|
|
 |
|
|
|
In the one minute files the position is treated somewhat differently then all the other data. The position given is the ship's position at the start of the one minute 'averaging' period. (All other data are a true average). The PMEL GPS was the primary source, the Ship's GPS was used when there were missing data in the ship's record. One second position, ship speed, course and gyro, and relative and true winds are available on the CalNex Position and Ship Data page.
The ship's GPS speed in knots (Speed Over Ground) and GPS Course in compass degrees (Course Over Ground) are the one minute averages from the GPS (the PMEL GPS as the primary source, the Ship's GPS was the secondary source). To make the one minute averages the 1-second recorded motion vector was separated into east and north components that were averaged into one minute bins. The one minute components were then combined into the ship Velocity Vector. One second position, ship speed, course and gyro, and relative and true winds are available on the CalNex Position and Ship Data page.
The GyroCompass in compass degrees is the one minute average heading. The primary source was the PMEL GPS compass (Si-TEX Vector Pro), the ship gyro compass data was used when the primary data were missing. The 1-second data was separated into an east and north component before averaging and then recombining. NOTE: The GPS-Course is the direction the ship is moving. The GyroCompass is direction the ships bow is pointing. When the ship is moving at 6 or more knots they generally are almost the same. Due to water currents, at slow speeds there can be quite a difference between the two. When the ship is stationary, the two are totally unrelated. One second position, ship speed, course and gyro, and relative and true winds are available on the CalNex Position and Ship Data page.
The primary source for the relative wind data was the PMEL Vaisala WX520 sonic anemometer that was located on the aerosol sampling mast. . For periods of missing data from the PMEL data source the ships starboard WX520 sensor at the bow IMET tower was used. We assume that the relative wind information is primarily used to determine periods of ship contamination, thus we are using the anemometer that is closest to the sample inlet. This anemometer (or the "Skyvan anemometer at the top of the mast) was also used as an input to the algorithm that turned off the sample pumps during periods of ship contamination. One second position, ship speed, course and gyro, and relative and true winds are available on the CalNex Position and Ship Data page.
The one second relative wind speed and direction data were separated into orthogonal components of "keel" and "beam". These components were averaged into 1 minute averages, and then recombined to relative wind vectors. Wind speed is reported meters per second and wind direction is in degrees with -90 being wind approaching the ship on the port beam, 0 degrees being wind approaching the ship directly on the bow, and +90 degrees being wind approaching the ship on the starboard beam. One second position, ship speed, course and gyro, and relative and true winds are available on the CalNex Position and Ship Data page.
True wind speed and direction were calculated from measurements obtained with the Ships IMET wind sensor. This sensor was mounted 14 meters above the sea surface on the ship's meteorological sampling mast at the bow and should be less affected by bending of streamlines as the air moves over the ship. (The PMEL “Skyvane” was on the top of the Aero-Van and in the ‘perturbed airflow’.) The true North and East components of the wind vector from the 10 second SCS data were calculated and then averaged into 1 minute intervals in m/s. The true wind vector was calculated from these components and is given as wind speed in m/s and wind direction in compass degrees. The WindU and WindV are the east and north components of the wind vector (in m/s). One second position, ship speed, course and gyro, and relative and true winds are available on the CalNex Position and Ship Data page.
One minute averages in degrees C. The following data sources were used. The PMEL rotronics sensor on the aerosol sampling mast, the PMEL Vaisala WXT520 seonsor on the sampling aerosol mast and the two ship owed Vaisala WXT520 sensors on the Ship's IMET bow tower. These 4 sensors generally agreed to better than 0.5 deg C and average of these four sensors was used. There were times when the PMEL data system was down and during those times the average of the Ship's WXT520 sensors was used. Likewise, during times that the Ship's data system was down, the average of the above two PMEL sensors was used.
Relative humidity:
One minute averages in %. The following data sources were used. The PMEL rotronics sensor on the aerosol sampling mast, the PMEL Vaisala WXT520 seonsor on the sampling aerosol mast and the two ship owed Vaisala WXT520 sensors on the Ship's IMET bow tower. These 4 sensors generally agreed to better than 5 (RH) % and average of these four sensors was used. There were times when the PMEL data system was down and during those times the average of the Ship's WXT520 sensors was used. Likewise, during times that the Ship's data system was down, the average of the above two PMEL sensors was used.
Barometric Pressure:
One minute averages in units of mb. There were two sources of raw data, the PMEL Vaisala sensor (in the Aerosol van) and the Ship's digital sensor. They both agreed within 0.5 mb. An average of these two signals was used and when only one source was available it is given.
One minute averages in units of watts per square meter. Total solar radiation was measured with an Epply Black and White Pyranometer (horizontal surface receiver -180, model 8-48, serial number 12946) and an Epply precision pyranometer (horizontal surface receiver -180, twin hemispheres, model PSP, serial number 133035F3) that were mounted on the top of AERO van. Both instruments were calibrated by The Epply Laboratory on October 11, 1994. There were times when the sampling mast shaded one or both sensors. There were also times when the ship's mast/bridge shaded the sensors. The shaded data have not been edited out of the 1 minute data record. The data reported here are from the model 8-48, serial number 12946 radiometer and are in watts per square meter and are the average value over the 1 minute sampling period.
Sea Surface Temperature (SST) in degrees C and Salinity in PSU were measured with the Ship's Thermosalinograph. The sample seawater for scientific uses was pumped in through an intake that was 5.3 meters below the water line. When the ship was in the Los Angels harbor, the San Francisco Bay and in the Sacramento River and Deep Water Ship Channel the ships sampling pump was turned off. The data data from these times is listed as "missing", -999.
PMEL/UW CN and UFCN:
Aerosol particles were sampled at 18 m above sea level through a heated mast. The mast extended 5 m above and forward of the aerosol measurement container. The inlet was a rotating cone-shaped nozzle that was automatically positioned into the relative wind. Air was pulled through this 5 cm diameter inlet nozzle at 1 m3 min-1 and down the 20 cm inner diameter mast. The lowest 1.5 m of the mast was heated to dry the aerosol to a relative humidity (RH) of of not less than 60% and partially dry the aerosol. Twenty one 1.6 cm inner diameter conductive tubes extending into this heated zone were used to subsample the main air flow for the various aerosol instruments at flows of 30 l min-1.
One of the twenty one 1.6 cm diameter tubes was used to supply
ambient air to TSI 3010 (CN_Direct)and TSI 3025A (UFCN_Direct)
particle counters. Another one of tubes was used to supply ambient
air to a TSI3785 (UFCN_Chem) particle counter. A separate 1/4"
line was used to supply air from the top of the mast directly to a
TSI 3760 particle counter. The 3760, 3010, 3025 and 3785 measure all
particles larger than roughly 12, 3 and 5 nm respectively. The total
particle counts from each instrument were recorded each second. The
data were filtered to eliminate periods of calibration and instrument
malfunction, SeaSweep and zero air periods, and periods of obvious
ship contamination from the R/V Atlantis (based on relative wind and
high CN counts). As one goal of the project was to measure other ship
emissions there may be periods where contamination from the Atlantis
is mixed in with other ships emissions. If the user is interested in
further filtering the CN data to remove these relative wind data can
be made available (contact Derek Coffman). The “best”
filtered values were chosen to represent CN>12 (CN) and ultra-fine
(UFCN) particle concentrations. The best CN values primarily include
data from CN_Direct and the data from CN_Stack were used to fill in
periods where the CN_Direct were not available. Similarly, the UFCN
values primarily include data from UFCN_Direct and the UFCN_Chem data
were used to fill in periods where UFCN_Direct were not available.
These "best" data were averaged into one minute periods.
One second data are available upon request. The value of -999
was
assigned to any one minute period without data.
Air was sampled from 18 m above sea level down the 20 cm ID powder-coated aluminum aerosol sampling mast (6 m) at approximately 1 m3min-1. At the base of the sampling mast a 1 Lmin-1 flow was pulled through a 0.32 cm ID, 2m long Teflon tube into a Thermo Environmental Instruments Model 49c ozone analyzer. The data are reported as one minute averages in units of ppb
PMEL SO2
Inlet and Instrument
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 m3min-1. At the base of the sampling mast a 2.8 Lmin-1 flow was pulled through a 0.32 cm ID, 1m long Teflon tube, a Millipore Fluoropore filter (1.0-um 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 the Thermo Environmental Instruments Model 43C Trace Level Pulsed Fluorescence Analyzer. The initial 1 m of tubing, filter and drier we located in the humidity controlled (60%) 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 2 Lmin-1. Data were recorded in 10 second averages. The data have not been filtered for periods when Atlantis ship exhaust entered the mast.
Standardization:
Zero air was introduced into the sample line upstream of the Fluoropore filter periodically throughout the cruise to establish a zero baseline. An SO2 standard was generated with a permeation tube held at 40ºC. The flow over the permeation tube, diluted to 4.6 ppb, was introduced into the sample line upstream of the Fluoropore filter periodically throughout the cruise. The limit of detection for 1 min averaged data, defined as 2 times the standard deviation of the signal during the zero periods, was 150 ppt. Data below detection limit are listed as 0 in the ACF and IGOR .itx files and -8888 in the ICARTT format file. Missing data are listed as -9999. Uncertainties in the concentrations based on the permeation tube weight and dilution flows are <5%.
Seawater entered the ship at the bow, 5.3 m below the ship waterline, and was pumped to the ship laboratory. Every 30 minutes a 5 ml water sample was valved from the ship water line directly into a Teflon gas stripper. The sample was purged with hydrogen at 80 ml/min for 5 min. DMS and other sulfur gases in the hydrogen purge gas were collected on a Tenax filled trap, held at -5 deg C. During the sample trapping period, 6.2 pmoles of methylethyl sulfide (MES) were valved into the hydrogen stream as an internal standard. At the end of the sampling/purge period the trap was rapidly heated to +120 deg C and the sulfur gases were desorbed from the trap, separated on a DB-1 megabore fused silica column held at 70 deg C, and quantified with a sulfur chemiluminesence detector. Between each water sample the system analyzed either a DMS standard or a system blank. The system was calibrated using gravimetrically calibrated DMS and MES permeation tubes. The precision of the analysis has been shown to be ± 2% based on replicate analysis of a single water sample at 3.6 nM DMS. The automated DMS system is described in greater detail in Bates et al. (J. Geophys. Res., 103, 16369-16383, 1998; Tellus, 52B, 258-272, 2000). The major improvements since these papers are a new automation-data system and a more reliable cold trap consisting of an electrically heated stainless steel tube embedded in an aluminum block that is cooled to -5 deg C with a thermoelectric cooling chip
Chlorophyll
Chlorophyll concentrations were recorded with fluorescence measurements of seawater from the ship's continuous underway scientific sampling system. The inlet for this system was located 5.3 meters below the water line at the bow of the ship. Most of the data were collected with a PMEL owned Turner model 10-AU fluorometer located in the seawater van. Data were also collected with a ship owned fluorometer, located in the bow chamber next to the water inlet pump. The raw signals from the two fluorometers had a correlation coefficient that had a maximum of 0.98 as the signal delay from the PMEL fluorometer was increased from zero to 4 minutes, indicating that the seawater flow time from the bow inlet to the seawater van was 4 minutes. 1 minute averages of the Ship fluorometer signal were shifted by 4 minutes and a linear transformation of the raw Ship fluorometer signal to the raw PMEL fluorometer signal was determined. For the final data set the 1 minute averages of the PMEL fluorometer signal was used. The time shifted and ship signal, transformed to the raw PMEL scale, was used for times when the PMEL signal was missing. Discreet filter samples from the ship's sampling seawater were taken approximately 6 times per day during the cruise and the absolute Chlorophyll amounts were determined in Seattle after the end of the cruise. The raw PMEL signal was standardized to the filter sample data and the resulting underway chlorophyll data, in mg/m-3 is given.
Aerosol Mass Spectrometer, AMS
Concentrations of submicrometer NH4+, SO4=, NO3-, and POM were measured with a Quadrupole Aerosol Mass Spectrometer (Q-AMS) (Aerodyne Research Inc., Billerica, MA, USA) [Jayne et al., 2000; Allan et al., 2003]. The species measured by the AMS are referred to as non-refractory (NR) and are defined as all chemical components that vaporize at the vaporizer temperature of 600°C. This includes most organic carbon species and inorganic species such as ammonium nitrate and ammonium sulfate salts but not mineral dust, elemental carbon, or sea salt. The ionization efficiency of the AMS was calibrated every few days with dry monodisperse NH4NO3 particles using the procedure described by Jimenez et al. [2003]. The instrument operated on a 5 min cycle with the standard AMS aerodynamic lens.
Version 0 data have a "Collection Efficiency" (CE) of 0.8 applied to the four “standard” AMS measurements of sulfate, nitrate, ammonium, and organic mass, based on simultaneous collection of filters for ion chromatography as reference standards. The detection limits from individual species were determined by analyzing periods in which ambient filtered air was sampled and are calculated as three times the standard deviation of the reported mass concentration during those periods. The detection limits during CalNEX were 0.02, 0.15, 0.02, and 0.16 ug/m3 for sulfate, ammonium, nitrate, and POM, respectively. Samples below these detection limits are listed as 0 in the ACF file and -8888 in the ICARTT format file. Missing data are listed as -9999 in the .acf and .ict files and NaN in the .itx file.
Jayne, J.T., D.C. Leard, X. Zhang, P. Davidovits, K.A. Smith, C.E. Kolb, and D.R. Worsnop, Development of an aerosol mass spectrometer for size and composition analysis of submicron particles, Aersol Sci. Technol., 33, 49-70, 2000.
Allan, J.D., J.L. Jimenez, P.I. Williams, M.R. Alfarra, K.N. Bower, J.T. Jayne, H. Coe, and D.R. Worsnop, Quantitative sampling using an Aerodyne aerosol mass spectrometer. Part 1: Techniques of data interpretation and error analysis, J. Geophys. Res., 108(D3), 4090, doi:10.1029/2002JD002358, 2003.
CalNEX ion chem data 10-18-10 -- Aerosol Ion Chemistry Data
Contact persons: Tim Bates, tim.bates@noaa.gov; Trish Quinn, patricia.k.quinn@noaa.gov
Two-stage multi-jet cascade impactors (Berner et al., 1979) sampling air at 60% RH were used to determine the sub- and supermicrometer concentrations of Cl-, Br-, NO3-, SO4=, methanesulfonate (MSA-), oxalate (Ox-), Na+, NH4+, K+, Mg+2, and Ca+2. Sampling periods ranged from 2 to 13 hours. The RH of the sampled air stream was measured a few inches upstream from the impactor. The 50% aerodynamic cutoff diameters, D50,aero, were 1.1 and 10 um. Submicrometer refers to particles with Daero < 1.1 um at 60% RH and supermicrometer refers to particles with 1.1 um < Daero < 10 um at 60% RH.
The impaction stage at the inlet of the impactor was coated with silicone grease to prevent the bounce of larger particles onto the downstream stages. Tedlar films were used as the collection substrate in the impaction stage and a Millipore Fluoropore filter (1.0-um pore size) was used for the backup filter. Films were cleaned in an ultrasonic bath in 10% H2O2 for 30 min, rinsed in distilled, deionized water, and dried in an NH3- and SO2-free glove box. Filters and films were wetted with 1 mL of spectral grade methanol. An additional 5 mLs of distilled deionized water were added to the solution and the substrates were extracted by sonicating for 30 min. The extracts were analyzed by ion chromatography [Quinn et al., 2000]. All handling of the substrates was done in the glove box. Blank levels were determined by loading an impactor with substrates but not drawing any air through it.
Concentrations are reported as ug/m3 at STP (25C and 1 atm). Values below the detection limit are denoted with a -8888 in the .acf file or zero in the .itx and .ict files, missing data are denoted with a -9999 in the .acf and .ict files and NaN in the .itx file.
Berner et al., Sci. Total Environ., 13, 245 - 261, 1979.
Quinn et al., J. Geophys. Res., 105, 6785 - 6805, 2000.
Contact persons: Trish Quinn, patricia.k.quinn@noaa.gov, David Covert, dcovert@u.washington.edu, Derek Coffman, derek.coffman@noaa.gov
NOAA Pacific Marine Environmental Laboratory
A suite of instruments was used to measure aerosol light scattering and absorption. Two TSI integrating nephelometers (Model 3563) measured integrated total scattering and hemispheric backscattering at wavelengths of 450, 550, and 700nm (Anderson et al, 1996; Anderson and Ogren, 1998). Sample flow was taken from the AeroPhysics sampling van inlet. One nephelometer (neph_sub10) always measured aerosols of aerodynamic diameter Dae < 10 micrometers; the second nephelometer (neph_sub1) measured only aerosol of aerodynamic diameter Dae < 1 micrometer. Both nephelometers were operated at a sensing volume RH of approximately 60%. The 10 and 1 micrometer cut-offs were made with Berner multi-jet cascade impactors. Two Radiance Research Particle Soot Absorption Photometers were used to measure light absorption by aerosols at 467, 530, and 660nm (Bond et al., 1999; Virkkula et al.,2005) under 'dry' (<25% RH) conditions for sub 10 (psap_sub10) and sub 1 (psap_sub1) micrometer aerosols at the outlet of the respective nephelometers.
On the PMEL Data Sever the ~60% RH, neph_sub10 data are in the TOTSCAT file, the ~60% RH, neph_sub1 data are in the SUBSCAT file. The psap_sub10 and psap_sub1 data are in the PSAP file.
A separate humidity controlled system measured submicrometric light scattering at two different relative humidities, approximately 25% RH and 85% RH (neph_sub1_lo and neph_sub1_hi) with two TSI integrating 3-wavelength nephelometers operated in series downstream of a Berner impactor. There are no backscattering values available from the _hi or _lo nephelometers as the backscatter shutter mode was set to "total" due to problematic backscatter shutters. The first nephelometer measured scattering of the ~60% conditioned aerosol from the AeroPhysics sampling van inlet at approximately 25% RH after drying of the sample flow using a PermaPure, multiple-tube nafion dryer model PR-94. Downstream of this nephelometer a humidifier was used to add water vapor to the sample flow (6 microporous teflon tubes surrounded by a heatable water-jacket). The sample was conditioned to approximately 85% RH, scattering was measured by the second TSI neph. Humidity was measured by using a chilled mirror dew point hygrometer downstream of the second neph.
On the PMEL Data Sever the neph_sub1_lo data are in the SUBSCATloRH file, the neph_sub1_hi data are in the SUBSCAThiRH file.
DATA COLLECTION AND PROCESSING
Data from both systems were collected and processed at 1 sec resolution but are reported as 60-second averages. Data from each instrument are corrected and adjusted as described below, allowing for derivation of extensive parameters (light scattering and absorption) and intensive parameters (single scatter albedo, Angstrom exponent). Light absorption is box-car averaged by the instrument over a window 10-seconds wide.
For all parameters, the bad value code is "NaN" (-9999 in the .acf fles). Intensive parameters are set to NaN when the extensive properties used in their calculation fell below the measurement noise threshold. Both extensive and intensive properties are set to NaN (-9999) during certain events, such as during filter changes, instrument calibration, obvious instrument failure etc. Negative values of absorption might occur during periods of absorption signals near or in the range of the instrument noise, and are partly shifted into the negative range due to scattering correction.
STP are p_STP=1013.2 hPa, T_STP=273.2 K.
DERIVATION OF MEAN VALUES
EXTENSIVE PARAMETERS
Data from the TSI integrating nephelometers, Neph sub10 and Neph sub1, and f(RH=low) and f(RH=high) are processed as follows:
Span gas (air and CO2) calibrations were made before the field campaign using the standard TSI program. During the campaign zero (particle free air at ambient ater vapor conc.) and CO2 span checks were made at three to four day intervals. The resulting zero offset and span factors were applied to the data.
The TSI nephelometers measure integrated light scattering into 7-170 degrees. To derive total scatter (0-180degrees) and hemispheric backscatter (90-180degrees) angular truncation correction factors were applied as recommended by Anderson and Ogren (1998).
Total and hemispheric backscatter were adjusted to STP. (NOTE: There are no backscattering values available from the f(RH=low) and f(RH=high) nephelometers as discussed above.)
Data from the Radiance Research Particle Soot Absorption Photometers, PSAPs sub1, sub10, and _lo,
are processed as follows:
Reported values of light absorption are corrected for spot size, flow rate, artifact response to scattering, and error in the manufacturer's calibration, all given by Bond et al. (1999). Except the spot size, all corrections were made after data collection, i.e. they are not integrated into the PSAP firmware. However, the PSAP's were flow-calibrated prior to the campaign, and a flow correction was applied based on routine flow checks during the cruise.
Light absorption is adjusted to STP
The f(RH) of scattering data is processed as follows:
Reported values of light scattering at low RH and high RH were corrected to STP.
the exponent describing the f(RH) dependence of
scattering was determined using the scattering values of
neph_lo_1min (fRH-optics) and neph_hi_1min (fRH-optics) and applying
a linear regression of the relationship
log(scat_hi/scat_lo) = -gamma*log((1-fracRH_hi)/(1-fracRH_lo))
based on the Kasten & Hanel formula
scat_hi=scat_lo(1-fracRH)^(-gamma) [Wang et. al.,2006]
The fRH values given on the data server (SUBFRH) are at the measured high and low RH values. The gamma factor calculated from the equation above is available upon request.
INTENSIVE PARAMETERS
The Ångström exponent for scattering at (450,550,700nm),
A_Blue = -log(Bs/Gs)/log(450/550)
A_Green = -log(Bs/Rs)/log(450/700)
A_Red = -log(Gs/Rs)/log(550/700)
where Bs, Gs and Rs are light scattering values that apply to 450, 550 and 700 nm, respectively and where these values have been smoothed by averaging over a 30-sec wide window.
The Ångström exponent for absorption at (467,530,660nm),
A_Blue = -log(Ba/Ga)/log(467/530)
A_Green = -log(Bs/Rs)/log(467/660)
A_Red = -log(Gs/Rs)/log(530/660)
where Ba, Ga and Ra are light absorbtion values that apply to 467, 530 and 660 nm, respectively and where these values have been smoothed by averaging over a 30-sec wide window.
The single scatter albedo of the sub-micron aerosol was calculated as follows:
SSA = Neph1_scat / (Neph1_scat + PSAP1_abs)
where light absorption values and scattering have been averaged over 60 seconds. SSA is given for 532nm, i.e. the nephelometer data was wavelength-shifted to match the PSAP wavelength using the nephelometer based Ångström exponent.
The sub 1 micron and sub 10 micron Scattering Ångström exponents can be found on the PMEL Data Server in the SUBSCATANG and TOTSCATANG files. The sub 1 micron and sub 10 micron Absorption Ångström exponents can be found in the SUBABSANG and TOTABSANG files. The sub 1 micron and sub 10 micron single scatter albedo values can be found in the SUBSSA and TOTSSA files.
REFERENCES
Anderson, T.L., D.S. Covert, S.F. Marshall, M. L. Laucks, R.J. Charlson, A.P. Waggoner, J.A. Ogren, R. Caldow, R. Holm, F. Quant, G. Sem, A. Wiedensohler, N.A. Ahlquist, and T.S. Bates, "Performance characteristics of a high-sensitivity, three-wavelength, total scatter/backscatter nephelometer", J. Atmos. Oceanic Technol., 13, 967-986, 1996.
Anderson, T.L., and J.A. Ogren, "Determining aerosol radiatve properties using the TSI 3563 integrating nephelometer", Aerosol Sci. Technol., 29, 57-69, 1998.
Bond, T.C., T.L. Anderson, and D. Campbell, "Calibration and intercomparison of filter-based measurements of visible light absorption by aerosols", Aerosol Sci. and Tech., 30, 582-600, 1999.
A. Virkkula, N. C. Ahquist, D. S. Covert, P. J. Sheridan, W. P. Arnott, J. A Ogren,"A three-wavelength optical extinction cell for measuring aerosol light extinction and its application to determining absorption coefficient", Aero. Sci. and Tech., 39,52-67, 2005
A. Virkkula, N. C. Ahquist, D. S. Covert, W. P. Arnott, P. J. Sheridan, P. K. Quinn,D. J. Coffman, "Modification, calibration and a field test of an instrument for measuring light absorption by particles", Aero. Sci. and Tech., 39, 68-83, 2005
Wang et. al, Aerosol optical properties over the Northwestern Atlantic Ocean during NEAQS-ITCT 2004, and the influence of particulate matter on aerosol hygroscopicity, submitted to J. Geo. Phys. Res., 2006
Contact: Trish Quinn (patricia.k.quinn@noaa.gov)
Aerosol Sampling Inlet:
Sample air for all aerosol measurements was drawn through a 6-m mast. The entrance to the mast was 18 m above sea level and forward of the ship's stack. The mast was automatically rotated into the relative wind to maintain nominally isokinetic flow and minimize the loss of supermicrometer particles. Air entered the inlet through a 5 cm diameter hole, passed through a 7° expansion cone, and then into the 20 cm inner diameter sampling mast. The flow through the mast was 1 m3 min-1. The transmission efficiency of the inlet 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 60 ± 5%. On average, the aerosol was heated 2.5°C above the ambient temperature. Stainless steel tubes extending into the heated portion of the mast were connected to downstream aerosol instrumentation with either conductive silicon tubing or stainless steel tubing for analysis of organic aerosol.
The data reported here are based on air that was sampled only when the particle number concentration, the relative wind speed, and the relative wind direction all indicated that there was no possibility of contamination from the ship’s stack.
CCN Measurements:
A Droplet Measurement Technologies (DMT) CCN counter was used to determine CCN concentrations at supersaturations, S, of 0.3, 0.4, 0.5, 0.6, and 0.7%. Details concerning the characteristics of the DMT CCN counter can be found in Roberts and Nenes [2005] and Lance et al. [2006]. A multijet cascade impactor [Berner et al., 1979] with a 50% aerodynamic cutoff diameter of 1 µm was upstream of the CCN counter. Each supersaturation level was sampled for 5 min. The first 2 or 3 min (depending on supersaturations involved) of each 5 min period were discarded so that only periods with stable supersaturations are included in the analyzed data set.
The CCN counter was calibrated before and during the experiment as outlined by Lance et al. [2006]. An (NH4)2SO4 aqueous solution was atomized with dry air, passed through a diffusional drier, diluted and then introduced to a Scanning Mobility Particle Sizer (SMPS, TSI). The resulting monodisperse aerosol stream was sampled simultaneously by the CCN counter and a water-based Condensation Particle Counter (WCPC, TSI) in order to determine the average activated fraction (CCN/CN). This procedure was repeated for a range of particle sizes and instrumental supersaturations. Using this procedure, the instrument supersaturation is equal to the critical supersaturation of the particle obtained from the activation curve for an activated fraction of 50%. The critical supersaturation for a given particle size was calculated from Köhler theory (e.g., Fitzgerald and Hoppel, 1984). The supersaturations reported in the text are based on the calibrations and not the instrumental readout which disregards thermal efficiency. The difference between the calibrated values and those reported by the instrument were similar to the difference found by Lance et al. [2006]. The uncertainty associated with the CCN number concentrations is estimated to be less than ± 10% [Roberts and Nenes, 2005]. Uncertainty in the instrumental supersaturation is less than ± 1% for the operating conditions of this experiment [Roberts and Nenes, 2005].
Bates, T.S., D.J. Coffman, D.S. Covert, and P.K. Quinn,
Regional marine boundary layer aerosol size distributions in the
Indian, Atlantic and Pacific Oceans: A comparison of INDOEX
measurements with ACE-1, ACE-2, and Aerosols99, J. Geophys. Res.,
107(D19), 10.1029/2001JD001174, 2002.
Berner, A., C. Lurzer, F. Pohl, O. Preining, and P. Wagner, The size distribution of the urban aerosol in Vienna, Sci. Total Environ., 13, 245 – 261, 1979.
Fitzgerald, J.W. and W.A. Hoppel, Equilibrium size of atmospheric aerosol particles as a function of relative humidity: Calculations based on measured aerosol properties, in Hygroscopic Aerosols, edited by L.H. Ruhnke and A. Deepak, pp. 21 – 34, A. Deepak, Hampton, VA, 1984.
Lance, S., J. Medina, J.N. Smith, and A. Nenes, Mapping the operation of the DMT continuous flow CCN counter, Aer. Sci. Tech., 40, 242 – 254, 2006.
Roberts, G.C. and A. Nenes, A continuous-flow streamwise thermal gradient CCN chamber for atmospheric measurements, Aer. Sci. Tech., 39, 206 – 221, 2005.
Organic Carbon (Sunset Laboratory thermal/optical analyzr)
Submicrometer samples were collected on pre-combusted quartz fiber filters using 2 stage impactors for organic carbon (OC) and elemental carbon (EC) analysis [Bates et al., 2004]. A charcoal diffusion denuder was deployed upstream of the submicrometer impactor to remove gas phase organic species. OC and EC concentrations were determined with a Sunset Laboratory thermal/optical analyzer. Three temperature steps were used to evolve OC under O2-free conditions for quantification. The first step heated the filter to 230C; the second step heated the filter to 600C (AMS vaporizer temperature); and the final step heated the filter to 870C. After cooling the sample down to 550C, a He/O2 mixture was introduced and the sample was heated in four temperature steps to 910C to drive off EC. The transmission of light through the filter was measured to correct the observed EC for 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 percentage of carbonate carbon is unknown. EC was below the detection limit for all samples.
U.S.Dept of Commerce / NOAA / OAR / PMEL / Atmospheric Chemistry