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 The INTEX-A field mission was conducted in the summer of 2004 (1 July to 15 August 2004) over North America (NA) and the Atlantic in cooperation with multiple national and international partners as part of a consortium called ICARTT. The main goals of INTEX-A were to (1) characterize the composition of the troposphere over NA, (2) characterize the outflow of pollution from NA and determine its chemical evolution during transatlantic transport, (3) validate satellite observations of tropospheric composition, (4) quantitatively relate atmospheric concentrations of gases and aerosols with their sources and sinks, and (5) investigate aerosol properties and their radiative effects. INTEX-A primarily relied on instrumented DC-8 and J-31 aircraft platforms to achieve its objectives. The DC-8 was equipped to measure detailed gas and aerosol composition and provided sufficient range and altitude capability to coordinate activities with distant partners and to sample the entire midlatitude troposphere. The J-31 was specifically focused on radiative effects of clouds and aerosols and operated largely in the Gulf of Maine. Satellite products along with meteorological and 3-D chemical transport model forecasts were integrated into the flight planning process. Intercomparisons were performed to quantify the accuracy of data and to create a unified data set. Satellite validation activities principally focused on Terra (MOPITT, MODIS, and MISR), Aqua (AIRS and MODIS) and Envisat (SCIAMACHY) to validate observations of CO, NO2, HCHO, H2O, and aerosol. Persistent fires in Alaska and NW Canada offered opportunities to quantify emissions from fires and study the transport and evolution of biomass burning plumes. Contrary to expectations, several pollution plumes of Asian origin, frequently mixed with stratospheric air, were sampled over NA. Quasi-Lagrangian sampling was successfully carried out to study chemical aging of plumes during transport over the Atlantic. Lightning NOx source was found to be far larger than anticipated and provided a major source of error in model simulations. The composition of the upper troposphere was significantly perturbed by influences from surface pollution and lightning. Drawdown of CO2 was characterized over NA and its atmospheric abundance related to terrestrial sources and sinks. INTEX-A observations provide a comprehensive data set to test models and evaluate major pathways of pollution transport over NA and the Atlantic. This overview provides a context within which the present and future INTEX-A/ICARTT publications can be understood.
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 INTEX-A was an integrated mission performed over NA and the Atlantic during the summer of 2004 in cooperation with multiple national and international partners under a consortium called ICARTT (http://cloud1.arc.nasa.gov/intex-na/; http://www.al.noaa.gov/ICARTT/). This effort had a broad scope to investigate the transport and chemistry of long-lived greenhouse gases, oxidants and their precursors, aerosols and their precursors, as well their relationship with radiation and climate. NASA's DC-8 and J-31 were joined by aircraft from a large number of European and North American partners (Table 1) to explore the composition of the troposphere over NA and the Atlantic as well as radiative properties and effects of clouds and aerosols in a coordinated manner. An important new advance has been the launch of new satellite instruments that have the capability to map pollution in the troposphere on regional to global scales [Borrell et al., 2004; Schoeberl et al., 2004; Richter et al., 2005]. INTEX-A performed the dual role of validating satellite observations of tropospheric composition as well as integrating them with in situ observations. Specific sampling goals were achieved with the aid of chemical transport model (CTM) forecasts and a meteorological support system that allowed targeted sampling of air parcels with desired characteristics.
Table 1. Principal Mobile Platforms in Operation During the Summer 2004 ICARTT Study
 INTEX-A/ICARTT was organized around a few major objectives aimed at (1) characterizing the large-scale composition of the NA troposphere, (2) characterizing the outflow of pollution from NA and determining its chemical evolution during transatlantic transport, (3) validating satellite observations of tropospheric composition, (4) quantitatively relating atmospheric concentrations of gases and aerosols with their sources and sinks, and (5) investigating aerosol radiative properties and their effects. Summer season was selected because it is a season that is photochemically active, biogenic emissions and boreal fires are at their peak, and aerosol radiative forcing is near its maximum. Cold fronts and warm conveyor belts, while reduced in frequency in summer, continue to lift surface pollution to the free troposphere where it can be carried by the prevailing westerlies toward the European continent. INTEX-A aimed to better understand the chemical and physical processes that control the evolution of pollution during transatlantic transport, provide the advantage of improved source characterization offered by concurrent measurements of a large number of chemical species, relate radiative and chemical properties, and ultimately improve predictive capabilities via accurate model simulations.
 Although several field experiments have been performed over NA in the past decades and results published and evaluated in special issues of journals [see Fehsenfeld et al., 1996; Singh et al., 1999; Russell et al., 1999; Schere et al., 2000], this was the most complex and integrated study done to date that spanned the midlatitudes from the eastern Pacific to the eastern Atlantic and permitted sampling of the entire troposphere over NA. This overview focuses mainly on the goals and activities of the NASA DC-8 and J-31 aircraft during the summer 2004 study.
2. Mission Design and Implementation
2.1. Measurement Platforms
 The principal mobile platforms that participated in the INTEX-A/ICARTT campaign, their bases of operation, campaign duration, and specifications are summarized in Table 1. Approximately 9 aircraft participated in this study for extended periods of time. One other aircraft (NSF Wyoming King Air) had a more limited role in this study and is not included in Table 1. These airborne platforms had the capability to sample much of the midlatitude troposphere over long distances. The bases of operation were selected to facilitate sampling over NA and during transatlantic transport. Complementing these airborne platforms were several fixed surface air quality stations that operated during this study and in some cases continued to collect data after the study to develop more robust statistics. Principal among these were several air quality ground stations located in New Hampshire, Chebogue Point (Canada), and Pico (Azores). The NOAA ship R/V Ronald H. Brown also focused on the boundary layer outflow of pollution in the Gulf of Maine.
 Data were also available from several satellites overhead both in real time and for post mission analysis. Satellite observations both guided flight planning and helped to extend the range of airborne observations. In return, the airborne measurements provided validations of satellite observations. Integration of data from these multiple platforms was one of the central goals of this study. The INTEX Ozonesonde Network Study (IONS), organized by pooling resources from Environment Canada, NASA, and NOAA, provided some 300 O3 profiles from 11 fixed stations and the R/V Ronald H. Brown [Thompson et al., 2006a, 2006b]. The IONS network covered a wide region over NA that extended from 30 to 45°N and 60 to 125°W. Ozonesondes were released on a daily basis from three fixed sites and the R/V Ronald H. Brown and 1–3 times a week from other stations. Additional details about these platforms are available from http://cloud1.arc.nasa.gov/intex-na/ and http://www.al.noaa.gov/ICARTT/.
2.2. Instrument Payload and Measurement Capability
 The payloads of a majority of airborne platforms in Table 1 (e.g., DC-8, WP-3D, G-1, BAe-146, Falcon) were designed to measure detailed gas and aerosol composition in the troposphere. Three aircraft (J-31, Twin Otter, and Convair) were more focused on radiative and microphysical properties of clouds and aerosols. The DC-8 and the J-31 had an additional role that involved validation of satellite measurements of gases and aerosol optical properties in the troposphere. Table 2a shows the DC-8 payload of in situ and remote sensors for measurements of both gases and aerosols. Major greenhouse gases, ozone and precursors, aerosols and precursors, and a large number of tracers were measured. Detailed VOC, OVOC, and reactive nitrogen (NOx, HNO3, HO2NO2, PAN) speciation was carried out. A large number of tracers that could be used to identify specific sources such as biomass combustion (HCN, CH3CN), biogenic emissions (isoprene), anthropogenic emissions (CO, halocarbons), and oceanic emissions (CHBr3) were measured. A nadir and zenith viewing UV lidar measured O3 and aerosols remotely. HOx (OH and HO2) free radicals, that are critical to understanding oxidative processes, were directly measured along with their precursors (peroxides and aldehydes). Spectral radiometers allowed direct measurement of actinic flux used to derive key photolysis frequencies. Multiple instruments measured physical, chemical, and optical properties of fine and coarse aerosol. While many of the instruments had previously flown aboard the DC-8, several new instruments were incorporated. Two new CIMS instruments were used to measure HO2NO2 and SO2 [Huey et al., 2006] and hydrogen peroxide [Crounse et al., 2006]. A new LIF instrument measured NO2 as well as total PANs and total alkyl nitrates via their thermal decomposition to NO2 [Day et al., 2002]. INTEX-A provided the first in situ measurements of HO2NO2 [Huey et al., 2006] and peracetic acid (J. Crounse et al., Peroxyacetic acid is ubiquitous in the upper troposphere, submitted to Geophysical Research Letters, 2006, hereinafter referred to as Crounse et al., submitted manuscript, 2006) in the troposphere.
A. Clarke, University of Hawaii, and B. Anderson, NASA LaRC
Actinic fluxes and photolytic frequencies
spectral radiometry, zenith and nadir
R. Shetter, NCAR
10−8 s/15 s (±5%)
 Specific science objectives of the J-31 included validating satellite retrievals of Aerosol Optical Depth (AOD) spectra and of water vapor columns, measuring aerosol effects on radiative energy fluxes, and characterizing cloud properties using visible and near infrared reflectance in the presence of aerosols. To achieve these objectives, the J-31 carried two instruments (Table 2b) that operated in close coordination with measurements by other platforms especially the DC-8 and the R/V Ronald H. Brown. The Ames Airborne Tracking Sunphotometer (AATS) measured aerosol and water vapor attenuation of the direct beam from the Sun. The Solar Spectral Flux Radiometer (SSFR) measured solar energy from all directions with sensors looking both above and below the aircraft. Both AATS and SSFR covered a wide range of wavelengths, from the ultraviolet, through the visible, and into the near infrared.
Instruments on board the DC-8 and J-31 also measured pressure, temperature, dew point, aircraft position and orientation.
Optical depth, water vapor column
tracking Sun photometer, 354–2138 nm
P. Russell, NASA ARC
slant OD ∼0.002/3 s (±0.01), slant WV ∼0.0005 to 0.006 cm−1/3 s (±10%)
Solar spectral flux
spectrometer (380–1700 nm) with nadir and zenith hemispheric collectors
P. Pilewskie, University of Colorado
absolute accuracy 3–5%, precision 1%/0.25 s
 Satellite instruments are becoming capable of air chemistry measurements in the troposphere and received a great deal of attention from the NASA DC-8 and the J-31 platforms. In INTEX-A our principal focus was on the CO vertical profiles obtained by AIRS and MOPITT, NO2 and HCHO trop-columns from SCIAMACHY, and aerosol columns from MODIS-Terra, MODIS-Aqua, and MISR (Table 3). Additional details about the payloads of other platforms listed in Table 1 have been summarized by Fehsenfeld et al. .
Table 3. Satellite Measurements Targeted for Validation in INTEX-Aa
Platform (Launch Date)
All validations were performed with instruments looking in the nadir.
 Satellite observations, along with meteorological and chemical forecasts from global and regional models were extensively used to plan, coordinate, and implement the INTEX-A field mission. Table 4 lists the major CTMs in use for planning this experiment. Meteorological products were derived from the NCEP reanalysis data (H. E. Fuelberg et al., Meteorological conditions and anomalies during INTEX-NA, submitted to Journal of Geophysical Research, 2006, hereinafter referred to as Fuelberg et al., submitted manuscript, 2006). Daily meteorological and chemical forecasts guided the design and execution of the mission. Typical model products included source-tagged CO tracers, aerosol tracers, O3, and PAN distributions. Meteorological products included satellite imagery (visible, infrared, and water channels), precipitation and cloud fields, backward and forward trajectories and multi day synoptic weather forecasts. Principal satellite instruments that provided near real-time data to guide flight planning were AIRS and MOPITT (CO columns), SCIAMACHY (NO2 columns), MODIS and AVHRR (aerosol, fire counts), and TOMS (tropospheric O3, absorbing aerosols). To monitor lightning activity, real-time data from the NLDN and LRLN were provided by the Global Hydrology and Climate Center at NASA Marshall Space Flight Center. These data were combined with forward trajectories by groups at Florida State and NASA Goddard to assess lightning influence for flight planning purposes.
Table 4. Models Used for Flight Forecasting and Analysis in INTEX-Aa
Underlying Meteorological Model
Flight planning process also used weather forecasts based on NCEP analysis and near real-time satellite data from AIRS and MOPITT (CO columns), MODIS and AVHRR (aerosol, fire counts), TOMS (tropospheric ozone, absorbing aerosols), and LIS (lightning). Potential vorticity and convective influence plots were also available.
 The DC-8 science activities were closely coordinated with the main air quality (WP-3D, BAe 146, and Falcon) and radiation (J-31) platforms. This coordination was necessary to accomplish intercomparisons among instruments, to achieve several objectives that were unattainable by a single platform, and to collect complementary data where appropriate. The bases of operation of these platforms (Table 1) were selected to sample North American atmospheric composition and transatlantic transport. In many cases quasi-Lagrangian studies were carried out when pollution plumes sampled over NA were subsequently sampled over the central and eastern Atlantic after several days of transport [Law et al., 2005; Methven et al., 2006].
2.4. Flight Tracks and Sampled Air Masses
Figure 1 shows the flight tracks of four major air quality platforms (DC-8, WP-3D, BAe-146, Falcon-20) and the radiation platform (J-31) that collectively covered an area from the eastern pacific to the eastern Atlantic. Each flight was carefully planned to achieve the INTEX-A targeted objectives of investigating outflow of gases and aerosols, source characterization, chemical evolution, direct/indirect effects of aerosols, satellite validation, and instrument intercomparisons. The DC-8 flew 170 hours in 18 science flights of 7–10 hour duration from its three bases of operation in Table 5a. The DC-8 started from the west coast of the United States and sampled air masses over the Pacific before moving to mid-America and ultimately joined other platforms at the eastern site. Figure 2 shows the DC-8 flight tracks for each of the science flights. The J-31 flew 60 hours in shorter science flights of 2–4 hour duration operating exclusively from the New Hampshire site. Tables 5a and 5b provide a brief summary of the flight objectives of each of the DC-8 and J-31 flights with more details available at http://cloud1.arc.nasa.gov/intex-na/flight_reps.html. Figure 3 is a plot of CO and O3 that clearly shows that air masses influenced by forest fires, anthropogenic pollution, and stratosphere were sampled during INTEX-A.
Table 5a. INTEX-A DC-8 Missions and Salient Activitya
AIRS validation, characterization of low-level California and high-level Asian outflow, background and inflow characterization, and stratospheric intrusions
6 Jul 2004
NASA DFRC to Mid-America Airport
California plume transported to the east, Arizona fire plumes, frontal crossings over midwest and convective outflow over Great Plains
8 Jul 2004
validation of the MOPITT and MODIS instruments aboard the Terra satellite, characterization of aged California outflow, mapping of midwestern boundary layer pollution, sampling of deep convection over the Carolinas, and exploring the relationship between biogenic emissions and their products over the Smoky Mountains
10 Jul 2004
validation of instruments aboard the Envisat (SCIAMACHY) and Aqua (AIRS, MODIS) satellites, mapping of midwestern boundary layer pollution, sampling of deep convection, exploring the relationship between biogenic emissions and their products, and characterization of outflow from Texas
12 Jul 2004
boundary layer mapping of south-central United States, aged convective/lightning outflow over southeast United States and validation of Park Falls FTS, SCIAMACHY, and AIRS
15 Jul 2004
Mid-America Airport to Pease International Tradeport Airport (ITA)
validation of Terra (MOPITT, MISR) and Aqua (AIRS, MODIS) instruments, validation of FTS CO2 column measurements at Park Falls, CO2 intercomparison with the NSF King Air aircraft, characterization of Asian pollution, Alaskan fires, and anthropogenic pollution
18 Jul 2004
validation of Envisat (SCIA) and Aqua (AIRS, MODIS) satellite instruments, a first attempt at the Lagrangian experiment, characterization of North American pollution outflow, characterization of Alaskan fires, and a flyby over the NOAA ship Ronald H. Brown
20 Jul 2004
validation of Aqua (AIRS, MODIS)instruments, characterization of smoke from Alaskan fires transported over the United States, boundary layer pollution over the southeast and midwest
22 Jul 2004
sampling polluted boundary layer outflow along the eastern seaboard both to the north and south of Pease; intercomparison between the NASA DC-8 and NOAA WP-3D aircraft; a coordinated validation profile with the J-31 over the NOAA research vessel Ronald H. Brown and beneath the MISR instrument on NASA's Terra satellite
25 Jul 2004
convective outflow from southeast United States, map Ohio River Valley emissions in northerly flow and Terra underflight
28 Jul 2004
sample the structure and chemical evolution of the U.S. continental outflow over the Atlantic and a comparison between the measurements on the DC-8 and the BAe146
31 Jul 2004
satellite underpass, aged air sampling/recirculation, low-level outflow, DC-8/P-3 intercomparison, and possible Asian influences
2 Aug 2004
underfly Terra (MOPITT/MODIS) and Aqua (AIRS) satellites, sample low-level North American outflow and aged air pollution aloft, conduct a coordinated closure experiment over Ronald H. Brown with the J-31, and a flyby over the ground Appledore island air quality station.
6 Aug 2004
Continental outflow, city plumes, boundary layer characterization, Terra underflight
7 Aug 2004
underfly Terra (MOPITT/MISR) and perform a predefined maneuver in coordination with the J-31, Ronald H. Brown and MISR, sample North American outflow, a stratospheric intrusion, and perform P-3 intercomparison.
11 Aug 2004
Pease ITA to Mid-America Airport
Terra underpass, Aqua underpass, NA outflow and WCB lifting, frontal crossing and low-level pollution
13 Aug 2004
satellite underpass and outflow from major industrial cities
14 Aug 2004
Mid-America Airport to NASA DFRC
satellite underpass, Asian outflow, and NA inflow
Table 5b. INTEX-A J-31 Missions and Salient Activitya
Aqua overpass at 1813 UT; good profile data, R/V Ronald H. Brown rendezvous
15 Jul 2004
over stratus clouds near Ronald H. Brown Terra overpass (1525 UT) and near surface Aqua (1705 UT); 2nd run above cloud past RB; two Ronald H. Brown sondes
16 Jul 2004
spiral descent and low-level legs around Aqua overpass time (1748 UT), increasing fog in the vicinity of low-level legs, profile near Ronald H. Brown for special sonde
17 Jul 2004
spiral descent and low-level legs around Terra overpass time (1513 UT), increasing Cirrus (Ci), about 5 min of Ci-free data, found Ronald H. Brown under Ci
20 Jul 2004
radiation legs over stratus near Ronald H. Brown with Terra overpass in MISR local mode swath; AOD profiles over stratus; brief AOD near surface
21 Jul 2004
low-level runs during Aqua OP at 1806 UT, two profiles in clear; aerosol gradient at several altitudes
22 Jul 2004
coordination with DC-8, Ronald H. Brown, and Terra in MISR local mode patch, collocated profile with DC-8 (18 min apart) over Ronald H. Brown
23 Jul 2004
aerosol profiles and near-surface legs with Aqua (1754 UT).
26 Jul 2004
Meteo and H2O profiles over Ronald H. Brown near shore; cirrus or lower clouds for most of flight
29 Jul 2004
spiral descent profile over Ronald H. Brown, multiple low-level flybys of Ronald H. Brown, Terra MISR local mode, Aqua, two Ronald H. Brown sondes; aerosol gradient two ways
29 Jul 2004
successful airborne Langley flight at constant press alt (∼6.4 km GPS) at sunset
31 Jul 2004
flew 297 nm E of Pease to find cirrus-free air; flew L-pattern at max alt of 20k ft, spiral down to 2k ft (stratus below), another L-pattern at 2k ft, and a final L-pattern at 7.5k ft above an aerosol layer; upon return, descent above Ronald H. Brown (3 miles offshore); Ronald H. Brown sonde; coordination with DC-3 lidar over Ronald H. Brown, but AATS got water vapor data only, because of cirrus
2 Aug 2004
Terra overpass; profile and legs in clear area; aerosol gradient two ways
2 Aug 2004
airborne Langley before descent to 200′, then low-level leg to Ronald H. Brown, ascent over Ronald H. Brown with its sonde, and continuation of airborne Langley
3 Aug 2004
coordination w/DC-3 w/legs at ∼200′ and ∼5200′, followed by Ronald H. Brown flyby and spiral ascent nearby with Ronald H. Brown sonde
7 Aug 2004
coordination w/DC-8, Ronald H. Brown and its sonde during Terra MISR local mode overpass; includes 5000′ and 200′ L-shaped legs and RB flybys, and spiral ascent w/DC-8
7 Aug 2004
airborne Langley before/after descent and ascent in clear air just east of Ronald H. Brown (∼68.75°W, 43°N); post Ronald H. Brown Langley measurements were ENE of Ronald H. Brown; Ronald H. Brown sonde
8 Aug 2004
Aqua overpass at 1354 UT; very low AOD along extended 220′ run from 70°W to 68°W, 42.75°N; profiles down/up to 11,000′; level leg at 11,000′; ramped descent from 68°W to ∼69°W
8 Aug 2004
aborted on runway
8 Aug 2004
airborne Langley E of Pease
 Intraplatform and interplatform comparisons of measurements were extensively performed to establish accuracies and precisions. On the DC-8 itself, several chemical measurements were duplicated using a variety of different techniques (Table 2). Salient among these were measurements of O3, H2O, HCHO, H2O2, HNO3, NO, and aerosol composition. Intercomparisons were also performed with other aircraft by coflying these in formation with a separation of less than 300 m in the horizontal and 100 or so meters in the vertical. Two to three levels were chosen between 0 and 8 km altitudes and the entire intercomparison including descent and ascent typically lasted one hour. Table 6 provides a summary of the intercomparisons carried out by the DC-8. The BAe 146 separately intercompared with the DLR Falcon-20 based on the European continent. The intercomparison data are available at http://www-air.larc.nasa.gov/missions/intexna/meas-comparison.htm.
 Satellites are increasingly able to measure key gases and aerosols in the troposphere. These sensors measure radiances from which information on atmospheric composition is retrieved using radiative transfer algorithms with substantial and often poorly characterized uncertainties. Validation with in situ vertical profiles is required, and this was a major activity during INTEX-A. The DC-8 focused on validation of CO, NO2, H2O, HCHO, and, together with the J-31, on aerosols and clouds (Table 3). Opportunities for satellite validation were sought daily during flight planning. Figure 2 shows all the satellite spirals undertaken by the DC-8. Specific times and locations for these spirals are given in Table 7. Validations were performed to test retrievals with a variety of underlying surfaces. A typical DC-8 validation flight involved measuring a profile from 0.2 to 11.5 km in about 20 km radius within ±30 min of the satellite overpass time under relatively cloud free conditions. This time coincidence was necessary for accurate validation and, in addition, provided a measurement of many chemical and physical parameters that could be used to improve satellite retrievals. In many cases it was possible to simultaneously validate multiple satellite sensors when the overpass times were similar and instrument swaths quite wide. In situ aerosol measurements on these DC-8 spirals have also been used to test aerosol retrievals from GOES (S. Kondragunta et al., unpublished manuscript, 2006). The satellite sensors of primary attention for the J-31 were MISR and MODIS (Table 3) and the focus was on validating satellite retrievals of AOD spectra and water vapor columns. Although the Aura satellite was launched in July 2004 and was not ready for validation, INTEX-A data for O3-CO correlations were used subsequently for TES/Aura validation [Zhang et al., 2006].
Table 7. DC-8 Satellite Validation Opportunities During INTEX-A
3. Overview of First Results
 We present here a brief overview and synthesis of the results presented in the first collection of INTEX-A/ICARTT papers assembled in this special section of Journal of Geophysical Research and published elsewhere to date. We primarily focus on the goals and activities of the NASA DC-8 and J-31 platforms. Complementary papers are also being published in a second special section of Journal of Geophysical Research: ICARTT/NA-Europe. The reader is referred to the specific papers for more information.
3.1. Long-Distance Transport of Pollution
 Fuelberg et al. (submitted manuscript, 2006) provide an overview of the meteorological environment during INTEX-A. Seven cold fronts passed Atlanta, GA during INTEX-A, the greatest number between 2000 and 2005. These fronts brought record low temperatures to portions of the Great Plains and south. Frontal passages over the northeast were above average and the short time interval between passages (averaging 4.6 days) precluded the formation of stagnant high-pressure centers that lead to pollution build up. The persistent high pressure over Alaska led to a record number of wildfires. Typically, the ridge transported the fire by-products southeastward toward the INTEX-A domain where they were sampled by the DC-8 during numerous flights. On some flights, forward trajectories and satellite imagery showed that the plumes were also carried to parts of Europe, Africa, and the Arctic. An analysis of cloud-to-ground lightning from 2003 to 2005 showed that both flash counts and horizontal patterns of lightning flashes during INTEX-A were similar to other years. Deep convection and lightning were important factors during INTEX-A. Analysis of data by Kiley and Fuelberg  shows that midlatitude cyclones were capable of producing vertical transport as great or greater than much stronger cyclones.
3.1.1. Forest Fire Emissions and Transport
 The summer of 2004 was one of the largest fire seasons on record in NA because of persistent wildfires in the boreal forests of Alaska and Canada resulting from exceptionally warm and dry conditions. According to the U.S. National Interagency Fire Center (http://www.cidi.org/wildfire), more than 2.6 million hectares burned in Alaska in 2004, which represents more than 8 times the 10-year average and the highest burning on record. In western Canada the fire season was also well above the 10-year average. Smoke plumes from these fires were identified by several satellites from their aerosol and CO signatures thousands of miles downwind and fire influences were widespread. As a result, HCN and CH3CN, both tracers of biomass combustion, were elevated nearly everywhere in the troposphere [Singh et al., 2006]. The interception of biomass burning plumes several thousand kilometers downwind by the DC-8 observations indicated that very limited mixing was occurring between the stretching filaments and the background. Tracers such as CO and HCN reached concentrations as high as 600 ppb and 3 ppb in the mid Atlantic. The smoke particles were also transported efficiently from the source regions to Europe without major removal of particles. The availability of extensive atmospheric CO observations from aircraft and satellite for this period provided an opportunity to evaluate our understanding of factors controlling boreal fire emissions and their impact on atmospheric chemistry.
 S. Turquety et al. (Inventory of boreal fire emissions for North America: Importance of peat burning and pyroconvective injection, submitted to Journal of Geophysical Research, 2006, hereinafter referred to as Turquety et al., submitted manuscript, 2006) constructed a daily bottom-up fire emission inventory, including consideration of peat burning and high-altitude (buoyant) injection, and evaluated it in a global CTM (GEOS-Chem) simulation of CO through comparison with MOPITT satellite and ICARTT aircraft observations. They estimate a total North American fire emission of 28.6 Tg-CO during the summer of 2004 consistent with the top-down estimate of 30 ± 5 Tg-CO derived by Pfister et al.  from inverse modeling of MOPITT observations. The fires represented a major perturbation to summertime North American emissions of the same magnitude as the anthropogenic source (18 Tg CO). It was further deduced that the largest fires injected a significant fraction of their emissions in the upper troposphere (UT) that could be easily transported long distances.
Cook et al.  sought to investigate whether the fires in Alaska in 2004 were only of regional importance or had an impact on a wider, even hemispheric scale. They used the Cambridge TOMCAT model and the emissions data from Turquety et al. (submitted manuscript, 2006) and investigated the long-range transport of pollution across the North Atlantic and its impact on O3 production. The simulated plumes closely match CO data from the aircraft and the MOPITT satellite instrument. Although uncertainties remain in the simulation of PAN and NOx, TOMCAT simulations showed increased O3 in the troposphere over much of NA, the north Atlantic, and western Europe from photochemical production and transport. Biomass burning plumes sampled by the DC-8 in the free troposphere over NA did not show this O3 enhancement. R. C. Hudman et al. (A multiplatform analysis of the North American reactive nitrogen budget during the ICARTT summer intensive, submitted to Journal of Geophysical Research, 2006, hereinafter referred to as Hudman et al., submitted manuscript, 2006) attribute this to rapid conversion of the emitted NOx to PAN.
 W. W. McMillan et al. (unpublished manuscript, 2006) investigated transport of CO from a large fire outbreak in the Alaskan/Canadian Yukon region on 11–14 July 2004 by following it downwind to the southeastern United States and Europe until 22 July 2004. Correlations between AIRS CO and MODIS AOD indicated changes in the vertical distribution of CO as supported by in situ measurements, meteorological data, and forward trajectory analyses. Ground based lidar observations showed smoke plume altitudes from 3 to 11 km over Wisconsin and 1 to 4 km over Maryland in agreement with the forward trajectories. Comparison of AIRS CO maps and forward trajectories from the fire locations illustrate the great variations in fire emissions, especially emission height, that must be accounted for if any forecast model is to correctly predict the impact of fire emissions days to weeks downwind.
 Aerosol measurements revealed statistically robust differences in aerosol size, composition, spectral optical properties, and humidity-dependent growth for fires compared to other sources (A. D. Clarke et al., Biomass burning and pollution aerosol over North America: Organic components and their influence on spectral optical properties and humidification response, submitted to Journal of Geophysical Research, 2006, hereinafter referred to as Clarke et al., submitted manuscript, 2006). Aerosol particles from fires were larger, had a greater fraction of organic carbon (OC), and had a lower growth in response to increasing humidity. They also exhibited unique spectral signatures that revealed that they could be separated optically using remote sensing approaches.
3.1.2. Asian Pollution Over North America
 Asian pollution transport to NA peaks in late spring and little influence in summer was expected from historical evidence. Contrary to this expectation, significant Asian pollution influences were encountered across NA. INTEX-A observations thus offered an unprecedented opportunity to quantify the role of transpacific transport of Asian pollution during summer. Asian air masses were identified via trajectory analysis, model simulations, and from their chemical composition. Q. Liang et al. (Summertime influence of Asian pollution in the free troposphere over North America, submitted to Journal of Geophysical Research, 2006, hereinafter referred to as Liang et al., submitted manuscript, 2006) identified five major Asian plumes sampled during INTEX-A and analyzed these observations to examine the summertime influence of Asian pollution in the free troposphere over NA. By applying correlation analysis and Principal Component Analysis (PCA) to the observations between 6 and 12 km, they find dominant influences from recent convection and lightning (13% of observations), Asia (7%), the lower stratosphere (7%), and boreal forest fires (2%), with the remaining 71% assigned to background. Typically Asian plumes contained high levels of CO, O3, HCN, PAN, acetylene, benzene, methanol, and SO4−. The partitioning of reactive nitrogen species in the Asian plumes was dominated by PAN (∼600 pptv), with varying NOx/HNO3 ratios in individual plumes consistent with different plume ages ranging from 3 to 9 days. Export of Asian pollution in warm conveyor belts of midlatitude cyclones, deep convection, and lifting in typhoons all contributed to the five major Asian pollution plumes. Liang et al. (submitted manuscript, 2006) point out that compared to past measurement campaigns of Asian outflow during spring, INTEX-A observations display unique characteristics: lower levels of anthropogenic pollutants (CO, propane, ethane, benzene) due to their shorter summer lifetimes; higher levels of biogenic tracers (methanol and acetone) because of a more active biosphere; as well as higher levels of PAN, NOx, HNO3, and O3.
3.1.3. North American Pollution Outflow and Transatlantic Transport
 Air mass trajectories and model predictions were used to establish the occurrence of events where chemical processing could be studied in a quasi-Lagrangian framework. Several such opportunities were identified and aircraft were successfully flown to the forecast locations of the previously sampled air masses. ICARTT studies provided evidence that this type of experiment can be successfully performed in the free troposphere on intercontinental scales. Attempts were made to intercept and sample air masses several times during their journey across the North Atlantic using four aircraft based in New Hampshire (USA), Faial (Azores) and Creil (France). Methven et al.  describe techniques to identify Lagrangian matches between flight segments and further study five clear Lagrangian cases. They find indications of slow O3 increases and CO decreases in the UT over the Atlantic.
 J. Al-Saadi et al. (unpublished manuscript, 2006) investigated source/receptor relationships to characterize the dominant contributions to O3 distributions over the continental United States and Europe during the summer of 2004 campaign. For both United States and Europe they find that (1) approximately half the air in the lower troposphere (LT) experiences O3 production within the domain upon exposure to local precursor emissions, (2) air influenced by the stratosphere and by strong mixing has origins primarily over the Pacific Ocean 10 days earlier, and (3) exposure to high levels of PAN in the midtroposphere characterizes 20–25% of all free tropospheric air. The largest differences between United States and Europe were associated with persistent deep convection over the Gulf of Mexico and southern United States. They find relatively little direct Asian influence on ozone over the United States during the summer of 2004 (2%) but a sizable U.S. influence on Europe (9%).
3.2. Composition and Chemistry in the North American Troposphere
 As can be seen from Figure 3, INTEX-A successfully characterized air masses of multiple origins over NA and the Atlantic at all different altitudes. Salient among these were pollution plumes from Alaskan fires and anthropogenic pollution, and stratospheric incursions in the UT. A number of papers in this special section analyze and interpret these observations.
 Ozone was measured via in situ instruments and a UV lidar aboard the DC-8 as well as from coordinated ozonesonde launches from IONS (http://croc.gsfc.nasa.gov/intex/ions.html). The latter provided nearly 300 O3 profiles from eleven North American sites and the R/V Ronald H. Brown in the Gulf of Maine [Thompson et al., 2006a, 2006b]. Analysis of daily ozone soundings show that northeastern NA during INTEX-A was dominated by weak frontal systems that mixed aged pollution and stratospheric ozone with ozone from more recent pollution and lightning. These sources were quantified to give tropospheric ozone budgets for individual soundings. Cooper et al.  utilize data from daily ozonesondes, commercial aircraft, and lidar at 14 sites during July-August 2004 and find that the UT above midlatitude eastern NA contained 15–16 ppb more tropospheric residual ozone than the UT at the westerly upwind sites, as well as the more polluted surface layers (0–2 km). Using observed data and simulations, they conclude that these O3 enhancements can be generated from photochemical synthesis given significant amounts of lightning NOx and only background mixing ratios of CO and VOCs.
3.2.2. Reactive Nitrogen
 INTEX-A provided the most detailed measurements of reactive nitrogen over the NA troposphere. Singh et al.  analyze the distribution and partitioning of this group of chemicals. They find that the North American UT was greatly influenced by both lightning NOx and surface pollution lofted via convection and contained elevated concentrations of PAN, ozone, hydrocarbons, and NOx. Under polluted conditions PAN was a dominant carrier of reactive nitrogen in the UT, while HNO3 dominated in the LT. Unknown alkyl nitrates appear to be present in the continental boundary layer in moderately large concentrations. L. W. Horowitz et al. (Observational constraints on the chemistry of isoprene nitrates over the eastern United States, submitted to Journal of Geophysical Research, 2006, hereinafter referred to as Horowitz et al., submitted manuscript, 2006) suggest that isoprene nitrate may be a major contributor. Huey et al.  performed the first direct in situ measurements of HO2NO2 in the troposphere. The highest mean HO2NO2 mixing ratios (76 ppt) were observed at altitudes of 8–9 km, with lower levels above and below. CTMs capture the broad structure of HO2NO2 but uncertainties among models are quite large. Aerosol nitrate appeared to be mostly contained in large soil-based particles in the LT. Observational data suggested that lightning was a far greater contributor to NOx in the UT than previously believed and a major source of uncertainty among models [Singh et al., 2006; Hudman et al., submitted manuscript, 2006; Porter et al., unpublished manuscript, 2006]. J. E. Dibb et al. (unpublished manuscript, 2006) use Beryllium-7 and the HNO3/O3 ratio to identify similar distinct stratospheric and tropospheric populations in the UT. Estimates based on 7Be suggest that the stratosphere was a major source of HNO3 in the UT. Hudman et al. (submitted manuscript, 2006) and R. B. Pierce et al. (Chemical data assimilation based estimates of continental U.S. ozone and nitrogen budgets during INTEX-A, submitted to Journal of Geophysical Research, 2006, hereinafter referred to as Pierce et al., submitted manuscript, 2006) calculate a NOy export efficiency of 16% and 24% respectively from eastern NA using two different models.
3.2.3. HOx, Peroxides, and OVOC
 Measurements of OH and HO2 in INTEX-A provided an excellent test of atmospheric oxidation chemistry in and out of pollution plumes throughout the troposphere. X. Ren et al. (unpublished manuscript, 2006) compared OH and HO2 measurements to predictions using a photochemical box model constrained by coincident measurements of long-lived tracers and physical parameters. For most of the troposphere, observed OH and HO2 were less than expected from model calculations (Figure 4). In the planetary boundary layer of forested regions, the observed-to-modeled OH ratio is a strong function of isoprene. The observed and modeled HO2 greatly deviated in the UT (∼2.5 at 11 km), implying, if the measurements are correct, a major flaw in current understanding of HOx chemistry. These lower-than-expected OH measurements throughout much of the troposphere in northern midlatitudes have wide implications for ozone formation, global oxidation capacity, and reactive nitrogen distribution.
 Peroxides and formaldehyde were measured by two separate instruments [Crounse et al., 2006; J. Snow et al., Hydrogen peroxide, methylhydroperoxide, and formaldehyde over North America and the North Atlantic, submitted to Journal of Geophysical Research, 2006, hereinafter referred to as Snow et al., submitted manuscript, 2006; A. Fried et al., unpublished manuscript, 2006]. Measurements of peroxides also revealed the presence of peroxyacetic acid (PAA), which was quantified for the first time in INTEX-A. On several flights, the concentration of PAA in the UT exceeded that of hydrogen peroxide. Simulations using a constrained box model greatly underestimated the observed concentrations of PAA. In the UT, PAA was often 10–100 times larger than expected (Crounse et al., submitted manuscript, 2006). Unusually high concentrations of formaldehyde in the UT support the role of rapid convection that lofted HCHO from the LT (A. Fried et al., unpublished manuscript, 2006). Bertram et al.  find that convective outflow was strongest between 9 and 11 km and that more than 33% of the air in the UT had been placed there by convection within the last 2 days. Snow et al. (submitted manuscript, 2006) report considerable variability in mixing ratios of H2O2, CH3OOH, and HCHO throughout the North American and North Atlantic troposphere. Convectively influenced air parcels were found to be enhanced in CH3OOH, HCHO, CO, NO, and NO2, while H2O2 and HNO3 were depleted by wet removal. Biomass burning was also shown to increase UT H2O2, CH3OOH, and HCHO mixing ratios even after 4–5 days of transit.
 Oxygenated volatile organic compounds (OVOC) comprise a large number of the species whose transport to the remote troposphere can impact radical budgets, sequester NOx in the form of nitrates, and play a role in the formation of organic aerosols. Kwan et al.  use measurements of atmospheric oxidants and aerosol size distributions performed on the DC-8 during INTEX-A to suggest that organic aerosol oxidation by OH can be a significant source of OVOC in the troposphere. They estimate that the potential magnitude of OVOC flux from organic aerosol to be as large as 70 ppt C/day in the free troposphere. The potential importance and highly uncertain nature of these estimates highlight the need for field and laboratory studies on organic aerosol composition and aging.
3.2.4. Aerosols and Radiation
 Clarke et al. (submitted manuscript, 2006) performed thermal analysis of aerosol size distributions and found that the “nonplume” regional haze was dominated by pollution characteristics near the surface and biomass burning aloft. Volatile OC included most water-soluble organic carbon. Refractory OC (stable at 400°C) dominated the enhanced shortwave absorption in plumes from Alaskan and Canadian forest fires. Biomass burning, pollution, and dust aerosol could be stratified by their combined spectral scattering and absorption properties. The measured response in light scattering to humidity revealed that ambient AOD typically included a 30–50% contribution from water due to its hygrosopicity.
 Dry aerosol mass measured on the lowest flight legs provided an estimate of PM2.5, such as would be measured at the surface. Y. Shinozuka et al. (Aircraft profiles of aerosol microphysics and optical properties over North America: Aerosol optical depth and its association with PM2.5 and water uptake, submitted to Journal of Geophysical Research, 2006) show that ambient column AOD is well correlated (R2 = 0.74) to near surface PM2.5 for most cases. This indicates satellite measurements of AOD could be used as a surrogate for PM2.5 for regions over NA where such measurements are not currently available. A related study by S. Kondragunta et al. (unpublished manuscript, 2006) demonstrated that GOES AODs capture variability in atmospheric aerosol loading and can be used as a proxy to monitor pollution associated with urban/industrial sources and biomass burning events.
 Other INTEX-A measurements of aerosols and radiation from the J-31 have been reported by Redemann et al. . They found high variability in the derived aerosol forcing efficiencies for the visible wavelength range (350–700 nm). They also estimated aerosol absorption by matching calculations with the measured gradients of aerosol optical depth and radiant flux. In this way, they found larger aerosol absorption earlier in the 12 July to 8 August period, consistent with independent trajectory analyses and satellite imagery that showed the stronger influence of smoke aerosols from Alaska fires during the early period.
3.2.5. Carbon Cycle and CO2
 North American terrestrial ecosystems are major sources and sinks of carbon. Quantifying their role in the continental carbon budget was a key objective in INTEX-A. Highly precise (±0.25 ppm), fast-response (1 s) measurements of CO2 were obtained aboard the NASA DC-8 during INTEX-A. These measurements provided extensive regional-scale information on carbon sources and sinks over sparsely sampled areas of NA and adjacent ocean basins. Y. Choi et al. (unpublished manuscript, 2006) and K. Vadrevu et al. (unpublished manuscript, 2006) use airborne and satellite data along with models to investigate the ecosystem-atmospheric CO2 exchange over NA and its relationship with the observed atmospheric variability of CO2 resulting from biospheric uptake, industrial activity, and atmospheric dynamics. An example of the observed drawdown of CO2 (approximately 3%) and short-term variability particularly in the free troposphere is shown in Figure 5a. Several products derived from the LANDSAT, NOAA AVHRR, and MODIS sensors were assimilated to specify spatiotemporal patterns of land use cover and vegetation characteristics to link the aircraft CO2 data with terrestrial sources of carbon. They find that the lowest CO2 mixing ratios were over agricultural fields in Illinois dominated by corn, a C4 plant having a high photosynthetic rate.
 To facilitate and test column measurements of CO2 from ground-based and spaceborne instruments, the DC-8 and The University of Wyoming King Air aircraft (S. Wofsy, Harvard, concurrent COBRA campaign) recorded eight in situ CO2 profiles over the Park Falls Wisconsin WLEF Tall Tower Site (45.945°N, 90.273°W) where an automated solar observatory for measuring atmospheric column abundances of CO2 was colocated. Comparison of the integrated aircraft profiles and ground-based CO2 column abundances (Figure 5b) showed excellent correlation but a small (0.7%) bias [Washenfelder et al., 2006]. The comparison to aircraft integrated columns allowed the CO2 retrievals to be corrected reducing the uncertainty in retrieved column average CO2 mixing ratio to ∼0.3% (±1.1 ppm).
3.3. Integrated Model Analysis of INTEX-A Observations
 INTEX-A provided a vast amount of observational data that was further analyzed with the help of a variety of models of chemistry and transport (Table 4). Figure 6 shows a comparison of the observed O3 to NOy ratio and its simulation by four state of the art models over a large grid over eastern NA (30–50°N; 260–320°W). This ratio is chosen to compare models as it requires simulations of both O3 and its precursors (reactive nitrogen and hydrogen) and provides a qualitative measure of O3 formation efficiency. It is evident that models disagree substantially with observations as well as with each other. Validation of existing models and their further improvement using INTEX-A data was a key focus in this campaign.
3.3.1. Synthesis of Results Using 3-D Models
 Pierce et al. (submitted manuscript, 2006) performed global ozone analyses, based on assimilation of stratospheric profile and ozone column measurements, and NOy predictions from the RAQMS model to estimate the ozone and NOy budget over the continental United States. Comparison with available observations show that RAQMS captured the main features of the global and continental U.S. distribution of tropospheric ozone, CO, and NOy with reasonable fidelity. This study suggests that the majority of the continental U.S. export occurred in the UT/LS poleward of the tropopause break. Continental U.S. photochemically produced ozone was found to be a small component of the total ozone export, which was dominated by stratospheric ozone during INTEX-A. The NOy export efficiency was estimated to be 24%, with NOx and PAN together accounting for half of the total NOy export.
 Hudman et al. (submitted manuscript, 2006) used the GEOS-Chem model to test current understanding of the regional budget of reactive nitrogen. They found that a 50% reduction in power plant and industry NOx emissions over the eastern United States between 1999 and 2004 due to the NOx abatement plans resulted in estimated ozone decreases of 1–6 ppb. Increasing the model lightning source to match the constraints of observed NOx had a 5–10 ppb positive impact on free tropospheric O3, but led to a significant overestimate of OH. A Lagrangian analysis of the fraction of North American emissions vented to the free troposphere using NOy-CO correlations indicated an NOy export fraction of 15% consistent with previous studies, but significantly lower than those estimated by Pierce et al. (submitted manuscript, 2006). They report that PAN was by far the dominant component of NOy (>50%) in pollution plumes above 3.5 km.
 Horowitz et al. (submitted manuscript, 2006) used a CTM simulation (Table 4) in conjunction with INTEX-A total alkyl nitrate and isoprene observations over the eastern United States to constrain uncertainties in the chemistry of isoprene nitrates. The best agreement between simulated and observed boundary layer concentrations of organic nitrates was obtained with a 4% yield of isoprene nitrate production from the reaction of isoprene hydroperoxy radicals with NO, recycling of 40% NOx when isoprene nitrates react with OH, and fast dry deposition of isoprene nitrates. Isoprene nitrates were shown to have a major impact on the NOx budget in the continental boundary layer, consuming 17–21% of the emitted NOx.
Pfister et al.  tested the MOZART-4 model with aircraft measurements and examined the ozone production from a case study of wildfires in Alaska and Canada. Modeled and observed enhancement ratios (ΔO3/ΔCO) were about 0.25 ppb/ppb and resulted in a global net ozone production of 13 ± 2 Tg O3 during summer 2004 in good agreement with the net ozone production calculated in the model for a region ranging from Alaska to the East Atlantic (9–11 Tg O3). On average, the fires increased the ozone burden (surface–300 hPa) over Alaska and Canada during summer 2004 by about 7–9%, and over Europe by about 2–3%.
3.3.2. Model Evaluations
 S. Yu et al. (A comprehensive evaluation of the Eta-CMAQ forecast model performance for O3, its related precursors, and meteorological parameters during the 2004 ICARTT study, submitted to Journal of Geophysical Research, 2006) tested the performance of Eta-CMAQ model forecasts with the surface observations available during this campaign. The model was able to capture the observed daily maximum 8-hour O3 within a factor of 1.5 but underestimated CO by ∼30% likely because of inadequate representation of the transport of pollution associated with Alaska forest fires. T. Chai et al. (Evaluating and improving regional emission estimates using the ICARTT observations, submitted to Journal of Geophysical Research, 2006) assimilated various O3 measurements during INTEX-A/ICARTT into the STEM regional CTM and evaluated the effects of assimilating O3 observations on model predictions. During the operational phase of the ICARTT field experiment in 2004, the regional air quality model STEM showed a strong positive surface bias, and a negative UT bias with respect to ozone (M. Mena-Carrasco et al., unpublished manuscript, 2006). After updating emissions from NEI 1999 to NEI 2001 (with a 2004 large point sources inventory update) and modifying boundary conditions, surface model bias was essentially eliminated. Observed and modeled ozone production efficiencies were in good agreement suggesting that the recurring ozone bias was due to overestimated NOx emissions.
3.4. Satellite Validation During INTEX-A
Figure 2 shows all the satellite spirals undertaken by the DC-8 during INTEX-A to validate principally AIRS (Aqua), MOPITT (Terra), and SCIAMACHY (Envisat) for CO, HCHO, NO2, O3 and H2O. The three satellite sensors of primary attention for the J-31 aircraft were MISR on Terra, MODIS on Terra, and MODIS on Aqua and the focus was on validating satellite retrievals of AOD spectra and water vapor columns.
3.4.1. Chemistry-Based Validations
 W. W. McMillan et al. (unpublished manuscript, 2006) used DC-8 coincident CO data to provide crucial assessment of AIRS midtropospheric CO retrievals (400–500 hPa). They find that AIRS 400–500 hPa CO retrievals were biased high by approximately 8 (±5)%. Emmons et al.  used INTEX-A and other available measurements of CO designed to be coincident with MOPITT overpasses for the continued validation of the CO retrievals from the MOPITT instrument onboard the Terra satellite. On average, MOPITT was 7–14% high at 700 hPa and 3% high at 350 hPa and generally consistent with the MOPITT design criteria of 10% accuracy. Figure 7 shows the agreement between MOPITT and DC-8 based CO columns in the troposphere. MOPITT is shown to be useful at describing CO during several meteorological scenarios involving Alaskan fires, urban plumes and a warm conveyor belt, in the middle to upper troposphere (C. M. Kiley et al., unpublished manuscript, 2006).
 A. Heckel et al. (unpublished manuscript, 2006) and Martin et al.  compared coincident airborne in situ measurements of NO2 from INTEX-A with those derived from SCIAMACHY. A reasonable correlation was indicated although random errors of the order of 50% were possible (Figure 7). Martin et al.  further analyzed SCIAMACHY and airborne NO2 data to derive top-down emission estimates via inverse modeling using GEOS-Chem (Table 4). Their global emission inventory for land surface NOx emissions (46 Tg N yr−1) was 22% larger than the GEIA-based bottom-up inventory for 1998 but was shown to improve the GEOS-Chem simulation of NOx, PAN, and HNO3 over eastern NA. Although the Aura satellite was launched in July 2004 and not ready for validation during INTEX-A, Ozone-CO correlations observed in INTEX-A were compared to summer 2005 correlations observed by the Aura TES instrument in North American outflow as a test of the ability of TES to observe these constituents [Zhang et al., 2006].
Millet et al.  performed a detailed error analysis to investigate if satellite retrieved HCHO columns from GOME (and OMI) could be used as a proxy for isoprene emissions over NA. They used aircraft data collected over NA and the Atlantic during INTEX-A and the GEOS-Chem model to draw conclusions about space-based mapping of VOC emissions. From observed HCHO and isoprene profiles they infer an HCHO molar yield from isoprene oxidation of 1.6 ± 0.5, consistent with current chemical mechanisms. Retrieval errors, combined with uncertainties in the HCHO yield from isoprene oxidation, resulted in an estimated 40% (1σ) error in inferring isoprene emissions from HCHO satellite measurements.
3.4.2. Optical Property–Based Validations
Russell et al.  report J-31 AOD measurements using AATS and compare them to retrievals by three satellite sensors: MISR on Terra and MODIS on both Terra and Aqua. Comparisons of MISR and AATS on days with strong AOD gradients over the Gulf of Maine showed that MISR (versions 15 and 16) captured the AATS-measured AOD gradient (R2 = 0.9) for all 4 MISR wavelengths (446 to 866 nm). Comparisons of MODIS and AATS AOD values on 8 overpasses using 61 grid cells yielded R2 = 0.97 and RMS difference = 0.03, with about 87% of the MODIS AOD retrievals agreeing with AATS values within the MODIS over-ocean uncertainty. In contrast the MODIS-AATS differences in Ångstrom exponent (–dlnAOD/dlnl) were rather large but improved substantially when cases with small AOD (<0.1 at 855 nm) were excluded. Water vapor measurements from AATS were in good agreement with those of the in situ Vaisala HMP243 sensor and sondes (J. Livingston et al., Comparison of water vapor measurements by airborne sunphotometer and near-coincident in situ and satellite sensors during INTEX-A 2004, submitted to Journal of Geophysical Research, 2006). However, MODIS infrared retrievals of column water vapor were poorly correlated with (R2 = 0.20) and significantly exceeded those of AATS. In comparison, AIRS layer water vapor agreed with AATS and Vaisala to within 10%.
 S. Kondragunta et al. (unpublished manuscript, 2006) determined that operational GOES AOD imagery provided accurate spatial extent of pollution plumes despite residual cloud contamination in certain pixels. Although the GOES-suborbital AOD differences were notably larger than MODIS-suborbital differences reported by Russell et al.  and others, S. Kondragunta et al. (unpublished manuscript, 2006) argue that that GOES AODs are a good proxy to monitor pollution associated with urban/industrial sources and biomass burning events. Results from these INTEX-A validation studies are helping to quantify the tradeoffs between the greater frequency of GOES retrievals from a geostationary operational satellite and the greater accuracy of more modern sensors on polar orbiting satellites such as MODIS and MISR.
 These INTEX-A/ICARTT measurements provide a rich data set for describing the composition and chemistry of the North American troposphere, for investigating the transformation of gases and aerosols during long-range transport, for the radiation balance of the troposphere, and for validating a variety of satellite observations as well as models of chemistry and transport. Some of the highlights of this study include the first airborne measurements of HO2NO2 and peroxyacetic acid, successful conduct of quasi-Lagrangian studies to investigate processes of chemical aging during transatlantic transport, characterization of Asian pollution over NA in the summer, and the finding of an unexpectedly large source of lightning NOx in the troposphere. The oxidation capacity of atmosphere, determined by the observed HOx concentrations, was found to be nearly a factor of two lower than predicted by models. The results presented in this special section of JGR represent only the initial analysis, and much more integrated analysis is expected in the near future. The data are available to all interested parties for further analysis.
International Consortium for Atmospheric Research on Transport and Transformation.
Phase 1 of INTEX-NA.
Phase 2 of INTEX-NA.
Intercontinental Chemical Transport Experiment–North America.
INTEX Ozonesonde Network Study.
Laser induced fluorescence.
Lightning imaging sensor.
Long-Range Lightning Network.
Multiangle Imaging SpectroRadiometer.
Moderate Resolution Imaging Spectroradiometer.
Measurement of Pollution in the Troposphere.
Measurement of Ozone on Airbus In-service Aircraft.
Model of Ozone and Related Tracers.
National Aeronautics and Space Administration
National Centers for Environmental Prediction.
National Oceanic and Atmospheric Administration.
National Lightning Detection Network.
Ozone monitoring instrument.
Oxygenated volatile organic chemicals.
Peroxy acetyl nitrates.
Real-Time Air Quality Modeling System.
Scanning Imaging Absorption Spectrometer For Atmospheric Chartography.
Solar Spectral Flux Radiometer.
Sulfur Transport and Deposition Model.
Tunable Diode Laser.
Tropospheric Emission Spectrometer.
Toulouse Offline Model of Chemistry and Transport.
Total Ozone Measuring Spectrometer.
 We thank all ICARTT participants and sponsoring agencies for making this project possible. DC-8 activities were supported by the NASA Tropospheric Chemistry Program. J-31 activities received partial support from NASA's Radiation Science and NOAA's Atmospheric Composition and Climate Programs. We very much appreciate the dedicated efforts of all personnel associated with DC-8 activities from NASA Dryden, NASA Wallops, University of North Dakota, and J-31 personnel from Sky Research in making this campaign a success.