meeting summaries

28

The Earth Observer

May - June 2014

Nitrogen Dioxide: Pollution precursor culprits revealed
NO2 is a brownish gas that is corrosive and toxic and isalso a precursor of ground-level small-sized particulatematter and of O3 through a complex path of chemical
reactions triggered by sunlight. In addition, NO2 is partof the highly reactive family of gases, generically referred
to as NOx, that result from combustion at high temperatures. Its principal anthropogenic sources are motorvehicle exhaust and power plants. NO2 is detectable fromspace because of its unique spectral signature that canbe differentiated from other atmospheric constituents.
However, the concentrations are small, sensitive to surface reflectivity, and highly variable, spatially and temporally. Therefore, remote sensing of NO2 is a continuing challenge. The presentations on this topic focused oncase studies, algorithm refinements, and the detection often-year trends.
A summary presentation described how NO2 concentrations over the tar sands in Alberta, Canada are increasing
at a rate of 10% per year—consistent with the increase inoil mining operations. Another important area of interest for OMI NO2 data use is to improve emission inventories—a measure of pollutants discharged into the atmosphere from combined individual sources. In some cases,
the data calculated from OMI observations agree reasonably well with standard inventories, but in others, they
do not. One aid to resolve these discrepancies is the useof chemical transport models that help researchers understand the inconsistencies between observed OMI NO2
trends and decreases in emissions.
Other studies are underway that attempt to use OMI
data directly, without models. By analyzing the downwind patterns of NO2 over megacities for different
wind conditions, lifetimes and (subsequently) emissions
amounts can be calculated. Several studies are underway
to improve NO2 algorithms by testing them with similar instruments on other satellites (e.g., GOME-2), andby developing algorithms with advanced capabilities forupcoming missions, as will be discussed below.

Volume 26, Issue 3

One technique, called cloud slicing, employs cloud
top heights and yields layer NO2 amounts in the freetroposphere (the upper part of the troposphere above theboundary layer), produced by lightning and broughtin via long-range transport. One analysis indicated thatthese amounts are independent of the NO2 concentrationin the lower troposphere, where the primary sourceis combustion. Another study relevant to future
instruments demonstrated how data artifacts due to
instrument peculiarities can be removed, which results
in better consistency among observing spectral windows.
Another presentation showed how cloud and aerosol
corrections, spectral window selection, and air massfactors were dependent on increasing amounts of NO2.
Ozone in the troposphere: The “hidden” ozone
Observing tropospheric O3, particularly in the boundary layer, is a continuing challenge for space-based
observations. O3 is the primary component of polluted
air at this level, but stratospheric O3 concentrations areabout 10 times larger than those observed in the lower
troposphere, and current instruments struggle to distinguish the small amounts at lower levels from total column values. There are several ongoing efforts to moreaccurately retrieve O3 in the troposphere, and in particular to be able to discriminate O3 in the boundary layer
and in the free troposphere, where O3 becomes a greenhouse gas. One presentation on measuring O3 above theboundary layer over OMI’s 10-year lifetime describedO3 stratospheric–tropospheric exchange and correlations with tropical cycles, observations in deep convective clouds, and anthropogenic O3 sources.
A complementary study showed a clear correlation ofOMI-observed tropospheric O3 over eastern China,
presumably in the boundary layer, with increased
amounts of CO measured by the Measurements ofPollution in the Troposphere (MOPITT) between the
0 and 3 km (0 and ~1.9 mi) altitude range, with strong
evidence from aircraft measurements, of transboundary
pollution to Korea and Japan—see Figure 4. TheFigure 4. The maps show monthly averages of O3 between 0 and 3 km (0 and ~1.9 mi) as measured by OMI for June 2005 [left], June 2009
[middle], and June 2013 [right]. Ozone builds up in the planetary boundary layer over eastern China; the prevailing westerlies in the free troposphere transport it into Korea and Japan. Image credit: Hayashida et al., manuscript in preparation for Geophysical Research Letters, 2014

May - June 2014

Volume 26, Issue 3

correlation of OMI andaircraft data at this altitude
range was on the order of0.9, indicating that OMI
can measure tropospheric O3
exiting the boundary layer
and transported into theupper troposphere.
A novel result that
demonstrated synergy ofAura instruments used Aura’s
Tropospheric Emission
Spectrometer (TES)-derived
tropospheric O3 and OMI
NO2 measurements, alongwith a chemical transportmodel for the period 2005 to2010. Tropospheric O3 wascalculated from the model
Figure 5. DISCOVER-AQ fourteen day observing strategy for July 2013 over the Baltimore, MD/using derived OMI NO2
Beltsville, MD/Washington, DC area. The aircraft ground track has been overlaid on a map of theemissions and then comparedregion showing major roadways. The vertical spirals represent locations where aircraft made descentswith the TES-derived O3
to measure atmospheric constituents at different levels—to construct profiles. Ground stations areindicated by the towers. Image credit: Timothy Marvel [NASA’s Langley Research Center]amounts; the results showed
good agreement. A byproductof this study showed that theincreased emissions of pollution over China increased
products. The main issues are with the latest versions ofthe free tropospheric O3 amount over the western
the KNMI cloud products, HCHO, bromine monoxide
U.S. by a factor of two due to the strong westerly(BrO), and chlorine dioxide (ClO2)—particularly overtranspacific winds during spring.
snow/ice and under partly cloudy conditions.
Using the OMI Algorithms on OMPS
Despite the fact that the priority for OMPS on Suomi
NPP was to measure stratospheric O3 only to meet
NOAA requirements, OMI algorithms for measuring
the array of atmospheric constituents discussed above
have been put to use on OMPS data. For example,
the application of the SO2 algorithm to OMPS clearly
showed SO2 hot spots and their daily variations over
the Ohio Valley, eastern China, and from most volcanic
emissions, worldwide. The OMI NO2 algorithm wasalso tested on OMPS, and also showed seasonal variations similar to those detected by OMI. Other results
showed excellent agreement of monthly zonal averages
between OMI and OMPS, thereby assuring the needed
consistency between the two datasets to detect global
trends in air quality across satellite missions.
Validation
Validation of OMI data products is an ongoingprocess, but significant progress already has been made.
Uncertainties have been quantified and systematiceffects characterized. Since 2011, 18 papers on
validation results have been published; several more
have been submitted or are in preparation. Gaps invalidation results have been identified for almost all

DISCOVER-AQ: A field campaign to validate OMI andother instruments
A major NASA aircraft-based field campaign is called
Deriving Information on Surface conditions from Column
and Vertically Resolved Observations Relevant to Air
Quality (DISCOVER-AQ7). It is designed to improvethe use of satellites to monitor air quality for publichealth and environmental benefit. Specific regions in theU.S. were selected to measure the horizontal, vertical,and temporal variations of key air-polluting substances.
Measurements were conducted using two NASA aircraft, several ground stations, and balloons, and thencorrelated with OMI and other instruments on theA-Train satellites. The key objectives of the four-mission
campaign are to quantify the relationships between tropospheric columns amounts as measured by the satellites
to ground-level amounts of air constituents of interest to
the air quality data user, improve satellite retrievals, andadvance regional air quality models and forecasts. The
aircraft deployment strategy and location of ground stations are illustrated in Figure 5. Key findings from theDISCOVER-AQ has been discussed previously, mostrecently in “NASA’s Venture Continues: An Update on theEV-1 Investigations,” The Earth Observer, July–August 2013[Volume 25, Issue 4, pp. 19-32].

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meeting summaries

The Earth Observer