pandora - air & waste management associationpubs.awma.org/flip/em-june-2019/szykman.pdf ·...

A look at how the NASA Pandora Project is being used in support of the

Canada–United States Air Quality Agreement.

by J. Szykman, R.J. Swap, B. Lefer, L. Valin, S.C. Lee, V. Fioletov, X. Zhao, J. Davies, D. Williams, N. Abuhassan, L. Shalaby, A. Cede, M. Tiefengraber, M. Mueller, A. Kotsakis, F. Santos, and J. Robinson

PandoraConnecting in-situ and Satellite

Monitoring in Support of the Canada–

U.S. Air Quality Agreement

em • The Magazine for Environmental Managers • A&WMA • June 2019

Pandora Monitoring System Overview by J. Szykman, et al.


Evaluating progress on transboundary air issues, in supportof the Canada–United States Air Quality Agreement, requiresscientifically credible methods that can span the 8,900-kmborder. Progress has been tracked by a combination of in-situdeposition sampling, source monitoring (e.g., ContinuousEmission Monitoring System), and atmospheric modeling.There is recognition that low-earth orbit UV/visible measurements and soon-to-be launched geostationary-orbitUV/Vis spectrometers can be an important tool for monitor-ing air pollutant abundance and transport. To enhance the in-tegrity of using satellite data products for interpreting impactson surface air quality, there is a need to develop an inte-grated air quality monitoring and satellite validation network.

The technology and science underpinning the ability to re-trieve trace gas column abundance from satellite-based spec-trometers have matured over the past several decades, resultingin higher resolution data (e.g. GOME-2 40 x 40 km resolution,OMI1with nominal 13 x 24-km resolution vs. Sentinel 5PTROPOMI2 with nominal 7 x 7-km resolution; see Figure 1).


While satellites in low-earth orbit provide once-a-day observations of an area of interest at best, remotely sensedair quality research and monitoring is on the cusp of a newera with the launch of a series of geostationary air quality instruments from 2020 to 2023: Korea–GEMS (Geostation-ary Environment Monitoring Spectrometer),3 NASA–TEMPO(Tropospheric Emissions: Monitoring Pollution),4 and EuropeanSpace Agency (ESA) Sentinel-4.5

As geostationary weather satellites have revolutionizedmesoscale weather forecasting, this next generation of geosta-tionary air quality instruments has the potential to revolution-ize how air quality and pollutant transport is monitored.However, with these advancements, a new set of challengesassociated with validation of their hourly measurements anddata products arises. Monitoring air quality from space is astrategic research area within both the United States andCanada, and is a focus area for technical cooperation underthe agreement. While there have been efforts to evaluate andvalidate satellite-based trace gas data products, routine and

Figure 1. NO2 measured by TROPOMI (a) and OMI (b) on April 9, 2018, over the surface mines of theAlberta Oil Sands Region (w/NW winds). Averages (March–May, 2018) with an averaging radius of 5 kmand 16 km were used for TROPOMI (c) and OMI (d), respectively. The black line traces the borders of theindividual mining operations.17



systematic validation of the geophysical parameters uncertaintyand representativeness in space and time on a large scale israre, with cost and technology being the main limiting factors.

Pandora Ground-Based SpectrometerIn 2005, NASA initiated an effort at Goddard Space FlightCenter (GFSC) to address the gap in validation measure-ments through the development of cost-effective, easy-to-deploy, ground-based spectrometer called Pandora.6 Pandorais a compact, modestly-priced sun/sky/lunar passive UV/Visible grating spectrometer system. Over the past decade,the NASA Pandora Project together with the ESA Pandoniaproject, have endeavored to mature and refine the Pandoraspectrometer system.

When the spectrometer and head assembly are carefully calibrated, Pandora provides high-quality spectrally resolved direct sun/lunar or sky scan radiance measurements in the UVand visible wavelengths. The Pandora radiance measurementscombined with trace gas spectral fitting routines, and (in thecase of sky-scan measurements) radiative transfer modelingprovide real-time data of key air quality relevant pollutants,which can be compared to similar measurements from satellites. These observations include total column ozone (O3),nitrogen dioxide (NO2), formaldehyde (HCHO), sulfur dioxide(SO2) and bromine oxide (BrO). Figure 2 shows the variouscomponents of the Pandora system, and their installation in different configurations, while Table 1 provides technical detailson the components and measurement quality.

Environment and Climate Change Canada (ECCC) and theU.S. Environmental Protection Agency (EPA) have partneredwith NASA and ESA in the deployment of Pandora instrumentsacross North America. While the spatial distribution of Pandorainstruments in the field is not explicitly guided by issues oftransboundary transport of air pollutants, data provided fromthe confederated network will be used to validate satellite dataproducts of direct relevance to transboundary transport.

Pandora NetworkWithin the United States and Canada, measurements fromPandora instruments have demonstrated a new observationalperspective of air quality. The O3 and NO2 products havebeen rigorously investigated over the course of a decade dur-ing a series of intensive field campaigns and also at long-termmonitoring sites.6-8

The United StatesIn the United States, NASA’s Deriving Information on Surfaceconditions from COlumn and VERtically resolved observa-tions relevant to Air Quality (DISCOVER-AQ) mission9 in-volved the deployment of Pandora spectrometers at local airquality sites in each study domain: Baltimore, MD (2011),San Joaquin Valley, CA (2013), Houston, TX (2013), andDenver, CO (2014). The local air quality sites, many of themeither part of EPA’s State and Local Air Monitoring Stations(SLAMS) or Photochemical Assessment Monitoring Station(PAMS) networks, served as measurement anchor points forthe coordinated 3D sampling strategy (airborne spirals,ground-, and aircraft-based remote sensing).

DISCOVER-AQ results highlighted both the ability and theimportance of routinely monitoring factors affecting the 3Ddistribution of NO2.10 NO2 is a precursor of tropospheric O3 production. Understanding its 3D distribution is thus important for understanding patterns of O3 production, exposure, and transport.

Figure 3 demonstrates the power of adding Pandora (i.e.,total NO2 vertical column measurements) to a conventionalair monitoring station for understanding the vertical distribu-tion of NO2. Pandora does not differentiate troposphericNO2 from stratospheric NO2 columns, so we must make twoassumptions. First, we assume that a stratospheric contribu-tion to the total column is spatially uniform over all of Denverarea and slowly increases through the day from 3.1×1015

molecule cm-2 at 6:00 a.m. MST to 4.4×1015 molecule cm-2

Figure 2. (a) environmental enclosure housing instrument electronics (left), spectrometer on thermal electriccooler (right), computer (not shown), (b) sensor head with camera and enhanced sun tracker, (c) anotherconfiguration with sensor head installed on the roof of a trailer, and the spectrometer and computer locatedinside the trailer.

a b c



at 6:00 p.m. MST, consistent with previous climatologicalmeasurements.11 The second assumption is that the entiretropospheric NO2 column is located within the boundarylayer and that the boundary layer is well mixed, a reasonableapproximation in urban environments near sources given therelatively short lifetime of NO2 in the atmosphere.

With these assumptions, we are able to infer that the decreaseof NO2 surface concentration after 11:00 a.m. MST, as shownin Figure 3 relative to the Pandora NO2 column reflects growthin the boundary layer mixing NO2 higher in the atmosphere,or over a greater volume. This inference is supported by thetemporal increase in the vertical extent of aerosol backscatterand increasing mixed layer height while the total NO2 columnremains relatively constant between 11:00 a.m. and 1:00 p.m.MST. The significance being that this behavior is not possible toobserve with surface in-situ sensors alone.

Additional field campaigns, such as the 2016 NASA/NIERKORUS–AQ study: An International Cooperative Air QualityField Study in Korea (https://www-air.larc.nasa.gov/missions/korus-aq/), the 2017 NASA/EPA Lake Michigan OzoneStudy (https://www.ladco.org/technical/projects/lmos-2017/),the 2017 and 2018 Ozone Water Land EnvironmentalTransition Study (OWLETS 1&2; http://pubs.awma.org/flip/EM-Aug-2016/crawford.pdf) and 2018 Long Island SoundTropospheric Ozone Study (http://www.nescaum.org/documents/listos), have effectively served as opportunities to continue the evaluation of the operational performance of the Pandora systems and to characterize column NO2

diurnal patterns.12-14

In 2015, EPA finalized changes to the PAMS Network, whichincluded the addition of an Enhanced Monitoring Plan (EMP)for certain regions of the United States with persistent O3

non-attainment issues. Informed by the use of Pandora

spectrometers in multiple field campaigns to monitor columnNO2, recent EMP guidance provides the opportunity forstate and local agencies to work with EPA and NASA to in-corporate Pandora spectrometers into select monitoring sitesas a component of the EMP. Under the new EMP monitoringrequirements, Pandora column NO2 combined with mixed-layer height measurements and surface in-situ measurementsof true NO2 will better aid in the characterization of NO2

and allow for a direct comparison with satellite-based NO2

column measurements.

CanadaIn Canada, ECCC’s Pandora program is focused primarily onlong-term monitoring. One of the first sites is Fort McKay(57.184°N, 111.64°W) in the Canadian oil sands region,where Pandora measurements began on August 15, 2013.Located in the province of Alberta, the oil sands region con-tains vast deposits of bitumen–oil mixed with sand, clay, andwater. Environmental and health concerns associated with theoil sands operations, including air quality and acid deposition,are well known. The SO2 emission sources in the oil sandsregion are among the largest in Canada, while NO2 emissionsare comparable to those from a small town.15 Due to thelarge area of the oil sands operations, satellite measurementsare an appealing approach for air pollution monitoring in thisregion. Pandora was deployed to provide total column SO2

measurements16 and validation of satellite NO2 observations.17

Pandora SO2 measurements compared with in-situ data during the “pollution events” at Fort McKay (see Figure 4)demonstrate Pandora’s capability for air quality monitoring.

ECCC deployed Pandora instruments at three sites in ornorth of the Greater Toronto Area between 2013 and 2018to validate TROPOMI measurements at high spatial resolu-tion. In 2018, an additional Pandora instrument was installednear Edmonton, AB, to study urban pollution, as well as

• System developed at NASA Goddard with a focus on satellite validation.

• Ground-based direct sun/moon and sky scanning remote sensing for air quality and atmospheric composition

• 1-Spectrometer (1S) unit – spectral range 270–530 nm, resolution 0.6 nm

• 2-Spectrometer (2S) unit: spectral range 400–900 nm, resolution 1 nm

• Initial measurement is a slant column

• NRT Standard Operational Products at high frequency (~ 2 mins): Total Column Ozone (+/-15 Dobsonunit, ~5%); Total Column NO2 (+/-0.05 Dobson unit, ~10%)

• Research products: HCHO; BrO, profile and near-surface NO2, tropospheric and near-surface O3.

• Two main parts to instrument: (1) sensor head and (2) spectrometer, thermoelectric cooler, electronics, computer contained with environmental housing case 23x16x39 inches or 8-inch rackmounted enclosure.

Table 1. Overview of Pandora Spectrometer.



pollution from large coal-burning power plants located in thearea. To withstand harsh winter conditions, all Canadian instal-lations are configured with the sensor head placed outside,while the spectrometer and computer are indoors in a temperature-controlled environment.

Although scientific analyses have been published on bothSO2 and HCHO,16,18 these products and their associated al-gorithms are still undergoing review and require verificationto ensure their integrity before becoming a standard dataproduct. These focused studies, along with others in Europe,have provided opportunities to evaluate instrument opera-tions and performance, which have resulted in engineering and software changes to increase operational reliability andlong-term stability of the Pandora system.

The current distribution of Pandora instruments deployed inNorth America is depicted in Figure 5, with ECCC, NASA,and EPA instruments color coded separately. With the launchof TROPOMI (October 2017) and the future launch of thegeostationary TEMPO instrument circa 2021, the Canada–U.S. Air Quality Agreement is providing a mechanism for scientific cooperation between ECCC, NASA and EPA to define a coordinated satellite validation strategy and measurement instrument suites at long-term sites, with thesesites also being within a larger global network.

The Pandonia Global NetworkNASA and ESA are collaborating to coordinate and facilitatean expanding global network of standardized, calibrated Pandora instruments focused on air quality and atmospheric

Figure 3. An example set of NO2 and aerosol backscatter measurements that will be part of the PAMS network: in-situ NO2 measurements (top panel, gray line) Pandora NO2 column measurements shown following estimation and subtraction of stratospheric NO2 contribution (top panel, red and black; see text formore details), and a vertically resolved aerosol scattering measurements (bottom panel) with derived mixedlayer (bottom panel, white line). Vertically integrated aircraft in-situ NO2 column measurements are alsoshown (top panel, colored triangle and circles).



composition. The Pandonia Global Network (PGN) endeavorsto ensure systematic processing and dissemination of thedata to the greater global community in support of in-situand remotely sensed air quality monitoring. A major joint objective is to support the validation and verification of morethan a dozen low-earth orbit and geostationary orbit basedUV-visible sensors, most notably Sentinel 5P, TEMPO,GEMS, and Sentinel 4.

PGN participants are primarily comprised of governmental andacademic researchers and technicians. The launch of the PGNin mid-2019 represents a programmatic shift by NASA andESA toward establishing long-term fixed locations that are fo-cused on providing long-term quality observations of total col-umn and vertically resolved concentrations of a range of tracegases. PGN provides real-time, standardized, calibrated, andverified QA/QC air quality data. PGN also seeks to coordinateand implement network standards regarding common algo-rithms and data processing, instrument operating routines,quality control, real-time data processing, and data archiving.

ConclusionThe scientific cooperation and partnership between ECCCand NASA/EPA on the Pandora Spectrometer System will bebeneficial to both countries with specific areas of collabora-tion focused on instrument development, standardization ofdeployment and operations, shared data processing tech-niques, and combined analysis of results. The cooperation on Pandora as a new routine and systematic monitoring

Aug 23, 2013

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Figure 5. The current distribution of deployed Pandora systems in Canada and the United States. Each site is color-coded to correspond to the agency owner of the Pandora with total numbers deployed indicated in the legend.



approach is expected to assist in providing an improved char-acterization and validation of satellite data, leading to greateruse of satellite data to monitor and assess emissions and transport of air pollution over the vast areas that contribute to

transboundary pollution. In addition, the PGN is expected to play a critical role through implementation of standardsacross the network, which will assist ECCC and EPA on thedevelopment of a common measurement platform. em

J. Szykman, L. Valin, and D. Williams are with the U.S. Environmental Protection Agency (EPA); R.J. Swap and A. Kotsakis arewith NASA Goddard Space Flight Center; B. Lefer is with NASA Headquarters; S.C. Lee, V. Fioletov, X. Zhao, and J. Davies arewith Environment and Climate Change Canada (ECCC); N. Abuhassan, L. Shalaby, and J. Robinson are with the JCET–UMBC NASAGoddard Space Flight Center; F. Santos, ESSIC, University of Maryland; and A. Cede, M. Tiefengraber, and M. Mueller are withLuftBlick and the Department of Atmospheric and Cryospheric Sciences, University of Innsbruck.

Acknowledgment: The authors thank NOAA ESRL Chemical Sciences Division for use of the ground based in-situ observations of nitrogen dioxide at BAO Tower collected during the DISCOVER-AQ and FRAPPÉ field campaigns in summer 2014.

Disclaimer: The research described in this article has been reviewed by the U.S. Environmental Protection Agency (EPA) and approved for publication. Approval does not signify that the contents necessarily reflect the views and the policies of the agency nor does mention of trade names or commercial products constitute endorsement or recommendation for use.

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