In the winter and spring of 2002, airborne and ground-based measurements of O3, aerosols, and their precursors were made in the eastern and western North Pacific regions. Three field studies were conducted by an international team of scientists collaborating as part of the Intercontinental Transport and Chemical Transformation (ITCT) program, an activity of the International Global Atmospheric Chemistry (IGAC) project of the International Geosphere-Biosphere Program (IGBP). Previous measurements have indicated that the transport of Asian emissions across the North Pacific Ocean influences the concentrations of trace tropospheric species over the Pacific and even the west coast of North America. In this special section, the recently acquired data are used to better characterize the contribution of continental sources to the aerosol, ozone, and related trace species concentrations over the North Pacific. This overview is aimed at providing the operational and logistical context of the study and introducing the principal findings and conclusions that have been drawn from the results.
 Over the past two decades there have been increasing indications that chemical pollutants, even compounds with reasonably short lifetimes, can be detected at great distances from their sources. To implement a systematic study of the intercontinental transport of anthropogenic emissions, their influence on the photochemistry of remote regions, and their possible impact on the air quality of downwind continents, a coordinated international research program was organized within the framework of the International Global Atmospheric Chemistry Project (IGAC): the Intercontinental Transport and Chemical Transformation (ITCT). The first field programs carried out as part of ITCT were the Pacific Exploration of Asian Continental Emission (PEACE)-A and PEACE-B campaigns and the ITCT 2K2 campaign. PEACE-A was conducted in January 2004, and PEACE-B and ITCT 2K2 were conducted in coordination in April-May 2004. Both PEACE campaigns used the Gulf-stream II (G-II) aircraft, chartered by Japan Aerospace Exploration Agency (JAXA) flying from Nagoya (35°N, 137°E) and Kagoshima (32°N, 131°E), Japan. The ITCT 2K2 campaign used the NOAA WP-3D aircraft flying from Monterey, California (36.6°N, 122°W), and instrumented a ground site at Trinidad Head, California where the NASA Advanced Global Atmospheric Gases (AGAGE) program measures various chlorofluorocarbons and CMDL presently maintains a baseline station and regularly launches O3 sondes. Also operating in close coordination, the Photochemical Ozone Budget of the Eastern North Pacific Atmosphere (PHOBEA) 2002 measured from a small research aircraft over the eastern North Pacific and the Cheeka Peak, Washington, surface site.
 The study of tropospheric photochemistry over the North Pacific has been initiated over the past two decades, although earlier studies documented the long-range transport of dust from Asia [Prospero, 1979; Duce et al., 1980]. The springtime is of particular interest since it is the season with the strongest outflow of Asian emissions to the Pacific, and also has the most extensive observational database. In this season in the source outflow region the NASA Global Tropospheric Experiment (GTE) missions 1994 PEM-West B [Hoell et al., 1997] and 2001 Transport and Chemical Evolution Experiment over the Pacific (TRACE-P) [Jacob et al., 2003] focused on oxidant photochemistry. The 2001 fourth Aerosol Characterization Experiment (ACE-Asia) [Huebert et al., 2003] quantified the spatial and vertical distribution of aerosol properties in this same region. The Mauna Loa Observatory Photochemistry Experiment (MLOPEX) was conducted in the central subtropical Pacific during May and June 1988 [Ridley and Robinson, 1992] and during four one-month-long intensive periods in each season of 1991/1992 [Atlas and Ridley, 1996] in the central North Pacific on Hawaii. Hess  analyzed the springtime MLOPEX results with respect to the transport and photochemical processing of the measured species.
 Studies over the eastern North Pacific have been more limited. The NASA GTE program conducted part of the Chemical Instrumentation and Evaluation (CITE) 1C Study over the western United States and the adjoining Pacific Ocean [Hoell et al., 1990]. Andreae et al.  found that sulfur dioxide and sulfate in aerosols were largely associated with emissions of sulfur dioxide from sources located on the Asian continent. Parrish et al.  found that levels of O3, peroxyacetyl nitrate (PAN), nitric acid (HNO3) and the light alkanes were enhanced during periods when trajectory analysis indicated rapid transport from Asia.
 More recently, the Photochemical Ozone Budget of the Eastern North Pacific Atmosphere (PHOBEA) surface and aircraft campaigns were conducted off the northwest coast of the United States in spring beginning in 1997. Surface observations made at the Cheeka Peak Observatory on the northwestern tip of Washington were used to quantify the in situ O3 production [Jaffe et al., 2001]. Airborne observations made in 1999 were used to derive the in situ O3 production rates for the free troposphere in that region and the significance of PAN decomposition [Kotchenruther et al., 2001]. In addition a number of episodes of transpacific long-range transport have been identified during these campaigns [Jaffe et al., 2003a, and references cited therein]. Industrial pollutants and mineral dust were the primary species found in air masses that arrived over the northeastern Pacific via transpacific transport. The GEOS-CHEM model has modeled transpacific transport and confirmed that Asian industrial sources are a dominant source for CO and O3 in the northeast Pacific region [Jaeglé et al., 2003].
 The PEACE and ITCT 2K2 coordinated studies were organized around four major objectives: (1) to investigate the seasonal variation in the mode of horizontal and vertical transport and its effect on the distributions of ozone precursors and aerosols in the North Pacific troposphere, (2) to quantify seasonal variation of the oxidizing capacity and ozone budget over the western North Pacific from winter to late spring as a result of changes in key photochemical and transport processes, (3) to characterize the chemical composition of the air masses coming ashore at the U.S. West Coast, and to determine its relation to the sources and sinks of ozone and aerosols and (4) to explore the composition of these air masses as they are transported inland, and investigate the alteration in composition associated with the addition of emissions from U.S. West Coast sources.
 The ITCT 2K2 study was primarily an exploratory mission with additional specific science questions designed to guide mission planning: (1) Will measurements of trace chemicals or aerosol chemical composition provide elemental and/or chemical speciation fingerprints for different source classes or source regions? (2) What mechanisms control the export of emissions from Asia (and Mexico-Central America, and North America) to the North Pacific Ocean? (3) How does the composition of the anthropogenic emissions change during transport, and what is the effect upon ozone, aerosols and other photochemical products? (4) What are the magnitude and ultimate impact of emissions from commercial shipping and air transport in the Pacific? (5) Can we find evidence of influence from continental sources upwind of Asia (Europe, tropical regions, or globally circulated North American emissions)?
2. Design and Measurements
 PEACE-A was conducted in January 2004 and PEACE-B and ITCT 2K2 were conducted in coordination in April-May 2004. The timing of the PEACE missions, combined with the March 2001 TRACE-P study, provides coverage of the seasonal progression of O3 photochemistry and O3 precursor transport processes from winter to late spring [Kondo et al., 2004]. We chose to focus on spring since it is the season with the strongest outflow of Asian emissions to the Pacific, has the most extensive past observational database, and encompasses the northern hemisphere tropospheric O3 maximum that is thought to largely result from the maximum in the net photochemical production of O3.
 The major sampling regions over the North Pacific are shown in Figures 1 and 2. The PEACE program operated over the western North Pacific off the coast of Asia, both to the southeast and northwest of Japan in the Asian outflow region, while ITCT 2K2 focused upon the receptor region over the eastern North Pacific near the North American west coast.
2.1. Aircraft Instrument Payload and Flights
 In situ chemical data obtained on board the G-II aircraft were used for the PEACE study. The measured quantities included O3, CO, NMHCs, NO, total reactive nitrogen (NOy), H2O, and photolysis frequency of NO2 (J(NO2)). The instruments used here are the same as those used for BIBLE-B campaigns [Kondo et al., 2002], with an addition of an SO2 instrument. Table 1 summarizes the techniques, uncertainties (accuracy and precision), and time resolutions of these measurements. Tables 2a and 2b summarize the flights for the winter and spring PEACE campaigns, and Figure 1 illustrates the flight tracks.
Table 1. Instruments Used Aboard the GII Aircraft During PEACE-A and PEACE-B
 The NOAA WP-3D Orion aircraft was instrumented for the ITCT 2K2 study with an ozone photochemistry payload that was augmented with measurements of aerosol composition. The aircraft could operate from the marine boundary layer (MBL) up to 8 km and had sufficient range to reach to the Canadian and the Mexican borders and well out into the North Pacific when stationed in Monterey, California. The payload used in previous studies measured ozone, its precursors (volatile organic compounds and nitrogen oxides), products and by-products of the chemistry that produces ozone, size resolved aerosol number density, emission tracers (SO2, CO2, CO and certain VOCs), actinic fluxes, aircraft and meteorological state parameters. New aerosol instruments added for ITCT 2K2 included a single particle aerosol composition measurement provided by Particle Analysis by Laser Mass Spectrometry (PALMS) and the soluble bulk ionic composition measurements through particle into liquid sampling (PILS). A chemical ionization mass spectrometer measured H2SO4 and OH radicals. Tables 3a and 3b summarize the WP-3D instrument characteristics. Table 4 and Figure 2 summarize the ITCT 2K2 flights.
Table 3a. Instruments for Gas-Phase Measurements Aboard the WP-3D Aircraft During ITCT 2K2
white light scattering (Climet & LasAir) with low turbulence inlet
280–400 nm spectrally resolved radiometer, zenith and nadir
Table 4. Dates and Locations of the ITCT 2K2 WP-3D Aircraft Flights
transit Tampa, Florida, to El Paso, Texas, sample Houston area chemical plants
transit El Paso to Monterey, California
transect of cutoff low SSW of Monterey
step profile on west side of cutoff low
Trinidad Head flyby, ship plume study, Richmond/San Francisco Bay Area
intercept Asian outflow NW of Monterey
follow Asian outflow ashore
ship plume study, Richmond/San Francisco Bay Area, California Central Valley
vertical distribution along longitudinal transect over Trinidad Head
vertical distribution along latitudinal transect off U.S. West Coast
Los Angeles basin
Asian outflow in high-pressure system SW of Monterey
intercept Asian outflow SW of Monterey
transit Monterey, California, to Jefferson County, Colorado, sample Los Angeles outflow, power plant plume in SW Colorado, and forest fire plume
2.2. Trinidad Head and PHOBEA Measurements
 The primary site instrumented for the ITCT 2K2 study operated at Trinidad Head, California (41°03′N, 124°09′W, 107 m asl) from 19 April to 22 May 2002. Volatile organic compounds were measured using a fully automated, in situ GC/MSD/FID system that is described in detail elsewhere [Millet et al., 2004b]. O3, CO, CO2 and met parameters were measured [Goldstein et al., 2004] along with NO and NOy, and 222Rn using a dual-flow loop, two-filter radon detector [Whittlestone and Zahorowski, 1998]. Time-resolved aerosol measurements at the site included chemical composition using an Aerodyne aerosol mass spectrometer (AMS, Aerodyne Research Inc.) [Jimenez et al., 2003; Allan et al., 2003] and a particle-into-liquid sampler (PILS) [Weber et al., 2001; Orsini et al., 2003], number density (7 nm to 2.5 μm) using a condensation particle counter (CPC, model 3022a, TSI Inc.), and elemental composition using an 8-stage drum impactor and synchrotron X-ray fluorescence [Bench et al., 2002; Cahill and Wakabayashi, 1993; Perry et al., 1999]. Balloon-borne sondes launched daily from the site measured O3 concentrations and meteorological parameters with 1.2 second resolution from the surface to approximately 35 km elevation. Direct, diffuse and total broadband solar irradiance (down welling), total down welling IR irradiance were also measured.
 In coordination with the ITCT 2K2 experiment, PHOBEA measurements of CO, O3, aerosol scattering (550 nm), elemental mercury, radon and meteorology parameters were made at the Cheeka Peak Observatory (48°18′N, 124°36′W, 480 m asl) from March through May 2002. The instrumental methods have been described previously [Jaffe et al., 2001; Weiss-Penzias et al., 2003]. From 29 March through 23 May 2002, 13 flights using a small research aircraft (Beechcraft Duchess 76) were conducted over the NE Pacific Ocean (47.8°–48.5°N and 123.9°–125.4°W, 0–6 km asl). On each flight, CO, O3 and aerosol scattering (red, green and blue) were measured. Bertschi et al.  describe the experimental methods, calibration procedures and other quality control measures for the PHOBEA airborne measurements.
2.3. Model Simulations
 The flight planning for the NOAA WP-3D was guided by forecasts from the CFORS-STEM, GEOS-CHEM, MOZART, and MATCH Eulerian chemical transport models (CTM), and the FLEXPART Lagrangian particle dispersion model. Most of the institutions that provided models also sent a scientist to the ITCT 2K2 operations center to interpret the model products and discuss the information with the flight coordinators.
 The NOAA Geophysical Fluid Dynamics Laboratory brought the MOZART-II CTM into the field. The model was run once per day in full chemistry mode using the forecast National Centers for Environmental Prediction (NCEP) global AVN T170 wind fields (0.7° × 0.7° resolution). Products included fossil fuel, biomass burning and tagged regional CO tracers, as well as ozone, NOx and a stratospheric ozone tracer, all calculated at 1.9° × 1.9° resolution [Horowitz et al., 2003].
 The University of Iowa ran the CFORS online tracer model based on forecast wind fields (200 km × 200 km resolution) from the RAMS model. Products included tagged regional CO tracers, and dust and sea salt tracers. Emissions from megacities were also tagged. In addition the STEM regional CTM was run once per day in full chemistry mode using the 60 × 60 km forecast wind fields of the RAMS model [Tang et al., 2004].
 Harvard University and the University of Washington provided forecasts from the GEOS-CHEM global CTM [Bey et al., 2001b], based on wind fields from the Goddard Earth Observing System (GEOS) of the NASA Global Modeling and Assimilation Office, regridded to 2° × 2.5° horizontal resolution. Products were provided once per day and included a total CO tracer using archived monthly OH fields [Bey et al., 2001a] as well as Asian, European and North American fossil fuel tracers and a biomass-burning tracer. All products were updated twice daily.
 The Max-Planck-Institut für Chemie, in Mainz, Germany provided products from the MATCH-MPIC CTM over the internet [Lawrence et al., 2003]. On the basis of the NCEP AVN wind fields, regridded to 2.8° × 2.8° resolution, the model produced 3-day forecasts of the global distributions of ozone and regional CO tracers. The tracer products were updated once per day.
 The FLEXPART Lagrangian particle dispersion model [Stohl et al., 1998; Stohl and Thomson, 1999] provided 3-day forecasts of Asian, European and North American CO tracers. The tracer forecasts were updated 4 times per day, were driven by the 1° × 1° wind fields of the NCEP AVN model, and were made available at 1° × 1° resolution. The accuracy of the FLEXPART forecasts during the campaign is described by Forster et al. .
 Finally, near-real-time geostationary satellite imagery were provided by the NOAA Aeronomy Laboratory. Data from the visible, infrared and water vapor channels of the Japanese GMS-5 and the NOAA GOES-West satellites were downloaded hourly from UNIDATA and the University of Wisconsin. Panoramic views of the North Pacific basin were produced at 5 km resolution, and used to verify the transport pathways of the model tracers and to identify the weather systems advecting Asian pollution to the United States [Cooper et al., 2004a, 2004b].
3. Overview of Results
3.1. Studies in the Source Region: Overview of the Pacific Exploration of Asian Continental Emission (PEACE)-A and PEACE-B Campaigns
Kondo et al.  present box model calculations constrained by the observed concentrations of the O3 precursors. These calculations show that the net O3 formation in the boundary layer over the western North Pacific makes a major contribution to the column integrated net O3 formation, especially in winter. The northwesterly wind in the boundary layer is highest over the western Pacific in winter because of the dominating Siberian high. It weakens with the progression of the season associated with the weakening of the Siberian high and strengthening of the Pacific high pressure. The net O3 formation rate in the boundary layer is largest in winter because of the highest NO concentrations, which are caused by efficient transport of NOx from the Asian continent due to high wind speeds and slow oxidation by OH. The net O3 formation rate decreases from winter to spring because of the increase in the O3 destruction rate associated with the increase in J(O1D) and H2O. From January to April/May the column integrated O3 formation rate is 6 times larger than the increase in the O3 column of 3.1 × 1017 molecules cm−2, indicating that O3 formed over the western Pacific is transported to regions over the Pacific outside the region of the present study.
 G. Chen et al. (A diagnostic analysis of winter/spring ozone budget based on ozonesonde and airborne observations from PEACE A/B and TRACE-P, submitted to Journal of Geophysical Research, 2004) (hereinafter referred to as Chen et al., submitted manuscript, 2004) present an analysis complementary to that of Kondo et al. . They construct an observation-based regional tropospheric ozone column (TOC) budget in the western North Pacific for the time period from January to May. All budget terms, except for stratospheric flux estimates taken from current literature, are evaluated from either ozonesonde measurements recorded at Japanese stations (Naha, Kagoshima, and Tateno) or airborne data from PEACE-A, PEACE-B, and TRACE-P. Ozonesonde data show a significant winter/spring time increase of TOC, ranging, depending on latitude, from 16 to 48% of the average burden. The net effect of advective processes, estimated as a residual of the mass balance equation, is found to be a major sink of ozone to offset the photochemical production. The derived eastward advective flux is 20 times higher than the photochemical production. Thus the observed TOC trend is not controlled by local photochemistry. Because of this rapid advection, Chen et al. (submitted manuscript, 2004) conclude that the regional TOC seasonal variation is strongly modulated by the hemispheric ozone seasonal changes, while the net ozone exported from the western North Pacific is an important source for downwind regions where photochemistry is a net sink.
Takegawa et al.  estimate the removal rates of NOx and NOy in the boundary layer during the PEACE-A period. Correlations of CO with CO2 and back trajectories are used to identify plumes strongly affected by Asian continental emissions. ΔCO/ΔCO2 ratios in the plumes generally fall within the variability range of the CO/CO2 emission ratios derived from the emission inventory of Streets et al. , demonstrating the consistency between the aircraft measurements and the emission characterization. The photochemical age of the plumes is derived from the observed ΔC2H4/ΔC2H2 ratios and the OH concentration calculated by a constrained photochemical box model. The average lifetime of NOx in the plumes is estimated to be 1.2 ± 0.4 days using the correlation with CO2 and the plume age. It is shown that NO2 + OH reaction and N2O5 hydrolysis can likely account for most of the NOx loss processes under PEACE-A conditions. The average lifetime of NOy in the plumes is estimated to be 1.7 ± 0.5 days, suggesting the importance of chemical processing near the source regions in determining the NOy budget.
 Transport processes that were responsible for export of anthropogenic emissions over the east Asia during the PEACE-B period are described by Oshima et al. . During this period a quasi-stationary frontal zone had formed over central China by low-level southerlies. Pronounced upward motion of air along the frontal zone followed by westerly transport by the subtropical jet over this system is found to be an efficient transport mechanism of pollutants emitted around central China. Episodic enhancements of convection play an important role producing the updrafts. Back trajectories of air parcels sampled onboard the aircraft show that 27% of air parcels sampled at altitudes above 4 km were likely influenced by convection over central China and other regions. These results are consistent with enhancements of CO and relatively high levels of Halon 1211 (a good tracer of Chinese anthropogenic emissions) observed at altitudes between 5 and 10 km during several flights.
 Measurements of O3 by ozonesondes were made over subtropical and midlatitude China during the PEACE-A period [Chan et al., 2004]. The O3 concentrations in the boundary layer were similar to those observed over the western Pacific, except near Beijing, where titration by NO reduced O3. Influx of O3 from the stratosphere likely caused occasional increases in O3 in the free troposphere.
3.2. Overviews of Transport and Photochemistry
 Six analyses describe the transport pathways from Asia to North America, two of which also model the photochemistry during transit. Forster et al.  used the FLEXPART particle dispersion model to estimate the quantity of Asian CO transported across the western edge of North America. On the basis of a 15-year climatology, CO transport has a springtime maximum and summer minimum, with the springtime flux centered at 35°N and 8 km altitude. During the ITCT period the CO flux was 8% less than the climatological mean for April and May.
Liang et al.  simulated global CO transport for the March 2001 through May 2002 time period with the GEOS-CHEM CTM and identified one winter and four spring cases when Asian transport events increased CO at Cheeka Peak Observatory by 20–40 ppbv. Most events reaching the eastern North Pacific lower troposphere were the result of boundary layer outflow behind cold fronts (for spring) or ahead of cold fronts (for other seasons) followed by low-level transpacific transport. In contrast, lifting ahead of cold fronts on the western side of the Pacific was associated with 78% of the events reaching the eastern North Pacific middle and upper troposphere.
Cooper et al. [2004a, 2004b] provide detailed case studies of transpacific warm conveyor belt (WCB) transport. The first analysis [Cooper et al., 2004b] showed that a WCB that formed over Japan entrained air from a variety of source regions including the marine boundary layer, the polluted lower troposphere of east Asia, the midtroposphere and an aged upwind WCB. The polluted WCB had decayed by the time it reached the U.S. West Coast and advected across the country with only minimal impact on the lower troposphere. The second study [Cooper et al., 2004a] described the manner in which stratospheric intrusions decay and disperse into the remnants of polluted WCBs above the Pacific, blurring the chemical distinction between anthropogenic and stratospheric influenced air masses.
Hudman et al.  used the global GEOS-CHEM CTM in full chemistry mode [Bey et al., 2001b] to interpret the ITCT 2K2 and PEACE-B aircraft observations, and to assess the impact of Asian emissions on U.S. surface ozone. They argue that PAN decomposition represents a major and possibly dominant component of the ozone enhancement in transpacific plumes carrying Asian emissions. They also found that strong dilution of Asian pollution plumes takes place during entrainment in the U.S. boundary layer, greatly reducing their impact at U.S. surface sites.
Tang et al.  modeled the photochemistry of the eastern North Pacific and U.S. West Coast using the STEM-2K3 regional CTM. Focusing on two flights they found the ozone net chemical budgets were negative in regions dominated by Asian influences and positive in polluted American air masses. The results also indicated that the low NOy levels in Asian air masses was partly due to the conversion of gaseous HNO3 to nitrate particle. Surface pollution episodes at Trinidad Head were the result of calm conditions that allowed the accumulation of local emissions.
3.3. Studies in the Receptor Region: PHOBEA Results
 The PHOBEA spring 2002 airborne data reveal numerous episodes of long-range transport (LRT) of pollutants from the Eurasian continent [Bertschi et al., 2004]. A substantial episode on 15 April 2002 was associated with LRT from industrial sources in Asia, similar in many respects to previous LRT episodes [Jaffe et al., 2003a; Price et al., 2003], in that it contained significant enhancements of CO, O3 and aerosols. However since the measurements continued later in the spring than in previous years, Bertschi et al.  observed evidence of LRT of biomass burning sources in Siberia. These plumes were transported to North America in 6–8 days. Confirmation of the biomass burning source for several LRT events in May was made by using satellite imagery, back trajectories, the GEOS-CHEM global chemical transport model, and the chemical signature of the observed plumes (e.g., CO to aerosol ratio).
Price et al.  describe the dilution and photochemistry during 11 transpacific LRT episodes from our observations taken between 1997 and 2002. This work identifies two key processes that help explain variations in O3 mixing ratios during LRT events: presence of mineral dust and transport in the boundary layer. For those LRT events with significant boundary layer transport or with large amounts of mineral dust, relatively low ozone enhancements were seen. Only when the air mass was transported in the free troposphere and in the absence of large amounts of mineral dust is an O3 enhancement identified.
 The GEOS-CHEM model was used both for forecasting LRT and in postmission analysis [Liang et al., 2004; Goldstein et al., 2004; Weiss-Penzias et al., 2004]. Liang et al.  quantified the Asian influence on CO mixing ratios in the eastern North Pacific atmosphere over a full seasonal cycle, including the ITCT 2K2 time period. They found that Asian sources were responsible for 24–30% of the CO in this region. Liang et al.  also carried out a systematic analysis of the meteorological mechanisms leading to transpacific transport events. Frontal lifting and zonal transport in the free troposphere was the dominant mechanism to bring pollutants to the middle and upper troposphere over the eastern North Pacific. The majority of events reaching CPO below 2 km altitude were the result of boundary layer export either behind or ahead of midlatitude cyclones, followed by low-level transport across the Pacific. Weiss-Penzias et al.  interpret a full year of observations from CPO using both the GEOS-CHEM model and back trajectories.
3.4. Gas-Phase Composition and Photochemistry
3.4.1. Gas-Phase Species in the Marine Boundary Layer
Millet et al. [2004a] characterize the composition of the volatile organic compounds (VOCs) in the eastern North Pacific marine boundary layer, and use the observed variability of the concentrations to infer many aspects of transport and transformation of species in the sampled air masses. They report hourly in situ measurements of C1-C8 speciated VOCs obtained at Trinidad Head, California, in April and May 2002. They utilize factor analysis to elucidate the dominant processes affecting the concentrations, and to characterize the sources of the measured species. Strong decreases in background concentrations were observed for several of the VOCs during the experiment due to seasonal changes in OH concentration. CO is the most important contributor to the total measured OH reactivity at the site at all times. Oxygenated VOCs are the primary component of both the total VOC burden and of the VOC OH reactivity. VOC variability exhibited a strong dependence on residence time (slnX = 1.55 × τ−0.43, r2 = 0.98; where slnX is the standard deviation of the natural logarithm of the mixing ratio), and this relationship is used, in conjunction with measurements of 222Rn, to estimate the average OH concentration during the study period (6.1 × 105 molecules cm−3). They also employ the variability-lifetime relationship defined by the VOC data set to estimate submicron aerosol residence times as a function of chemical composition. Two independent measures of aerosol chemical composition yield consistent residence time estimates, 3–7 days for aerosol nitrate, organics, sulfate, ammonium, and sea salt. The lifetime estimate for methane sulfonic acid (∼12 days) is slightly outside of this range, and the lifetime of the total aerosol number density is estimated at 9.8 days.
Goldstein et al.  measured a wide suite of trace gases and aerosols in concert with the VOC measurements of Millet et al. [2004a]. They use a combination of in situ ground-based measurements from Trinidad Head, California, chemical transport modeling, and backward trajectory analysis to examine the impact of long-range transport from Asia on the composition of air masses arriving at the California coast at the surface. The impact of Asian emissions is explored in terms of both episodic enhancements and contribution to background concentrations. The variability in CO concentrations was largely driven by North American emissions, and episodic enhancements due to Asian plumes were not observable. Nevertheless, model simulations suggest that Asian emissions were responsible for 33% of the measured CO, a larger mean contribution than direct emissions from any other region of the globe. Surface ozone levels are found to depend primarily on local atmospheric mixing, with surface deposition leading to low concentrations under stagnant conditions. Model simulations suggest that on average 4.5 ± 1.1 ppb of ozone (11% of average observed) was transported from Asia.
 The study by de Gouw et al.  used the measurements of acetonitrile (CH3CN) from the NOAA WP-3D aircraft to demonstrate the utility of that species as a biomass-burning tracer. Acetonitrile was strongly enhanced in the plumes from two forest fires, confirming its presence in biomass burning emissions. The emission ratios of acetonitrile relative to CO in the two plumes were slightly higher than previously reported values for fires burning in other fuel types. No significant acetonitrile release was observed in the Los Angeles basin or from other point sources (ships and a power plant). Acetonitrile concentrations were significantly reduced in the marine boundary layer indicating the presence of an ocean uptake sink. Increased loss of acetonitrile was observed close to the coast, suggesting that acetonitrile was efficiently lost by dissolving in the upwelling ocean water, or by biological processes in the surface water.
3.4.2. Trends in Marine Boundary Layer Photochemistry
 Partially on the basis of data collected during ITCT 2K2, Jaffe et al. [2003b] conclude that O3 in air arriving at the North American coast from the eastern Pacific in spring has increased by approximately 10 ppbv, i.e., 30% from the mid-1980s to the present. Parrish et al. [2004a] provide substantial evidence that the increased O3 has resulted from a marked change in the photochemical environment in the springtime troposphere of the North Pacific. They note (1) larger increases in the minimum observed ozone levels and much more modest increases in the maximum levels, (2) increasing PAN levels that parallel trends in NOx emissions, and (3) decreasing efficiency of photochemical O3 destruction (less negative P(O3)). This changing photochemical environment is hypothesized to be due to increasing anthropogenic emissions from Asia, which are believed to have increased substantially over the 2 decades preceding the study. The influence of the increased emissions is proposed to have changed the spring time Pacific tropospheric photochemistry from predominately ozone destroying to more nearly ozone producing. However, chemical transport model calculations indicate the possible influence of a confounding factor; unusual transport of tropical air to the western North Pacific may have played a role in this apparent trend in the photochemistry.
3.4.3. Gas-Phase Species in the Eastern North Pacific Midtroposphere
Nowak et al.  investigate the gas-phase chemical characteristics of emission plumes transported from Asia across the Pacific Ocean. Plumes measured from an aircraft are separated from background air by using 1-s measurements of carbon monoxide (CO), total reactive nitrogen (NOy) and other gas-phase species along with back trajectory analysis. Asian transport plumes with CO mixing ratios greater than 150 ppbv are identified on seven flights. Correlations between CO and O3 and NOy are used to characterize the plumes. The NOy/CO ratios were similar in each plume, but significantly lower than those derived from Asian emission inventories. Despite the similar NOy/CO ratios the three strongest transport plumes exhibited differing NOy partitioning. PAN dominated NOy in the two plumes that were transported in cold, high-latitude and high-altitude regions, whereas HNO3 dominated in the one plume transported in warmer, lower latitude and altitude regions. Additional gas-phase species enhanced in one or more of these plumes include sulfuric acid, methanol, acetone, propane, and ethane. The O3/CO ratio varied among the plumes and was affected by the mixing of anthropogenic and stratospheric influences. The complexity of this mixing prevents the determination of the relative contribution of anthropogenic and stratospheric influences to the observed O3 levels.
 During research flights on 5 and 17 May, strong enhancements of carbon monoxide (CO) and other species were observed in air masses that had been transported from Asia [de Gouw et al., 2004]. The hydrocarbon composition of the air masses indicated that the highest CO levels were related to fossil fuel use. During the flights on 5 and 17 May and other days, the levels of several biomass-burning indicators increased with altitude. This was true for acetonitrile (CH3CN), methyl chloride (CH3Cl), the ratio of acetylene (C2H2) to propane (C3H8), and, on 5 May, the percentage of particles measured by the PALMS (particle analysis by laser mass spectrometry) instrument that were attributed to biomass burning on the basis of their carbon and potassium content. An ensemble of back-trajectories, calculated from the U.S. West Coast over a range of latitudes and altitudes for the entire ITCT 2K2 period, showed that air masses from Southeast Asia and China were generally observed at higher altitudes than air from Japan and Korea. Emission inventories estimate the contribution of biomass burning to the total emissions to be low for Japan and Korea, higher for China, and the highest for Southeast Asia. Combined with the origin of the air masses versus altitude, this qualitatively explains the increase with altitude, averaged over the whole ITCT 2K2 period, of the different biomass burning indicators.
 Measurements of PANs, peroxyacetic nitric anhydride (CH3C(O)OONO2; PAN), and peroxypropionic nitric anhydride (CH3CH2C(O)OONO2; PPN) were made in the spring of 2002, off the west coast of North America, as part of the Intercontinental Transport and Chemical Transformation project [Roberts et al., 2004]. Long-range transport events from Asia were observed in which PAN and PPN were as high as 650 pptv and 90 pptv respectively. Moreover, these two species constituted as much as 80% of the odd-nitrogen (NOy) in those air masses, and median PAN/NOy was more than 60% at altitudes of 4 km and above. Mixing ratios of PAN and PPN were also elevated in the marine boundary layer (MBL) close to the west coast of California, probably as a result of maritime NOx emissions, which provide the raw materials to photochemically produce PANs.
 A ship plume experiment was carried out about 100 km off the California coast during the recent NOAA airborne field campaign ITCT 2K2 (G. Chen, An investigation of the chemistry of ship emission plumes during ITCT 2002, submitted to Journal of Geophysical Research, 2004). Observations demonstrate a very short NOx lifetime (∼1.8 hours) inside the ship plume compared to ∼6.3 hours in the background marine boundary layer. An analysis with a photochemical model suggests that more than 80% of the NOx loss was due to the NO2 + OH reaction; the remainder was by PAN formation, which can be considered as a NOx reservoir species for the conditions encountered. The model underestimated in-plume NOx loss rate by about 30%. A comparison of measured to predicted H2SO4 in the plumes suggests the model may underestimate OH by as much as a factor of 2. However, model simulation of in-plume O3 production agrees well with the observations, suggesting that model predicted peroxy radical (HO2 + RO2) levels are reasonable. The largest model bias was seen in the comparison of HNO3. The model overestimated in-plume HNO3 by about a factor of 6. This is most likely caused by underestimated HNO3 sinks, such as particle scavenging of HNO3. However, available data did not allow a conclusive test of this possible loss process. Future studies should revisit these issues to further test our understanding of the ship plume chemical processes.
3.5. Aerosols in the Eastern Pacific and Western North America
 Measurements of particle properties obtained during ITCT 2K2 in westerly flow conditions show a complex and dynamic aerosol affected by a variety of sources and chemical and physical processes during transport. Measurements of particle and trace gas characteristics at the surface site at Trinidad Head during periods of unambiguous transpacific flow indicate that the MBL aerosol was generally decoupled from that aloft. The particles at the coastal site were dominated by sea salt in the super micron (coarse) mode, and by sulfate, ammonium, nitrate, and organics in the submicron (fine) mode [Allan et al., 2004]. The organics present in the particles appear to be highly oxidized, probably because of the gradual oxidation and condensation of organic precursor gases during long-term transport from transpacific sources. The lifetime of the sulfate, nitrate, organic, and number concentration component of the marine fine aerosol has been estimated using variability-lifetime relationships with VOCs to be ∼4–10 days, while the lifetime of the ammonium and sea-salt components is shorter because of larger oceanic sources and sinks [Millet et al., 2004a]. There is some evidence for displacement of the MBL aerosol by air with free tropospheric aerosol characteristics, in particular, enhanced concentrations of Asian crustal material, during the first few days of operations at the Trinidad head site (R. A. VanCuren et al., Continental aerosol dominance above the marine boundary layer in the eastern North Pacific: Continuous aerosol measurements from the 2002 Intercontinental Transport and Chemical Transformation (ITCT 2K2) experiment, submitted to Journal of Geophysical Research, 2004) (hereinafter referred to as VanCuren et al., submitted manuscript, 2004). This free tropospheric influence is not unambiguously identified in the gas-phase data [Millet et al., 2004a]. Episodes influenced by coastal North American sources were evident in most of the aerosol data sets from Trinidad Head despite the relatively remote coastal location; their influence emphasizes that short instrumental sampling times and careful data evaluation are necessary to exclude contamination by local and regional sources.
 Above the MBL, a diverse range of particle properties were observed over the eastern Pacific and western North America during westerly flow conditions. Elemental analysis of size-segregated aerosol samples collected at two high-altitude surface sites, Trinity and Lassen, show a dominance of crustal components with peak mass loadings in the coarse size range, from 2.5 to 5 μm diameter (VanCuren et al., submitted manuscript, 2004). Submicron S, Pb, Zn, and K were associated with the crustal material and are characteristic of the aged Eurasian continental aerosol [VanCuren, 2003]. Airborne observations of particle size distributions, bulk submicron ionic composition, and single particle composition show that particle chemical and physical properties above the boundary layer were highly inhomogeneous in space and time. The aerosol could be characterized as a background of mixed sulfate-organic particles present at quite low concentrations, on which were superimposed layers of aerosol originating from industrial, biomass-biofuel, and crustal sources [Brock et al., 2004; Hudson et al., 2004]. The layers associated with these sources were often discrete and decoupled. For example, on 5 May 2002, a single vertical profile showed the presence of three distinct layers between 4 and 8 km; the upper one from Asian biomass-biofuel combustion, the middle one from Asian industrial-urban-biofuel sources, and the lower one containing coarse crustal particles from an unknown source [Brock et al., 2004]. Throughout the study, particles identified as biomass-biofuel combustion origin contributed a surprising 30–40% of the total particle number concentration detected by the PALMS single-particle mass spectrometer in the free troposphere [Hudson et al., 2004]. These particles were distinct from recently formed biomass-biofuel particles by the presence of sulfur compounds, presumably deposited during aging of the particles over long transport times.
 Analyses of the transport of the aerosol layers [Cooper et al., 2004b; Brock et al., 2004] are generally consistent with the findings from PEACE- B, TRACE-P, and other studies [Oshima et al., 2004; Miyazaki et al., 2003; Moore et al., 2003; Jacob et al., 2003; Crutzen and Lawrence, 2000] namely, that prefrontal lifting in WCBs and through frontal and postfrontal convection are the primary mechanisms for lifting polluted air from the Asian PBL into the free troposphere, where it may be transported across the Pacific. The importance of cloud processing of the polluted air is evident in the data from 17 May 2002. In this case, the insoluble or partially soluble gases SO2 and PAN were transported upward in the WCB of a Pacific wave cyclone, while existing aerosol particles and soluble gases were scavenged and removed by precipitation. In the ensuing several-day transport time in the free troposphere, thermal decomposition and photochemical conversion of the gases led to the formation of an aerosol layer rich in sulfuric acid particles and in gas-phase nitric acid [Brock et al., 2004; Nowak et al., 2004].
 Within the PBL over North America, local sources quickly dominated the characteristics of the Pacific air transported eastward over the continent. Over southern California, local and regional NOx and ammonia sources produced rapid conversion of gas-phase nitric acid to the particle phase [Neuman et al., 2003]. The spatial and vertical distribution of these ammonium nitrate particles is often governed by the source locations and by the atmospheric temperature profile. Local and regional biomass combustion sources can produce concentrated and dilute plumes with aerosol characteristics dominated by combustion-generated particles [Hudson et al., 2004]. Thus within the continental PBL, local and regional sources dominate over long-range transport from transpacific sources. However, within the free troposphere, layers of particles of Eurasian origin remain and may affect midtropospheric cloud and chemical processes. Frontal activity and convection will add a North American component to the free tropospheric aerosol during further eastward transport.
 The ITCT 2K2 and PEACE data sets comprise a remarkably rich data set for investigating the outflow from the emission regions on the Asian continent, their photochemical processing during transport across the Pacific, and their properties and impact upon arrival over North America. The results presented in this special section of JGR represent only the initial analysis; the data set will be available to the atmospheric chemistry community for further analysis in the coming years.
Appendix A:: Comparison of Aircraft- and Sonde-Measured O3 Profiles
 During the ITCT 2K2 study the NOAA CMDL Laboratory launched daily O3 sondes from their site at Trinidad Head, California (see Figure 1). On six flights the NOAA WP-3D aircraft also measured O3 during vertical profiles flown in the vicinity of Trinidad Head. Figure A1 shows the measured profiles for the aircraft flight most nearly coincident in time and location with a sonde launch. In general there is excellent agreement in magnitude and structure between the two profiles; the few ppbv differences can be attributed to differences in time and/or location of the profiles. The other 5 profiles showed larger differences, but they were also further apart in time (≤5 hours) and/or location (≤150 km). For five of the six flights the aircraft and the Trinidad Head ground site were in similar air masses at the lowest flight level of the aircraft. Excellent agreement was found between the aircraft and ground O3 instruments.
 To compare the data from vertical profiles from all six flights, O3 was averaged over 0.25 km altitude increments for both profiles on each day. These data were averaged for each 0.25 km increment for only those days when data were available from both measurements for that increment on that day. This procedure is deemed to give the most closely comparable data sets for the two techniques. Figure A2 shows the resulting comparison. The data scatter about the 1:1 line, and a linear regression with the intercept forced to zero (equivalent to assuming no offset in either technique) gives a slope statistically equivalent to unity.
 From these comparisons we conclude that the measurement of O3 at the surface site, from the NOAA WP-3D aircraft, and from the CMDL sondes, from the surface to the midtroposphere are consistent with each other, with no relative errors larger than 2 to 3 percent or offsets larger than 2 ppbv.
 Chen et al. (submitted manuscript, 2004) reached similar conclusions when they compared O3 sonde data from three Japanese stations with aircraft data collected during the PEACE and TRACE-P studies at the same latitude. Good agreement was found between these data, and these workers concluded that both measurements are representative of the atmosphere, and that they could use the data from both techniques in their analysis.
 The Climate and Global Change Program of the National Oceanic and Atmospheric Administration (NOAA) supported the ITCT 2K2 aircraft measurements. The Earth Observation Research and Application Center (EORC) of JAXA supported the PEACE aircraft measurements. The ITCT 2K2/PEACE campaigns were conducted under the framework of the International Global Atmospheric Chemistry (IGAC) project (http://www.igac.noaa.gov/).