The Contribution of Fires to TES Observations of Free Tropospheric PAN over North America in July

Tropospheric PAN over North America in July Emily V. Fischer, Liye Zhu, Vivienne H. Payne, John R. Worden, Zhe Jiang, Susan S. Kulawik, Steven Brey, Arsineh Hecobian, Daniel Gombos, Karen Cady-Pereira, and Frank Flocke Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA Bay Area Environmental Research Institute Moffett Field, Moffett Field, CA, USA 4 National Center for Atmospheric Research, Boulder, CO, USA 5 Atmospheric and Environmental Research (AER), Lexington, MA, USA 6 National Center for Atmospheric Research (NCAR), Boulder, CO, USA 7 MORSE Corp, Cambridge, MA, USA


Introduction
PAN is considered to be the largest reservoir for nitrogen oxide radicals (NO x = NO + NO 2 ) in the troposphere, and it plays a major role in the redistribution of NO x from sources to remote regions (Singh, each retrieval is uncertain.Figure 1 demonstrates the limitations in sensitivity of TES PAN measurements, which provide broader spatial and temporal coverage than in situ measurements, but with a compromise on sensitivity.However, the measurements can be used to validate models, provided the averaging kernel and prior are applied to model fields before comparison with the retrievals.The averaging kernels associated with the panels presented in Figure 1 are provided in the Supplemental Information (Figure S1).

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The peak sensitivity for PAN is generally between 400 -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 (Figure 2) show that TES can be sensitive to PAN in the boundary layer when boundary layer PAN is extremely elevated.As an example, Figure 2 presents in situ observations from a flight during FRAPPÉ made with a thermal dissociation chemical ionization mass 120 spectrometer (TD-CIMS) (Zheng et al., 2011).In situ data show mixing ratios up to 2.2 ppbv were observed within the boundary layer.Afternoon mixing ratios > 1 ppbv were also observed at the Boulder Atmospheric Observatory (BAO) ground site on this day (Zaragoza et al., 2017).The overlaid TES data in Figure 2a (parallelograms) show that TES is sensitive to the elevated boundary layer values despite the presence of high clouds (dashed line Figure 2c).Figure 2 also shows that TES has sensitivity to PAN below 125 800 hPa, but the retrieval places the additional PAN higher up in the atmosphere.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.This is what is plotted in Figure 2a and used throughout the paper.
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 to ensure that the retrievals are dominated 130 by real observed information rather than the a priori.This conservative choice means that we are primarily basing our analysis on retrievals with high PAN.The impact of this choice can be seen when we compare the PAN distribution observed by TES under different conditions later in Section 3.2

NOAA Hazard Mapping System (HMS) Smoke Plume Extent
We segregate the TES PAN retrievals by whether or not the TES footprint coincides with a smoke 135 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 smokeplumes in the atmospheric column over North America (Rolph et al., 2009;Ruminski et al., 2006).Visibleband geostationary (~15 minute refresh rate) imagery, occasionally assisted by infrared, is used to detect 140 smoke plumes in the atmospheric column (Ruminski et al., 2006); because smoke plumes are primarily identified with visible imagery, the analyzed smoke plume extent is only representative of local daylight hours.
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 145 dataset does not contain information about the vertical location or depth of smoke in the atmospheric column.As discussed in Brey et al. (2017), the number and extent of smoke plumes in this HMS dataset is Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2017-1025Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 6 November 2017 c Author(s) 2017.CC BY 4.0 License.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 (Supplemental Figure 1).The number of major wildfires over the U.S. has large seasonal and interannual variability (Brey et al., 2017).Wildfires in summer 2008 were particularly intense over California associated with record-breaking lightning and aggravated drought.Figure 3a shows a cluster of 180 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;Singh et al., 2012), and we show this data in Section 3.3.Elevated smoke was Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2017-1025Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 6 November 2017 c Author(s) 2017.CC BY 4.0 License.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 also associated with special observations from TES, providing a relatively high number of attempted retrievals this month (red line in Supplemental Figure 2).Figure 3f 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 Figure 3e suggests that this was not a notably high percentage of smoke-impacted retrievals.A much higher percentage of DOF > 0.6 retrievals were smokeimpacted in July 2008.Panels a and b of Figure 4 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 Figure 4a) and the not-in-smoke cases (Blue-Grey box plot in Figure 4a).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 Figure 3 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 Supplemental Figure 4).The other two red distributions in Figure 4a 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 510hPa CO > 120 ppbv and TES 510hPa CO > 150 ppbv.There are differences between these subsets of data and the not-in smoke cases.As discussed further in Section 3.3, background CO in July in the northern mid-latitudes is expected to be ~85 ppbv.Both criteria (510 hPa CO > 120 ppbv or 510 hPa CO > 150 ppbv) represent conservative indicators of smoke in the free troposphere.The latter subset is shown because this designation has been used previously (Alvarado et al., 2011), and we use this subset in our calculation of enhancement ratios in Section 3.3.Figure 4c and 4d 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 = 100 ppbv versus 92 ppbv for both day and night retrievals), and the upper tail of the CO distribution includes retrievals with tropospheric average CO above 200 ppbv.The difference in CO distributions in and out of smoke provides confidence in the use of the HMS smoke product as a smokeimpact filter.The tropospheric average CO distributions are shown for reference because we combine tropospheric average CO with tropospheric average PAN to calculate PAN enhancement ratios in Section 3.3.There are several other factors that may also contribute to the patterns shown in Figure 4 that are worth noting.In general, TES is more sensitive to CO than PAN in the lowermost atmosphere, and the HMS smoke product, which contains no vertical information, includes smoke plumes near the surface and higher in the column.Though the sensitivity to clouds appears to be modest in our data, the TES CO Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2017-1025Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 6 November 2017 c Author(s) 2017.CC BY 4.0 License.retrievals are even less sensitive overall to the presence of cloud than the TES PAN retrievals.Third, many of the smoke-impacted TES retrievals are located substantially downwind of the source fires.PAN has a substantially shorter lifetime than CO in the warm lower atmosphere in summer.

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TES observations allow measurements of smoke plumes over North America at various ages, even in the same day.Figure 5 shows the spatial distribution of TES retrievals with DOF > 0.6 over the U.S. 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 Figure 3, 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 230 calculate PAN enhancement ratios in Section 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. (2017), smoke plumes vary in size substantially.Small plumes cover < 100 km 2 and smoke plumes from major fire complexes can spread over several Western States or entire Canadian Provinces.

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For example, Figure 5 shows elevated PAN both directly over and east of Hudson Bay in late July 2008 associated with fires in northern Saskatchewan.
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 6 presents 240 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., 2006;Giglio et al., 2003) 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 245 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 250 the retrieval.The trajectories show that the smoke observed over South Dakota is likely older (2-3 days of atmospheric ageing).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 Supplemental Figure S3.The smoke filled a relatively thick layer based on available CALIPSO data.A CALIPSO overpass on 23 July 2007 (lower panel of Figure 6) shows an aerosol layer identified largely as elevated smoke 255 extending from the surface to ~5 km over this region.

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PAN enhancement ratios were estimated using tropospheric average PAN and tropospheric average CO.
We performed this calculation using 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 (90 265 ppbv) reflects the median tropospheric average CO (91 ppbv) in the PAN TES retrievals not overlapping HMS Smoke Polygons (blue-grey points in Figure 3).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 Figure 7.Even with our conservative CO criteria applied, the TES PAN data offer the opportunity to calculate tropospheric average PAN enhancement relative to CO for a large number of smoke samples (N 270 =159) over a variety of regions and distances downwind from fires.The median PAN enhancement ratio relative to CO calculated using a background PAN mixing ratio of 0.1 ppbv and a background CO mixing ratios of 90 ppbv is 0.43 %.When we assume a higher PAN background mixing ratio of 0.2 ppbv with this background CO mixing ratio, the median PAN enhancement ratio from the TES data is 0.29 %.As we show next, these values are similar to that reported from in situ measurements.

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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 x 8 km footprint of TES combined with the lack of vertical sensitivity makes it difficult to establish the age of the smoke contributing to the enhanced PAN and CO.There could be multiple layers 280 of smoke in the column, of various ages.Tracking plumes with aircraft allows for a more precise determination of plume age.In addition, PAN does not simply dilute proportionally to CO because its dissociation is also a function of temperature, which also depends on altitude.
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 285 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 ± 0.35 % (range = 0.09 % to 1.43 %) for fresh plumes and 0.28 ± 0.36 % (range = 0.16 % to 0.68 %) for old plumes.In Alvarado et al. (2010), fresh plumes were designated as those where propene was correlated with CO, and aged plumes were designated as plumes

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where CO was correlated with more long-lived species, like butane, benzene and propane.The enhancement ratios were calculated using aircraft data from plume crossings using the average withinplume PAN and CO mixing ratios and assuming background mixing ratios equal to the 25 th percentile of all measurements in the boundary layer (140 ppbv for CO and 180 pptv for PAN).To calculate enhancement Atmos.Chem.Phys.Discuss., https://doi.org/10.5194/acp-2017-1025Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 6 November 2017 c Author(s) 2017.CC BY 4.0 License.ratios presented in Figure 8, we used the 25 th percentile for each trace gas for each day.For simplicity, we used observations at all altitudes, not just boundary layer points.Figure 8 shows that there is a range of in situ enhancement ratios.Similar to the tropospheric average enhancement ratio from TES, the majority of these enhancement ratios fall below 1%.There are retrievals with PAN enhancement ratios greater than 1%, but the number of these depends on the assumed background PAN used in the calculation.The appropriate value to use is difficult to determine from the TES data alone, which is why a range of estimates is presented in Figure 7. Figure 8 presents enhancement ratios calculated from in situ measurements.This data shows that there is a higher median enhancement for plumes from fires in the northwestern U.S., than the boreal plumes, though there are vastly different numbers of samples.
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 -1 (0.146 -0.625 %).This range overlaps with the majority of the column average enhancement ratios from TES.All of the plumes identified in Briggs et al. (2016) were from fires in northern California or southeastern and central Oregon, so they differ from the fires intercepted during ARCTAS.

Conclusions
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.Glatthor, N., von Clarmann, T., Fischer, H., Funke, B., Grabowski, U., Höpfner, M., Kellmann, S., Kiefer, M., Linden, A., Milz, M., Steck, T., and Stiller, G. P.: Global peroxyacetyl nitrate (PAN) retrieval in the upper troposphere from limb emission spectra of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), Atmos. Chem. Phys., 7, 2775-2787, 10.5194/acp-7-2775-2007, 2007.445 Gyawali, M., Arnott, W. P., Lewis, K., and Moosmüller, H.: In situ aerosol optics in Reno, NV, USA during and after the summer 2008 California wildfires and the influence of absorbing and nonabsorbing organic coatings on spectral light absorption, Atmos.Chem. Phys., 9, 8007-8015, 10.5194/acp-9-8007-2009, 2009.Hecobian, A., Liu, Z., Hennigan, C. J., Huey, L. G., Jimenez, J. L., Cubison, M. J., Vay, S., Diskin, G. S., July 2010     The purple dots are the enhancement ratios for the 6 retrievals on 23 July 2007 plotted in Figure 5 associated with transported smoke.These specific enhancement ratios were calculated using an assumed 700 CO background of 80 ppbv, similar to the solid red line.The dashed orange 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.

Figure 7
Figure 7 presents a histogram of PAN enhancement ratios in the subset of retrievals shown in Figure 7 (colored dots), as well as these values for the entire suite of retrievals that overlap HMS smoke polygons and also are likely to have elevated PAN and CO in the free troposphere (TES CO > 150 hPa).

Figure 2 :
Figure 2: a) Map showing FRAPPÉ aircraft and TES tropospheric average satellite observations of PAN

Figure 3 :
Figure 3: Panels a) through d): PAN TES retrievals with DOF> 0.6 co-located with NOAA Hazard Mapping System smoke polygons (red), and PAN TES retrievals with DOF > 0.6 not co-located with NOAA Hazard Mapping System smoke polygons (grey).The black dots indicate PAN TES retrievals with DOF> 0.6 during times with no NOAA HMS data.The blue lines surround the regions included in the calculations in Figures 2 and 4: 125 o W -70 o W, 30 o N -50 o N and 130 o W -60 o W, 50 o N -70 o N. e) . Phys.Discuss., https://doi.org/10.5194/acp-2017-1025Manuscript under review for journal Atmos.Chem.Phys.Discussion started: 6 November 2017 c Author(s) 2017.CC BY 4.0 License.

Figure 5 :
Figure 5: 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 point is used to calculated PAN enhancement ratios relative to CO in Figure 7.

Figure 8 :
Figure 8: Histogram of estimated PAN enhancement ratios based on in situ measurements of fire plumes described in Hecobian et al. (2011).Enhancement ratios were calculated using the 25 th percentile for each trace gas during the corresponding flight day.These ratios were calculated using the 1-minute merged data.