3.1. Aerosol Sample Inlet
 Aerosol particles were sampled 18m above the sea surface through a heated mast that extended 5 m above the aerosol measurement container. The mast was capped with a horizontal inlet nozzle that was rotated into the relative wind to maintain nominally isokinetic flow and minimize the loss of supermicron particles. Air entered the inlet through a 5-cm-diameter hole, passed through a 7 degree expansion cone, and then into the 20-cm-inner-diameter sampling mast. The flow through the mast was 1 m3 min−1. Wind tunnel tests have shown that the transmission efficiency for particles with aerodynamic diameters less than 6.5 μm (the largest size tested) is greater than 95% [Bates et al., 2002].
 The bottom 1.5 m of the mast were heated to establish a stable reference relative humidity (RH) for the sample air of 55 ± 5%. A stable reference RH allows for constant instrumental size segregation in spite of variations in ambient RH and results in chemical, physical, and optical measurements which are directly comparable. In addition, measurement at a constant reference RH makes it possible, with the knowledge of appropriate growth factors, for end users of the data set (process, chemical transport, and radiative transfer models) to adjust the measured parameters to a desired relative humidity. A reference RH of 55% was chosen because it is above the crystallization humidity of most aerosol components and component mixtures [Carrico et al., 2003]. For the atmospheric conditions encountered during ACE Asia, it was possible to maintain 55% RH without excessive heating of the aerosol. On average, the aerosol was heated 5.1°C above the ambient temperature. All results of the in situ measurements are reported at 55 ± 5% RH.
 Twenty-three 1.6 cm outer diameter stainless steel tubes extended into the heated portion of the mast. These were connected to the aerosol instrumentation and impactors with conductive silicon tubing to prevent electrostatic loss of particles. An exception to this was the lines connected to the impactors used for collection of carbonaceous aerosol, organic functional groups and the aerosol-time-of-flight mass spectrometer (ATOFMS); they were constructed of stainless steel.
 The airflow to the impactors was controlled so that air was sampled only when the concentration of particles greater than 15 nm in diameter indicated the sample air was free of local contamination (i.e., there were no rapid increases in particle concentration), the relative wind speed was greater than 3 m s−1, and the relative wind was forward of the beam.
3.2. Aerosol Chemical Composition
3.2.1. Impactor Sample Collection for Chemical Analysis
 Two-stage multijet cascade impactors [Berner et al., 1979] sampling air at 55 ± 5% RH were used to determine submicron and supermicron concentrations of inorganic ions, organic and elemental carbon (OC and EC), and inorganic oxidized material (IOM). The 50% aerodynamic cutoff diameters of the impactors, D50,aero, were 1.1 and 10 μm. The RH of the sampled air stream was measured a few inches upstream from the impactors. Throughout the paper submicron refers to particles with Daero < 1.1 μm at 55% RH and supermicron refers to particles with 1.1 μm < Daero < 10 μm at 55% RH.
 A 12 μm grease cup at the inlet of each impactor was coated with silicone grease to prevent the bounce of larger particles onto the downstream stages. A Tedlar film placed on the impactor jet plate having a D50,aero of 10 μm was sprayed with silicone lubricant for the same reason. Since films placed on downstream jet plates were used for chemical analysis, they were not sprayed to avoid contamination. All handling of the substrates was done in an NH3- and SO2-free glove box. Blank levels were determined by loading an impactor with substrates but not drawing any air through it. Sampling periods ranged from 2.5 to 7 hrs during the day and 4 to 12 hours at night.
3.2.2. Inorganic Ions
 Submicron and supermicron concentrations of Cl−, NO3−, SO4=, methanesulfonate (MSA−), Na+, NH4+, K+, Mg+2, and Ca+2 were determined by ion chromatography (IC) [Quinn et al., 1998]. Tedlar films were used as the collection substrate in the impaction stage and a Millipore Fluoropore filter (1.0-μm pore size) was used for the backup filter. Prior to sampling, films were cleaned in an ultrasonic bath in 10% H2O2 for 30 min, rinsed in distilled, deionized water, and dried in the glove box. Postsampling, filters and films were wetted with 1 mL of spectral grade methanol. An additional 5 mL of distilled deionized water were added to the solution and the substrates were extracted by sonicating for 30 min.
 Non-sea-salt sulfate concentrations were calculated from Na+ concentrations and the ratio of sulfate to sodium in seawater. Sea-salt aerosol concentrations were calculated as
where 1.47 is the seawater ratio of (Na+ + K+ + Mg+2 + Ca+2 + SO4= + HCO3−)/Na+ [Holland, 1978]. This approach prevents the inclusion of non-sea-salt K+, Mg+2, Ca+2, SO4=, and HCO3− in the sea-salt mass and allows for the loss of Cl− mass through Cl− depletion processes. It also assumes that all measured Na+ and Cl− is derived from seawater. Results of Savoie and Prospero  indicate that soil dust has a minimal contribution to measured soluble sodium concentrations.
 Sources of uncertainty in the ionic mass include the air volume sampled (5%), the extract liquid volume (3.3%), 2 times the standard deviation of the blank values measured over the course of the experiment, and the precision of the method (2 times the standard deviation of repeated injections of a single sample).
3.2.3. Organic and Elemental Carbon
 Submicron and sub-10 μm samples were collected using 2 and 1 stage impactors, respectively. Aluminum foils were used as substrates on the 1.1 μm jet plate. Each impactor had 2 quartz backup filters. All substrates were prebaked at 500°C prior to sampling. The sub-10 μm and one submicron impactor were run without a denuder upstream. A second submicron impactor was run with a denuder upstream. The denuder was 30 cm long, contained 18 parallel strips of carbon-impregnated glass (CIG) fiber filters separated by 1.8 mm, and had a cross-sectional area of 9.6 cm2.
 The submicron OC data are from the denuder/impactor sampler. The last backup quartz filter in line was used as the blank. Submicron EC data are the average of the two submicron impactor samples (with and without denuders). The supermicron OC data are the difference between the sub-10 μm and submicron impactors run without denuders. Impactors without denuders upstream were used for the supermicron OC determination in order to avoid losses of large particles in the denuder. OC concentrations from both impactors were corrected for blanks and artifacts using the last quartz filter in line. Supermicron EC data are the difference between the sub-10 μm and the average of the two submicron impactor samples (with and without denuders).
 OC and EC concentrations were determined with a Sunset Labs thermal/optical analyzer [Birch and Cary, 1996]. The thermal program was the same as that used by other groups in ACE Asia [Schauer et al., 2003]. Four temperature steps were used to achieve a final temperature of 870°C in He to drive off OC. After cooling the sample down to 550°C, a He/O2 mixture was introduced and the sample was heated in four temperature steps to 910°C to drive off EC. The transmission of light through the filter was measured to separate EC from any OC that charred during the initial stages of heating.
 No correction was made for carbonate carbon so OC includes both organic and carbonate carbon. The carbonate carbon was never more than 10% of the total OC, however. On the basis of an interlaboratory comparison of punches from four high-volume samples and two blanks, the agreement between 8 Sunset Labs carbon analyzers was within 4% for moderate level OC, within 13% for low-level OC, and within 13% for EC [Mader et al., 2003; Schauer et al., 2003].
 The mass of particulate organic matter (POM) was determined by multiplying the measured organic carbon concentration in μg m−3 by a factor of 2.1 in the marine region and 1.6 elsewhere. The POM factor is an estimated average of the molecular weight per carbon weight and is based on a review of published measurements of the composition of organic aerosol in urban and nonurban regions [Turpin and Lim, 2001]. On the basis of the range of values given by Turpin and Lim , the POM factor has an uncertainty of ±31%.
 The uncertainties associated with positive and negative artifacts in the sampling of semivolatile organic species can be substantial [Turpin et al., 1994, 2000]. An effort was made to minimize and assess positive (adsorption of gas-phase species) and negative (volatilization of aerosol organic species which may have resulted from the pressure drop across the impactor and filter) artifacts by using a denuder upstream of the impactor and by comparing undenuded and denuder-filter samplers. Results from these comparisons have shown that after correcting for sampling artifacts, measured OC concentrations can vary by 10% between samplers [Mader et al., 2003]. Other sources of uncertainty in the POM mass include the air volume sampled (5%), the area of the filter (5%), 2 times the standard deviation of the blanks measured over the course of the experiment (0.44 μg/cm2), the precision of the method (5%) based on the results of Schauer et al. , and the POM factor (31%).
 Sources of uncertainty in the EC mass include the air volume sampled (5%), the area of the filter (5%), 2 times the standard deviation of the blank values measured over the course of the experiment (0.22 μg cm−2), and the precision of the method (5%) based on the results of Schauer et al. .
3.2.4. Inorganic Oxidized Material (Dust)
 Total elemental composition (Na, Mg, Al, Si, P, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Ba, As, and Pb) was determined by thin-film X-ray primary and secondary emission spectrometry [Feely et al., 1991, 1998]. Submicron and sub-10 μm samples were collected on 1.0 μm pore size Teflo filters using 2 and 1 stage impactors, respectively. Supermicron elemental concentrations were determined by difference between the submicron and sub-10 μm samples. This method of sample collection allows for the sharp size cut of the impactor while collecting a thin film of aerosol necessary for the X-ray analysis. Filters were weighed before and after sample collection as described below.
 A component composed of inorganic oxidized material (IOM) was constructed from the elemental data. The IOM most likely was composed of soil dust and/or fly ash. These two components are difficult to distinguish on the basis of elemental ratios. To construct the IOM component, the mass concentrations of Al, Si, Ca, Fe, and Ti, the major elements in soil and fly ash, were combined. It was assumed that each element was present in the aerosol in its most common oxide form (Al2O3, SiO2, CaO, K2O, FeO, Fe2O3, TiO2) [Seinfeld, 1986]. The measured elemental mass concentration was multiplied by the appropriate molar correction factor as follows [Malm et al., 1994; Perry et al., 1997]:
This equation includes a 16% correction factor to account for the presence of oxides of other elements such as K, Na, Mn, Mg, and V that are not included in the linear combination. In addition, the equation omits K from biomass burning by using Fe as a surrogate for soil K and an average K/Fe ratio of 0.6 in soil [Braaten and Cahill, 1986]. Noncrustal K was calculated using the K/Al ratio (0.31) of Asian loess [Jahn et al., 2001].
 Sources of uncertainty in the IOM mass concentration include the volume of air sampled (5%), the area of the filter (5%), the molar correction factor (6%), 2 times the standard deviation of the blank values measured over the course of the experiment for each element, and the precision of the X-ray analysis [Feely et al., 1991].
3.2.5. Individual Particle Analysis (ATOFMS)
 The size and chemical composition of individual aerosol particles were measured with an aerosol-time-of-flight mass spectrometer (ATOFMS). The ATOFMS couples aerodynamic particle sizing with time-of-flight mass spectrometry in a single instrument, providing both positive and negative ion mass spectra for each detected particle [Prather et al., 1994; Noble and Prather, 1996; Gard et al., 1997]. Particles were classified into 17 exclusive classes (e.g., sea salt with sulfate, dust with sulfate) by carrying out searches for ions of interest with specific threshold values [e.g., Gard et al., 1998; Silva et al., 1999; Liu et al., 2000; Angelino et al., 2001; Guazzotti et al., 2001]. The presence of nitrate in sea salt and dust particles was evaluated by the occurrence of signals at mass-to-charge (m/z) ratios −46 [NO2]−, −62 [NO3]−, +30 [NO]+, or +108 [Na2NO3]+ with the generic spectral characteristics of sea salt or dust. Similarly, sulfate was identified by peaks at m/z −97 [HSO4]−, −80 [SO3]−, −96 [SO4]−, or +165 [Na3SO4]+ [Guazzotti et al., 2001]. Further details about the ACE-Asia ATMOFMS measurements are given by S. A. Guazzotti et al. (Regional and temporal variability in the chemical composition of individual particles during ACE-Asia, submitted to Journal of Geophysical Research, 2003).
3.2.6. Organic Functional Groups
 Forty L min−1 of the 55% RH sample stream from the sampling mast were pulled through an impactor (D50,aero = 1.0 μm) and two virtual impactors [Sioutas et al., 1994] in series to increase the concentration of the sample aerosol by up to a factor of 5 prior to collection on 37 mm stretched Teflon filters (Teflo 1 μm pore size, Pall Corporation, Ann Arbor, Michigan). A size-dependent concentration factor was determined in the laboratory by using NaCl aerosol for a broad size range by comparing particle number size distributions upstream and downstream of the concentrator using a differential mobility analyzer (TSI model 3071) and an aerodynamic particle sizer (TSI model 3320). A mass average concentration factor was determined for each sample using the laboratory measured size-dependent concentration factors and ambient size distributions measured aboard the ship. The mass averaged concentrations factors during the project ranged from 3 to 4.
 All sample handling was done with powder-free vinyl gloves and Teflon-coated tweezers. After collection, samples were placed in polystyrene petri dishes (Pall Corporation), sealed with Teflon tape and stored frozen until analysis. The filter samples were analyzed using a Mattson Research Series 100 FTIR spectrometer with a deuterated triglycine sulfate (DTGS) detector and a He–Ne laser. FTIR spectra were collected in transmission mode for each filter by averaging 200 absorbance scans at wave numbers from 400 to 4000 cm−1 with a resolution of 4 cm−1. Teflon filters were scanned prior to use, and the resulting spectra were subtracted from scans after sampling to obtain the absorbance of the sampled aerosol. Filter support rings were etched to ensure that alignment was maintained during consecutive scans. Further details on the sampling protocol are given by Maria et al. .
 The combined uncertainty for FTIR detection, peak integration, and adsorption artifacts are between 5% and 22% [Russell, 2003]. Uncertainties associated with the impactor sampling are not included here since only relative concentrations of the functional groups are reported.
3.3. Sulfur Dioxide
 Air was pulled from 18 m above sea level down the 20-cm-ID powder-coated aluminum aerosol sampling mast (6 m) at approximately 1 m3 min−1. At the base of the sampling mast a 0.5 L min−1 flow was pulled through a 0.32-cm-inner-diameter, 1-m-long Teflon tube, a Millipore Fluoropore filter (1.0-μm pore size) housed in a Teflon filter holder, a Perma Pure, Inc., Nafion Drier (MD-070, stainless steel, 61 cm long) and then through 2 m of Telfon tubing to a Thermo Environmental Instruments model 43C trace level pulsed fluorescence analyzer. The initial 1 m of tubing, filter and drier were located in the humidity controlled (55%) chamber at the base of the mast. Dry zero air (scrubbed with a charcoal trap) was run through the outside of the Nafion Drier at 1 L min−1. The analyzer was run with two channels (0–20 ppb full scale and 0–100 ppb full scale) and a 20 s averaging time. Data were recorded every minute.
 Zero air was introduced into the sample line upstream of the Fluoropore filter for 10 min every hour to establish a zero baseline. An SO2 standard was generated with a permeation tube held at 50°C. The flow over the permeation tube, diluted to 17.7 ppb, was introduced into the sample line upstream of the Fluoropore filter for 10 min every 6 hours. The limit of detection for the 1 min data, defined as 2 times the standard deviation of the signal during the zero periods, was 170 ppt. For 30 min data the limit of detection was reduced to 30 ppt. Uncertainties in the concentrations based on the permeation tube weight and dilution flows is <5%.
3.4. Number Size Distributions
 One of the 23 mast tubes was used to supply ambient air to an ultrafine differential mobility particle sizer (UDMPS), a differential mobility particle sizer (DMPS) and an aerodynamic particle sizer (APS, TSI model 3320). The two DMPSs were located in a humidity-controlled box (RH = 55 ± 5%) at the base of the mast. The UDMPS was a short-column instrument [Winklmayr et al., 1991] connected to a TSI 3025 particle counter (TSI, St. Paul, Minnesota) operating with a positive center rod voltage to sample particles with a negative charge. Data were collected in 17 size bins from 3 to 26 nm geometric diameter. The UDMPS operated with an aerosol flow rate of 1 L/min and a sheath airflow rate of 10 L/min. The DMPS was a medium length column instrument connected to a TSI 3760 particle counter operating with a positive center rod voltage to sample particles with a negative charge. Data were collected in 17 size bins from 20 to 671 nm diameter. The DMPS operated with an aerosol flow rate of 0.5 L/min and a sheath airflow rate of 5 L/min. The relative humidity of the sheath air for both DMPSs was controlled resulting in a measurement RH in the DMPSs of approximately 55%. With this RH control the aerosol should not have effloresced if it was hydrated in the atmosphere [Carrico et al., 2003]. Mobility distributions were collected every 15 min.
 The mobility distributions from the UDMPS and DMPS were inverted to a number distribution assuming a Fuchs-Boltzman charge distribution from the Kr85 charge neutralizer [Stratman and Wiedensohler, 1997]. The overlapping channels between the two instruments were eliminated in the inversion. The data were corrected for diffusional losses [Covert et al., 1997] and size-dependent counting efficiencies based on pre-ACE-2 intercalibration exercises [Wiedensohler et al., 1997]. During ACE-Asia, the ratio of the total particle number measured by an independent TSI 3760 particle counter (Dp > 13 nm) to the integrated number concentration from the UDMPS and DMPS of particles with Dp > 13 nm averaged 1.15 ± 0.21% (one standard deviation). The estimated uncertainty in the number concentration in each bin, based on flow uncertainties, is ±10%.
 The APS was located in the lower humidity controlled box at the base of the mast. Although the APS inlet humidity was maintained at 55 ± 5% RH, internal heating of the sample in the APS by its sheath flow and waste heat may have reduced the measurement RH below 55% RH. The sheath flow was conditioned outside the instrument case before reintroduction into the sheath and acceleration nozzle but the temperature at the APS sensing volume was not measured. The instrumental temperature sensors near the sensing volume of the APS showed a temperature difference of about +2°C compared to the inlet temperature, but the sensing volume temperature may have been more or less than this. The effect of an RH change on the large aerosol particles at this point is difficult to quantify because the time for equilibration in the inlet jets is short. The RH is assumed to be above the efflorescence point of sea salt.
 Number size distributions were collected with the APS every 15 min averaged over the 13 min of the DMPS scan. The APS data were collected in 51 size bins with the nominal manufacturers aerodynamic diameters ranging from 0.542 to 20 μm. Data were corrected for phantom counts assuming that the counts in the largest 4 channels (Daero = 16 to 20 μm) were phantom counts and that value was subtracted from the entire APS distribution. This correction resulted in a Junge slope of the number distribution that was nearly constant for Dp > 5 μm and a volume concentration that varied randomly about zero for Dp > 10 μm. The APS data were corrected for ultra-Stokesian conditions in the instrument jet and nonspherical shape [Wang and John, 1987; Cheng et al., 1990, 1993; Wang et al., 2002]. On the basis of the coarse-mode chemical concentration, the aerosol measured in source regions 1, 2, and 4 were assigned a spherical shape, source regions 3 and 5 were assigned a doublet-sphere shape factor and source regions 6 and 7 were assigned a triplet-sphere shape factor [Quinn et al., 2004]. Finally, the APS data were converted from aerodynamic diameters to geometric diameters using calculated densities and the water masses associated with the inorganic ions at 55% RH. The densities and associated water masses were calculated with a thermodynamic equilibrium model (AeRho) using the measured chemical data [Quinn and Coffman, 1998]. The estimated uncertainty in the supermicron size distribution include particle sizing (±3%), the instrumental counting efficiency (±5%), and the shape factor (±17% for a doublet and ±22% for a triplet). The error in the shape factor was based on the percent change in size resulting from its use and, hence, most likely is an upper bound.
3.5. Additional Measurements
 Also measured were meteorological parameters including surface temperature, RH, wind speed and direction, as well as vertical profiles of these parameters from radiosondes. Air mass back trajectories were calculated at three arrival altitudes (500, 2500, and 5500 m) for the ship's position at 6 hour intervals. In cases where a temperature inversion occurred at a sub-500 m height (Polluted – Japan and Volcano + Polluted), additional trajectories were calculated for 50 and 100 m arrival heights. Trajectories were calculated with the hybrid-coordinate model HY-SPLIT 4 (Draxler  and http://www.arl.noaa.gov/ready/hysplit4.html) using wind fields from the U.S. National Centers for Environmental Prediction Global Forecast System; these fields were at 191 km horizontal resolution and were available at 6 hour intervals. All references to time are reported here in UTC. Dates are given as day of year (DOY), where noon on 1 February equals DOY 32.5.