Peroxyacyl nitrate (PAN) is a critical atmospheric reservoir for
nitrogen oxide radicals, and plays a lead role in their redistribution in
the troposphere. We analyze new Tropospheric Emission Spectrometer (TES) PAN
observations over North America from July 2006 to July 2009. Using aircraft
observations from the Colorado Front Range, we demonstrate that TES can be
sensitive to elevated PAN in the boundary layer (
PAN is considered to be the largest reservoir for nitrogen oxide radicals
(NO
In situ observations from aircraft show rapid conversion of NO
Aside from a handful of long term observational datasets (e.g., Brice et al.,
1988; Pandey Deolal et al., 2014; Fischer et al., 2011; Tanimoto et al.,
2002; Mills et al., 2007), much of our understanding of the distribution of
PAN outside urban areas rests on data from aircraft missions interpreted with
global chemical transport models (Alvarado et al., 2010; Fadnavis et al.,
2014; Fischer et al., 2014; Pope et al., 2016). Given the limited set of
long-term in situ measurements, satellite measurements are a potential tool
that can be used to investigate the seasonal cycle and interannual
variability of PAN in the troposphere along with which processes contribute
to these features. Limb-sounding satellite instruments have provided global
distributions of PAN in the upper troposphere and lower stratosphere
(Glatthor et al., 2007; Moore and Remedios, 2010; Ungermann et al., 2016;
Wiegele et al., 2012). Analysis of new observations of PAN from the
Tropospheric Emission Spectrometer (TES) can be used to look lower in the
troposphere (Payne et al., 2014). TES PAN observations confirm the important
role that high latitude fires play in the interannual variability of PAN
during spring at high latitudes (Zhu et al., 2015), support estimates of the
role of PAN in the transpacific transport of O
Here we present an analysis of TES PAN observations over North America during the month of July between 2006 and 2009. We focus on understanding the contribution of smoke to enhanced PAN by segregating TES PAN retrievals based on smoke impact through comparisons to NOAA Hazard Mapping System (HMS) smoke plumes.
TES is a nadir-viewing Fourier transform spectrometer that measures thermal
infrared radiances at a high spectral resolution (0.1 cm
Specific details of the TES PAN retrieval algorithm are provided in Payne et
al. (2014). TES PAN retrievals are being processed routinely for the whole
TES dataset and are publicly available in the TES v7 Level 2 product. The
retrievals use an optimal estimation approach (Bowman et al., 2006; Rodgers,
2000) . An important diagnostic output of the optimal estimation retrieval is
the averaging kernel (
Simulated TES PAN retrievals for four different hypothetical
conditions where the black dashed line shows the prior, the two red lines
show two different true profiles, and the two blue lines show the retrieved
profiles. The true profile exhibits a maximum in the volume mixing ratio
(vmr) close to the surface
in the upper panels (
At the time of this work, the v7 product was not yet available. The TES PAN
retrievals shown here were processed using a prototype algorithm for the area
and time periods of interest. The v7 PAN algorithm was built from this
prototype, using the same state vector representation, microwindows, and prior
constraints. The a priori profiles are based on GEOS-Chem simulations for
the year 2008, with six possible prior profiles for any given month, as
described in Payne et al. (2014). We have verified, using a subset of v7 data
processed so far, that v7 retrievals are consistent with those from the
prototype. On a single footprint basis, TES is capable of measuring elevated
PAN (detection limit
Figure 2 shows the July 2006–2009 tropospheric average PAN. Because of the
lack of vertical information, we define the tropospheric average for a given
retrieval as the average retrieved PAN between 800 hPa and the tropopause.
The PAN spectral feature at 1140–1180 cm
Average tropospheric PAN in retrievals (DOF > 0.6)
during July 2006–July 2009
When all the existing TES data is gridded (Fig. 2b), there are several large
patterns that emerge. (1) Average tropospheric PAN mixing ratios in the TES
observations generally increase with latitude during the month of July over
North America. (2) Average tropospheric PAN mixing ratios generally decrease
from west to east. (3) As can be seen in later figures, there are relatively
few retrievals per grid box over the southwestern US Though there are
relatively few samples (
The peak sensitivity for PAN is generally between 400 and 800 hPa (Payne et al., 2014), but a comparison between TES PAN transect observations coincident with Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) observations (Fig. 3) show that TES can have some degree of sensitivity to PAN in the boundary layer when boundary layer PAN is elevated. As an example, Fig. 3 presents in situ observations from a flight during FRAPPÉ made with a thermal dissociation–chemical ionization mass spectrometer (TD-CIMS) (Zheng et al., 2011). Mean PAN observed by the C-130 below 3 km during the field campaign was 481 pptv (Zaragoza et al., 2017). This particular day (29 July) was one of the four days identified by Zaragoza et al. (2017) with the highest surface PAN mixing ratios observed at the Boulder Atmospheric Observatory. The overlaid TES data in Fig. 3a (parallelograms) show an enhancement in the TES PAN (as shown by the TES observation highlighted by a black square) in the vicinity of aircraft measurements of highly elevated PAN values in the boundary layer, indicating that in this case TES is weakly sensitive to the elevated boundary layer values despite the presence of high clouds (dashed line Fig. 3c). Figure 3 also shows red and blue lines corresponding to the application of the averaging kernel for this case to hypothetical ”true” profiles with and without the enhancement in the boundary layer. The red and blue lines show that TES has some sensitivity to PAN located at altitudes below 800 hPa, but the retrieval places the additional PAN higher up in the atmosphere. While the difference between the red and the blue solid lines in Fig. 3d is small, it is non-zero indicating that TES has some sensitivity to the boundary layer enhancement in this case.
For the analysis presented below, we use PAN observations from TES over North
America in July, from 2006 to 2009. We only include data with
DOFS > 0.6. More specifically, this threshold value of
DOF > 0.6 was chosen to be consistent with a signal-to-noise
ratio (SNR) greater than 1 (Payne et al., 2014), and this criteria has been
used in all the papers that have presented TES PAN data thus far (Jiang et
al., 2016; Payne et al., 2017; Zhu et al., 2015, 2017). This conservative
choice means that we are primarily basing our analysis on retrievals with
high PAN. The mean (standard deviation) of the retrieved tropospheric average
PAN mixing ratios for DOFS > 0.6 for the region shown in the
figures presented here (125–70
We segregate the TES PAN retrievals by whether or not the TES footprint
coincides with a smoke plume identified by the NOAA Hazard Mapping System
(HMS). NOAA HMS is an interactive satellite image and graphics system
developed by the National Environmental Satellite, Data, and Information
Service (NESDIS). Using satellite imagery, trained analysts identify the
geographic extent of smoke plumes in the atmospheric column over North
America (Rolph et al., 2009; Ruminski et al., 2006). Visible-band
geostationary (
Plumes are analyzed multiple times on a given day and can be nested. For this work all overlapping plumes (either nested or analyzed at different times) are merged into a single plume. This dataset does not contain information about the vertical location or depth of smoke in the atmospheric column. As discussed in Brey et al. (2018), the number and extent of smoke plumes in this HMS dataset is a conservative estimate. In particular, it becomes challenging to identify smoke as it dilutes during transport or mixes with anthropogenic haze. Thus our estimate of the number of PAN retrievals impacted by smoke may be a lower bound. For this work, we follow the overlap methods described in Brey et al. (2018). We matched all TES PAN retrievals based on UTC day. This means that overnight retrievals are paired with the plume from the prior day. As discussed in Brey et al. (2018), most of the large wildfire plumes occurring in July over the western US are very large and last several days. So we would expect that pairing the overnight retrievals with the plume from the prior day (i.e., matching based only on UTC day) is not likely to change our results, and that is the case. We have repeated all our calculations using only the daytime retrievals, and the choice to use all the retrievals does not change the results.
Panels
As part of a case study presented in Sect. 3.3, we use the Hybrid
Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model (Draxler and
Hess, 1998)
(
The first four panels of Fig. 4 show the spatial distribution of TES PAN retrievals over the US and southern Canada for the month of July 2006 to 2009. All retrievals plotted in this figure have DOF > 0.6. The retrievals are colored red when they fall within a NOAA HMA smoke plume. A large fraction of the TES retrievals (15–32 %) during this month overlap smoke plumes; the largest percentage of retrievals associated with smoke occurred in July 2008 (32 %), though this year does not display a high percentage of detection compared to other years and the average tropospheric PAN measured by TES is not larger than other years (Fig. S2). Of all the retrievals attempted in July 2006 to July 2009, 18 % were associated with smoke. We expect a higher fraction of overlap in the subset of data with DOF > 0.6 (28 %). This threshold value of DOF > 0.6 is consistent with a signal-to-noise ratio greater than 1 (Payne et al. 2014), and this subset of data only reflects conditions with elevated PAN in the atmospheric column. The number of major wildfires over the US has large seasonal and interannual variability (Brey et al., 2018). Wildfires in summer 2008 were particularly intense over California associated with record-breaking lightning and aggravated drought. Fig. 4c shows a cluster of TES PAN retrievals over California associated with this smoke. The dense smoke, which spread substantially downwind, was sampled from the NASA DC-8 aircraft as part of the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS-CARB) campaign (Hecobian et al., 2011; Singh et al., 2010, 2012), and we show this data in Sect. 3.3. Elevated smoke was also observed at surface sites downwind throughout the month of July (Gyawali et al., 2009). As part of ARCTAS-B, Alvarado et al. (2010) also documented major PAN enhancements in fresh wildfire plumes sampled over Canada during July 2008. July 2008 was additionally associated with special observations from TES, providing a relatively high number of attempted retrievals this month (red line in Fig. S2). Figure 4f presents the seasonal transition for 2006 in smoke-plume polygon overlap from late spring (May) to early autumn (September). During this example year, the percentage of TES PAN retrievals with DOF > 0.6 associated with smoke peaked in July (20 %), but Fig. 4e suggests that this was not a notably high percentage of smoke-impacted retrievals. A much higher percentage of DOF > 0.6 retrievals were smoke-impacted in July 2008.
Panels a and b of Fig. 5 show the distribution of tropospheric average TES
PAN in the subset of retrievals overlapping HMS smoke plume polygons in
July 2006–2009. The distributions of tropospheric PAN in the subset of
retrievals with DOF > 0.6 is not different between the in-smoke
cases (leftmost red box plot in Fig. 5a) and the not-in-smoke cases
(blue-grey box plot in Fig. 5a). The choice to only include data with
DOFS > 0.6, pushes the median tropospheric average PAN
substantially higher than using all the available TES data. Thus the percent
of retrievals impacted by smoke shown in Fig. 4 reflects only situations with
substantially elevated PAN in the atmospheric column. Imposing an additional
cloud optical depth filter does not substantially change the distribution of
tropospheric average PAN (see Fig. S4). The other two red distributions in
Fig. 5a reflect additional criteria designed to ensure that the PAN
associated with smoke in the atmospheric column exists in the free
troposphere where we expect TES to be most sensitive. We show the PAN
distribution for in-smoke cases that also coincide with TES 510 hPa
CO > 120 ppbv and TES 510 hPa CO > 150 ppbv. As
discussed further in Sect. 3.3, background CO in July in the northern
mid-latitudes is expected to be
Figure 5c and d present the distribution of tropospheric mean CO associated
with the successful PAN measurements. There is higher CO associated with TES
retrievals that overlap HMS smoke polygons (median
TES observations allow measurements of smoke plumes over North America at
various ages, even in the same day. Figure 6 shows the spatial distribution
of TES retrievals with DOF > 0.6 over the US and southern
Canada for the month of July 2006 to 2009 that overlapped HMS smoke plume
polygons. These points are the red colored retrieval locations in Fig. 4, but
here they have been colored by the day of the month. The filled dots
represent points where TES 510 hPa CO > 150 ppbv, and these
are the points used to calculate PAN enhancement ratios in Sect. 3.3. The
presence of same colored dots demonstrate that wide swaths of North America
can have smoke located somewhere in the atmospheric column on a given day,
and that the smoke is associated with elevated PAN (> 200 pptv)
in the atmospheric column. As discussed in Brey et al. (2018), smoke plumes
vary substantially in size. Small plumes cover < 100 km
Successful TES PAN retrievals overlapping NOAA HMS smoke polygons for July 2006 to July 2009 colored by the day of the month. Filled circles denote the set of retrievals that also coincide with 510 hPa CO greater than 150 ppbv. This set of points is used to calculated PAN enhancement ratios relative to CO in Fig. 8.
Top panel: case study of TES PAN retrievals overlapping HMS smoke
polygons 22–23 July 2007. Orange triangles represent FIRMS MODIS hotspots
for 22 July (Product MCD14ML;
Next we present a case study of fires in Idaho and Montana during July 2007
that connects PAN enhancements associated with HMS smoke plumes to regions
impacted by fires, indicating that the TES sensitivity is often sufficient to
measure elevated PAN several days downwind of a fire. Figure 7 presents the
locations of TES retrievals with elevated (DOF > 0.6) PAN on 22
and 23 July 2007, red and purple dots respectively, along with FIRMS MODIS
hotspots (Giglio et al., 2003, 2006) on those two dates. The TES PAN
retrievals are located almost directly over active fires in Idaho on 22 July,
but this does not absolutely ensure that the PAN is from fresh smoke. As
discussed in Payne et al. (2014), TES is most sensitive to PAN in the
mid-troposphere, and we do not have injection height information for these
specific fires. The TES PAN retrievals on 23 July (located over rural areas
in North and South Dakota) are not located directly over active fires, but
they do overlap HMS smoke polygons. The purple lines show HYSPLIT backward
trajectories initialized from 4 km at the locations of the retrievals on
23 July. The trajectories show that the major fire complexes in Idaho and
Montana likely contributed to the smoke observed by TES on 23 July (purple
dots). If so, this smoke was approximately 1–2 days old at the time of the
retrieval. The trajectories show that the smoke observed over South Dakota is
likely older (2–3 days of atmospheric aging). We initialize the trajectories
from various heights (2, 4, and 6 km) because the TES PAN retrievals offer no
vertical information, and all these trajectories are plotted in Fig. S3. The
smoke filled a relatively thick layer based on available CALIPSO data. A
CALIPSO overpass on 23 July 2007 (lower panel of Fig. 7) shows an aerosol
layer identified largely as
Histogram of estimated PAN enhancement ratios based on tropospheric mean PAN and CO from July 2006–2009 North American TES PAN retrievals overlapping HMS smoke plume polygons. The solid black line represents enhancement ratios calculated using an assumed PAN background of 0.1 ppbv with an assumed CO background of 80 ppbv. The dotted black line represents enhancement ratios calculated using an assumed PAN background of 0.1 ppbv with an assumed CO background of 90 ppbv. These specific enhancement ratios were calculated using an assumed CO background of 80 ppbv, similar to the solid black line. The dashed line represents enhancement ratios calculated using a significantly higher assumed PAN background of 0.2 ppbv with an assumed CO background of 90 ppbv. In all cases, negative values are not shown. The purple dots are the enhancement ratios for the two circled retrievals on 23 July 2007 plotted in Fig. 7 associated with transported smoke.
Enhancement ratios relative to CO or another tracer (e.g., acetonitrile for biomass burning specifically) are a common way to characterize the composition of pollution plumes (Yokelson et al., 2013). Enhancement ratios are calculated from samples made from within and outside a given plume (i.e., background air). This section presents enhancement ratios calculated from TES PAN retrievals located within smoke plumes. We show that the tropospheric PAN enhancement ratios from TES fall within the range of relevant aircraft measurements over North America. We also show that there are many pitfalls associated with using enhancement ratios as observed from TES to study the evolution of PAN in the smoke plumes we have identified here.
Equation (4) indicates how the enhancement ratio of PAN relative to CO is
calculated here.
Figure 8 presents a histogram of PAN enhancement ratios in the subset of
retrievals that overlap HMS smoke polygons and are also likely to have
elevated PAN and CO in the free troposphere (TES CO > 150 hPa).
The purple dots designate the two retrievals shown in Fig. 7 that meet
these strict criteria. PAN enhancement ratios were estimated using
tropospheric average PAN and tropospheric average CO. We performed this
calculation using Eq. (4) and a CO background of 80 and 90 ppbv.
Background CO in the Northern Hemisphere is generally between 80 and 90 ppbv
(e.g., Parrish et al., 1991) with significant year-to-year variability
largely driven by boreal forest fire emissions (Wotawa et al., 2001). Thus
the lower mixing ratio (80 ppbv) is closer to estimates of background CO in
the Northern Hemisphere. The upper mixing ratio (90 ppbv) reflects the
median tropospheric average CO (91 ppbv) in the PAN TES retrievals not
overlapping HMS smoke polygons (blue-grey points in Fig. 4). Though we
repeated this calculation with various assumptions of background CO mixing
ratios, this choice does not impact the major key point we draw from Fig. 8.
Even with our conservative CO criteria applied, the TES PAN data offer the
opportunity to calculate tropospheric average PAN enhancements relative to CO
for a large number of smoke samples (
We have not been able to identify a case study where the TES data can be used
to examine the evolution of the enhancement ratio of PAN relative to CO in a
plume. Restricting ourselves to the conservative criteria of 510 hPa
CO > 150 ppbv severely reduces the sample size (from 1151 to
159). In addition, the 5 km
We compare the TES column PAN enhancement ratios to enhancement ratios of PAN
relative to CO observed during July 2008 during the ARCTAS/CARB field
campaign (Hecobian et al., 2011). Smoke identification within the aircraft
dataset is discussed in detail in Hecobian et al. (2011) and not repeated
here. Alvarado et al. (2010) report mean PAN enhancement ratios for boreal
plumes using this same dataset. They report enhancement ratios of 0.34
Histogram of estimated PAN enhancement ratios based on in situ measurements of fire plumes described in Hecobian et al. (2011) from the ARCTAS campaign. Enhancement ratios were calculated using the 25th percentile for each trace gas during the corresponding flight day. These ratios were calculated using the 1 min merged data.
A second chance for a qualitative comparison of PAN enhancement ratios in
smoke plumes is presented in Briggs et al. (2016); summertime observations of
23 different plumes from the Mount Bachelor Observatory indicate PAN
enhancement ratios of 1.46–6.25 pptv ppbv
We present the first detailed analysis of TES PAN measurements over North
America. Recent aircraft observations over Colorado offer the most direct
overlap of the TES PAN product with in situ aircraft observations to date.
This comparison indicates that TES can be sensitive to PAN in the boundary
layer when PAN in the boundary layer is elevated, though peak sensitivity is
in the free troposphere. We use a period with a large number of TES PAN
observations (2006–2009) to investigate the contribution of fire smoke to
elevated PAN over North America in July. This type of multi-year synthesis
is not possible with any other observational dataset, and demonstrates how
satellite measurements of PAN can be used to frame new questions that cannot
be answered with existing in situ measurements.
We segregate and examine the abundance of tropospheric average PAN
relative to CO in TES retrievals located within smoke plumes identified by
the NOAA Hazard Mapping System (HMS). We find that a large fraction of the
TES retrievals (15–32 %) during the month of July overlap smoke plumes
during the period 2006–2009, while the largest percentage of retrievals
associated with smoke occurred in July 2008 (32 %). Tropospheric average
CO is clearly enhanced in retrievals impacted by smoke, but a difference in
PAN between smoke-free and smoke-impacted retrievals is insignificant. We compare the tropospheric average PAN enhancement relative to CO in
smoke-impacted samples and find that our satellite-based estimates largely
fall within the range of enhancement ratios that have been observed from
recent aircraft and surface campaigns over western North America. While in
situ measurements represent samples from a select number of plumes, the
satellite measurements offer more samples of different plumes and
observations over regions and time periods that have not been sampled by
aircraft. We use a case study to illustrate that PAN enhancements associated with
HMS smoke plumes can be connected to regions impacted by fires, indicating
that the TES sensitivity is often sufficient to measure elevated PAN several
days downwind of a fire. Case studies of specific smoke events do not show a systematic pattern in
PAN enhancements relative to CO as a function of distance downwind from
presumed source fires. We also do not observe any consistent evolution in
the PAN enhancement ratio when this calculation is done using the
tropospheric maximum PAN and CO from the TES retrievals, rather than the
tropospheric averages. The TES PAN data are not useful in this context
because of large limitations associated with evaluating smoke age within the
TES data.
PAN is considered to be the most important reservoir for NO
TES PAN retrievals are being processed routinely for the
whole TES dataset and will be publicly available in the TES v7 Level 2
product. However, at the time of submission, the v7 processing is still
underway. For netCDF files containing TES PAN data used in this study, please
contact Vivienne H. Payne at vivienne.h.payne@jpl.nasa.gov. When the paper is
accepted for final publication, we will add a text file containing the
latitude, longitude, time, HMS smoke overlap status, and tropospheric average
PAN and CO to the CSU digital repository
(
The supplement related to this article is available online at:
EVF led the majority of the analysis and writing associated with this manuscript. LZ provided basic statistical analyses of the TES data for the region of interest. VHP led the processing and development of the TES PAN data. JRW and ZJ provided guidance on the use of the TES PAN data. SSK supported the algorithm development for the TES PAN retrieval. SB led the overlap analysis of the TES retrievals with HMS smoke plumes. AH provided the smoke designation associated with the ARCTAS aircraft data. DG and KCP performed data analysis and visualization of TES PAN distributions, concentrations, and averaging kernels from the FRAPPE aircraft and satellite data; FF was responsible for the FRAPPE aircraft PAN measurements.
The authors declare that they have no conflict of interest.
This work was supported by NASA Award Number NNX14AF14G. Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. PAN data from ARCTAS was provided by Greg Huey supported by NASA Award Number NNX08AR67G. We thank Glenn Diskin for the use of the ARCTAS CO data. Edited by: Ronald Cohen Reviewed by: two anonymous referees