 hPa wind velocities (flags: 25 m s-1;
barbs: 5 m s-1; half barbs: 2.5 m s-1) from before (a) and after (b) the
passage of a cold front at 06:00 and 21:00 UTC, respectively. Middle row shows
mean sea-level pressure (black contours) with near-surface wind velocities
(flags and barbs as above) from before (c) and after (d) the
cold-frontal passage. Standard frontal notation is used. (e) Ten-day air mass backward
trajectories initiated every 6 h at 500, 1000, and 2000 m a.m.s.l. during the
time period surrounding event 1 (15–18 August). Trajectories in red correspond
to the haziest day (17 August), and the blue dashed trajectories show the
remaining. (f) Time–height section of hourly-averaged wind profiles from the
449 MHz wind profiler at BAO between 06:00 UTC on 17 August and 06:00 UTC on 18 August
(flags and barbs are as above). The bold black line denotes the approximate
frontal shear boundary. The pair of red dashed lines shows the RAP analysis
times before (“B”) and after (“A”) the cold-frontal passage at BAO. Time
increases from right to left to portray the advection of upper-level synoptic
features from west to east.
Further, the satellite retrievals generally corroborate the air quality
observations on the ground along the Front Range in terms of when large
concentrations of aerosols might be expected. More fires were detected
across the Pacific Northwest by MODIS during the second event (678 fires, on
average), when PM2.5 was largest, as compared to the first event (231
fires, on average), which had the smallest maximum PM2.5 out of the
three haze events. The third event had PM2.5 values in between the
first and second, while also having 607 fires on average. Thus, the number
of fires likely influenced the relative amount of smoke produced and
transported to the Front Range. However, meteorological conditions as
described below also played a vital role in enabling transport of the smoke.
Synoptic- and regional-scale meteorology fuel long-range aerosol
transport from the Pacific Northwest
The transport of the enhanced AOD plume from the Pacific Northwest to
Colorado during each of the three events and the relationship between the
AOD column and ground-based in situ observations are supported by the
meteorological features present on both the synoptic and regional scales.
Plan-view synoptic analyses aloft and at the surface during the first air
quality event along Colorado's Front Range on 17–18 August 2015 are shown in
Fig. 7. At 500 hPa
(Fig. 7a and b), a transient shortwave trough
embedded in baroclinic zonal flow aloft migrated eastward across the
northern Rocky Mountains (i.e., north of Colorado), with westerly
(northwesterly) flow preceding (following) the passage of the trough axis.
These flow patterns are corroborated by the HYSPLIT air mass back
trajectories during the first event, shown in Fig. 7e. On average,
air mass back trajectories passed over the fire plume
region 40 % of the time; i.e., 19 of the 48 trajectories passed over
regions of enhanced AOD and fire hot spot locations from MODIS. At the
surface, high pressure and shallow cool air initially resided primarily
north of Colorado at 06:00 UTC on 17 August (Fig. 7c).
However, by 21:00 UTC on 17 August (Fig. 7d), the
shallow cool air moved southward across eastern Colorado. A companion
time–height section of hourly wind profiles at BAO
(Fig. 7f) shows low-level southerly flow ahead of
the frontal passage at ∼ 11:00 UTC on 17 August and generally
westerly to northwesterly flow aloft for the duration of the plot. The
observed flow aloft is represented in many of the back trajectories, which
show west-to-northwest flow reaching Boulder during this event. Following
the frontal passage at the wind profiler, the shallow cool air mass deepened
to ∼ 3 km a.m.s.l. by 18:00 UTC on 17 August in generally
northerly-component flow. Thereafter, the depth of the cool air decreased as
the low-level flow shifted to southeasterly. Operational rawinsonde data
from Denver (not shown) capture the top of the frontal inversion at 2.1 km a.m.s.l.
at 12:00 UTC on 17 August and at 2.7 km a.m.s.l. at 00:00 UTC on 18 August, consistent with
the wind-profiler analysis of the time-varying frontal altitude at BAO. For
plan-view context, the times of the synoptic analyses are marked on the
time–height section. The high PM2.5 values
(Fig. 3) on 17 August are corroborated by the
transition of air arriving from enhanced AOD regions (see air mass backward
trajectories in Fig. 7e) over and off the coast
of the Pacific Northwest and northern California
(Fig. 4c). PM2.5 increased markedly after
the passage of the shallow front, thus suggesting the postfrontal air
mass – which originated over Wyoming downstream of the Pacific Northwest
fires – contained a large concentration of particulates from those fires.
The evolution of the shallow cold front described above is typical of
southward-propagating cold fronts more generally across eastern Colorado,
and the frontal propagation is influenced heavily by the complex regional
topography depicted in Fig. 1. Specifically, the
blocking effect of the Rocky Mountains accelerates cold air southward along
the eastern side of the high terrain (e.g., Colle and Mass, 1995; Neiman et
al., 2001). Additionally, the postfrontal northerly-component airstream
flowing across the west–east-oriented Cheyenne Ridge in southeastern
Wyoming induces an anticyclonic gyre to the lee (south) of this ridge,
subsequently shifting the postfrontal flow from northerly to easterly and
driving the front westward against Colorado's Front Range (e.g., Davis,
1997; Neiman et al., 2001).
Same as Fig. 7 but for event 2 (23 August 2015). Before and after
the cold-frontal passage correspond to 18:00 UTC on 22 August and 12:00 UTC on 23 August,
respectively. Trajectories were initiated for the time period surrounding
event 2 (20–23 August). Time–height section measurements were between 17:00 UTC
on 22 August and 01:00 UTC on 24 August.
Same as Fig. 7 but for event 3 (29 August 2015). Before and after
the cold-frontal passage correspond to 18:00 UTC on 27 August and 18:00 UTC on 28 August,
respectively. Trajectories were initiated for the time period surrounding
event 3 (26–29 August). Time–height section measurements were between 13:00 UTC
on 27 August and 21:00 UTC on 28 August.
CALIPSO swath data from the night prior to event 1. Swath data
contained in the CAL_LID_L2_VFM_ValState1-V3-30 file are from 16 August 2015 at 09:57:00 UTC.
(a) Map showing CALIPSO coverage, with the purple markers representing
locations in the column measurement where dust, smoke, or polluted dust were
observed. (b) Vertical profile (in km a.m.s.l.) for all aerosol subtypes of the
swath corresponding to (a).
The meteorology during the second air quality event, on 22–23 August
(Fig. 8), was qualitatively similar to its
predecessor, although the transient shortwave trough aloft was more amplified
during the latter event (Fig. 8a and b).
Consequently, during the second event, the terrain-trapped cold front and
its trailing shallow cool air mass east of the Rockies surged much farther
southward across eastern New Mexico (Fig. 8c and
d). The corresponding air mass back trajectories
(Fig. 8e) traveled southeastward from the
Pacific Northwest fires to Colorado and passed over the fire plume region
96 % of the time, leading to the worst event along the Front Range in
terms of PM2.5 and total-column extinction
(Fig. 3). The wind-profiler analysis at BAO
(Fig. 8f) shows an abrupt low-level wind shift
from westerly to easterly with the frontal passage at 19:00 UTC on 22 August,
followed by a rapid deepening of the shallow cool air mass to nearly 3 km a.m.s.l.
Thereafter, the depth of this air mass ranged between ∼ 2.2 and 3.4 km a.m.s.l.
Nearby rawinsonde observations at Denver from 00:00 UTC on
23 August to 00:00 UTC on 24 August (not shown) document a strong frontal inversion
ranging between 3.3 and 3.8 km a.m.s.l., consistent with the wind-profiler
analysis. Above the shallow cool air mass, the profiler shows westerly flow
aloft, shifting to northwesterly with the passage of the transient
shortwave trough. The largest PM2.5 values observed during this event,
on 23 August, correspond to the most direct transport of air
(Fig. 8e) from over the enhanced AOD regions over
the Pacific Northwest fires (Fig. 5). As with the
previous case, the PM2.5 increased markedly with the passage of the
shallow front (Fig. 3). Significantly, air
quality was considerably poorer with the second event, perhaps due partly to
a stronger cold-frontal push across Colorado's Front Range that originated
near the smoke source region and partly due to northwesterly (rather than
westerly) flow aloft that could transport the smoke through a deeper layer
toward Colorado. Further, more fires were detected during the second event
(678, on average) compared to the first event (231 fires, on average); thus
the larger number of fires could result in more smoke production and thus a
denser smoke plume transported to the Front Range.
The synoptic-scale conditions on 27–28 August (Fig. 9) associated with the
third air quality case differ considerably from
those of the two earlier events. Most significantly, a broad ridge aloft
covered the intermountain West for the duration of this final event, while an
embedded weak shortwave trough migrated eastward through the ridge from
Wyoming–Colorado to the Great Plains (Fig. 9a and
b). A surface reflection of the upper-level shortwave trough was manifested as
a weak low-pressure center over western Nebraska and Kansas at 18:00 UTC on 27
August (Fig. 9c). This low migrated eastward during
the subsequent 24 h (Fig. 9d) in tandem with the
upper-level shortwave. Because this surface low resided beneath a mean ridge
aloft, the temperature contrast across this trailing cold front was weaker
than its earlier counterparts (not shown). Nevertheless, the southward
migration of the front east of the Rockies suggests that terrain blocking
may have influenced its evolution. The air mass back trajectories show
parcels originating from the region of the fires and enhanced AOD 85 % of
the time, similar to the trajectories from the earlier two events
(Fig. 9e). Companion observations from the BAO
wind profiler (Fig. 9f) capture the shallow
frontal passage at 20:00 UTC on 27 August, when westerly flow shifted abruptly to
northerly. Above 3 km a.m.s.l., the wind field exhibited a more gradual transition
from westerly to northwesterly as the weak shortwave trough moves across
the wind profiler. The Denver rawinsondes at 00:00 and 12:00 UTC on 28 August
observed a frontal inversion at ∼ 2.1 km a.m.s.l. (not shown). It
was less prominent than the frontal inversions during the earlier events,
largely because the temperature contrast across this front was weaker than
its predecessors. The subsequent rawinsonde profile at 00:00 UTC on 29 August (not
shown) captured a deep, dry-convective boundary layer extending up to 4 km a.m.s.l.,
despite persistent low-level northerly flow. Sensible heating eroded
the remnant low-level cool air east of the Rockies. PM2.5 increased
following the initial shallow cold-frontal passage at 20:00 UTC on 27 August and
continued to increase for the remainder of the wind-profiler time–height
section, as deep northerly-component flow behind the weak shortwave trough
transported smoke particulates across Colorado.
Aerosol extinction profiles at 294 nm observed with the TOPAZ lidar
on 9 days during the smoke pollution episodes. The numbers next to each day's
observations represent the daily mean AOD from the surface up to
2.5 km a.g.l. computed from the lidar measurements.
Mineral dust and smoke arrive along the Front Range
The types of aerosols present in the enhanced AOD plumes that were
transported towards the Front Range via the aforementioned synoptic
conditions were evaluated using additional satellite-based measurements and
support the interpretation of transport of aerosols from the wildfires in
the Pacific Northwest to Colorado. Figure 10 shows
aerosol subtype data from the CALIPSO satellite in planar (a panel) and
vertical-profile (b panel) views during event 1. CALIPSO data were
strikingly similar for events 2 and 3 and are provided in the Supplement. Only the
worst day or the day prior to the worst day of each haze
event is shown, although aerosol subtype data were examined anytime CALIPSO
passed over the Pacific Northwest or Colorado from 15 August to 2 September. CALIPSO
demonstrates the presence of smoke, dust, or polluted dust (dust mixed with
smoke in each profile) during times that intersect the enhanced AOD plume
propagating from the Pacific Northwest or when over Colorado. Dust and smoke
plumes from the fires extended up to 10 km a.m.s.l. over the western US. The
mineral dust and smoke detected by CALIPSO in transit to the Front Range were
also detected with the TOPAZ lidar and the in situ aerosol particle mass and
speciation monitor at the DSRC. Figure 11 shows
aerosol extinction profiles from the surface to 2.5 km a.g.l. measured with the
TOPAZ lidar on 9 days during the smoke episodes. The time resolution of the
extinction profiles is 5 min, and the vertical resolution is 1 m at the
lowest altitudes, increasing to 6 m above 500 m a.g.l. The observations on 14
August and 2 September, which bracket the smoke episodes, indicate very clean
conditions with AOD from the surface up to 2.5 km a.g.l. (AOD2.5km) of
0.05 and 0.04, respectively. Aerosol extinction coefficients and
AOD2.5km were significantly larger during the smoke episodes with an
approximately sevenfold increase in AOD2.5km on 20 and 21 August. This time
period also corresponds to increasing extinction at DESCI (Fig. 3).
Aerosol extinction was enhanced over the entire 2.5 km column, but the
largest aerosol extinction values were observed in the boundary layer in the
lowest few hundred meters up to 1.5 km a.g.l. Also, the lidar measurements
reveal that on most days aerosol extinction varied significantly over the
course of the day (e.g., 20 August). The largest aerosol extinction values,
around 1–1.5 km a.g.l., observed on 19 August were primarily due to swelling of
aerosol particles in the moist relative-humidity environment beneath cumulus
clouds at the top of the boundary layer. However, aerosol extinction in the
lower part of the boundary was still significantly larger than on 14 August,
which is consistent with the larger aerosol particle concentrations in the
smoke plumes. The lidar measurements are consistent with the atmospheric
extinction measurements from DESCI and the in situ PM2.5 and MODIS AOD
observations. When comparing lidar AOD2.5km with MODIS AOD, one has to
be cognizant of the fact that the TOPAZ observations only cover a portion of
the atmospheric column and that the two AOD measurements were made at
different wavelengths. A comparison between the near-surface TOPAZ and DESCI
extinction observations also needs to take into account that the
measurements were made at different wavelengths.
Figure 12 shows the time series of PM2.5, soil
mass concentrations, and elemental mass concentrations (data from the PX-375
were not available prior to this time period due to instrumental
complications). Soil concentrations were calculated by following the
Interagency Monitoring of Protected Visual Environments (IMPROVE) convention
using concentrations of specific metals: soil = 2.2[Al] + 2.49[Si] + 1.63[Ca] + 2.42[Fe] + 1.94[Ti]
(Malm et al., 1994; Hand et al., 2011).
Both PM2.5 and soil mass concentrations increased during the worst haze
event days (i.e., 26 and 29 August), when the Pacific Northwest fires were
influencing air along the Front Range and when CALIPSO showed the presence
of smoke and dust over the western US. The diurnal pattern is likely caused
by the upslope/downslope flow patterns due to proximity from the base of the
foothills, which is particularly pronounced in the summer (Toth and Johnson,
1985). Further, select metals also increased in concentration during haze
events, particularly those typically sourced from mineral dust (i.e., in the
IMPROVE soil convention equation) and S and K, which are metal tracers that
have been observed in smoke or biomass burning aerosols originating from
fires (Artaxo et al., 1994; Gaudichet et al., 1995; Yamasoe et al., 2000;
Pachon et al., 2013). It is important to note that K may also originate from
soil. We calculated the soil K and non-soil K based on the methods of
Kreidenweis et al. (2001), which are shown in the Supplement.
Concentrations of both soil K and non-soil K were highest during the
influence from the fires. Additionally, IMPROVE measurements at the Rocky
Mountain National Park location showed higher concentrations of soil, S, and
K during event days in August, corroborating our measurements (see
Supplement).
(a) Time series of hourly PM2.5 and soil mass concentrations as
measured by PX-375 between 27 August and 2 September 2015 and (b) hourly mass
concentrations of select individual metals relative to their maximum
concentration observed during the study time period, including an error of
±2 %. Only data higher than the LDLs are shown. PX-375 data
overlapped with event 3.
Averages of (a) PM2.5 and soil concentrations and
(b–e) select metal mass concentrations during non-event days (i.e., cleaner
conditions) compared to averages from haze event days (i.e., influence from
fires haze) for 26 August–2 September 2015. “Low” and “high” correspond to the
PM2.5 concentration values. Error bars represent the 90 % confidence
intervals. Concentration averages were statistically significant based on
t tests of two samples of unequal variances.
Figure 13 shows the average concentrations of
mineral dust or biomass burning metal tracers from the PX-375 from 26 August to
2 September, during conditions influenced by the Pacific Northwest fires (days
with enhanced PM2.5, 29–30 August) and days with cleaner, normal Front
Range conditions (days with low PM2.5, remaining days during this time
period). PM2.5 and soil mass, biomass burning metal (S and K), and
mineral dust marker (Al, Si, Fe, and Ca) concentrations were all larger, on
average, during influences from the Pacific Northwest fires, corroborating
the CALIPSO observations. It is important to note the possibility that some
small concentration of Ca, Al, and Fe could also originate from biomass
burning, although the apportionment of this source remains in question, and
their contribution from biomass burning aerosol is likely minor in
comparison to their concentrations in mineral dust (Chang-Graham et al.,
2011). Also included are metals that are typical of industrial tracers As
and Pb (Fig. 13e) (Paciga and Jervis, 1976;
Hutton and Symon, 1986; Thomaidis et al., 2003), which were actually lower
in concentration during influences from wildfires and enhanced during
normal, regionally sourced influences. The average PM2.5 mass
concentration from the CDPHE data was almost 3 times larger on 29–30 August as
compared to the remaining days in the 26 August–2 September time period (15.9
vs. 5.7 µg m-3, respectively). This result demonstrates how
influences from typical regional industrial sources is disrupted by the
synoptic conditions that introduced the long-range-transported biomass
burning plumes. Although Zn and Cu have been shown to originate from
wildfires (Yamasoe et al., 2000), the averages were similar – within
1 ng m-3 – and thus a distinct comparison could not be made within certainty.
Further, these metals can also be derived from vehicular emissions; thus
their concentrations may additionally be influenced by local traffic
(Sternbeck et al., 2002). These results demonstrate the transport of mineral
dust and biomass burning aerosol species to the Front Range, which were
indeed larger in concentration during poor-air-quality/haze events.
Interestingly, mineral dust mixed within a smoke plume from fires has
predominantly been observed originating from more arid regions along the
global dust belt and using modeling or remote-sensing data only (e.g.,
Radojevic, 2003; Tesche et al., 2009; Yang et al., 2013; Nisantzi et al.,
2014). To our knowledge, this co-lofting of dust and smoke has not been
shown to occur in the US, particularly in a region as densely covered in
vegetation as the Pacific Northwest.
Conclusions
We have demonstrated the transport of mineral dust and smoke/biomass burning
aerosols from wildfires in the Pacific Northwest to the Colorado Front Range
using a combination of in situ, remote-sensing, and air parcel modeling
techniques (M. Severijnen, personal communication, 2015). These aerosols were
transported under synoptic conditions that contributed to three different
haze events, inducing poor air quality in the Denver metro area. Three
separate poor-air-quality events with enhanced PM2.5 were likely
dependent on the number of fires and observed to occur with cold-frontal
passages along Colorado's Front Range, enabling the enhanced AOD plumes
originating from the Pacific Northwest wildfires to propagate southeastward
to Colorado's Front Range. Air masses were shown to originate from over the
region dense with wildfires and followed through satellite-detected aerosol
plumes, which were rich in a mixture of mineral dust and smoke. Tracers for
these aerosol types were also detected in situ along the Front Range and were
shown to be enhanced during periods of influence from the fires.
Overall, these unique observations were demonstrated using a complete suite
of in situ and remote-sensing aerosol measurements in the context of in situ
meteorological observations and air mass trajectory modeling. In tandem, we
utilized a real-time X-ray fluorescence spectroscopy technique using the
novel and field-portable PX-375 from HORIBA, Ltd., demonstrating the utility
of the instrument. Although the haze events were short lived, they
demonstrate how quickly (i.e., on the order of 2 to 3 days from the fire
region to the Front Range) aerosols can be transported long distances and
affect air quality in regions thousands of kilometers away. Interestingly,
mineral dust was observed to be co-lofted and transported within the smoke
plumes, an observation not previously reported for vegetated regions such as
the Pacific Northwest.
Mineral dust and smoke aerosols have disparate implications for health and
climate, particularly at the levels observed along the Front Range. These
unique observations should be taken into account when developing health
standards, seeing as not only regional urban and industrial emissions
contribute to poor air quality conditions. Additionally, dust and smoke are
efficient cloud-forming nuclei – which impacts cloud lifetime, radiative
effects, and precipitation formation mechanisms – particularly when
orographically lifted along barriers such as the Front Range into the upper
atmosphere, where cloud formation is prominent. Thus, transport of these
aerosols from wildfires has broad implications for altering aerosol
composition in regions far from the source.