TITLE: Aircraft C-130 Aerosol in-situ scattering and absorption, coarse/fine, RH-dependence AUTHORS: Theodore L. Anderson (tadand@atmos.washington.edu ph: 206-543-2044) Sarah J. Masonis* (sarahd@atmos.washington.edu ph: 206-543-6674) Dept. of Atmospheric Science University of Washington Box 351640 Seattle, WA 98195 USA fax: 206-543-0308 * contact for questions about the data Antony D. Clarke (tclarke@soest.hawaii.edu) Steven G. Howell (showell@soest.hawaii.edu) Cameron S. McNaughton (cameronm@soest.hawaii.edu) Dept. of Oceanography University of Hawai'i at Manoa 100 Pope Rd. Honolulu, HI 96822 USA 1.0 DATA SET OVERVIEW: Introduction: This README file contains a description of in-situ aerosol optical data acquired on board the NCAR C-130 aircraft during the intensive field phase of the ACE-Asia field project. These data are contained in files named "UWrf(##)_(t)sec(v).txt", where: ## = research flight # t = averaging time of data, in seconds v = version of program used to generate data file (versions are a,b,c...) *NOTES: **All data files have been compressed using "gzip", as indicated by the suffix ".gz" in the filenames. Time period covered: March 30 through May 4, 2001 Physical location: National Center for Amospheric Research C-130 aircraft based out of Marine Corps Air Station in Iwakuni, Japan. 2.0 INSTRUMENT DESCRIPTION: A suite of instruments was used to measure light scattering and absorption. Two TSI, Inc. integrating nephelometers (Model 3563) measured integrated total scatter and hemispheric backscatter at 450, 550, and 700nm wavelengths (Anderson et al, 1996; Anderson and Ogren, 1998). One nephelometer ("NTOT") always measured all aerosol; the second nephelometer ("NSUB") usually measured only aerosol of dry aerodynamic diameter D<1micron. Periodically we switched NSUB to measuring the total aerosol as a check that the two nephelometers were in agreement and to provide field data on instrument precision. These times are indicated by a change in the "sysstat" flag (see Section 4.0 DATA FORMAT below). A third, modified single-wavelength TSI, Inc. nephelometer ("N180") was used to measured 180-degree backscatter by all aerosol at 532nm wavelength (Doherty, 1999). Two Radiance Research Particle Soot Absorption Photometers were used to measure light absorption by aerosols at 550nm (Bond et al., 1999). For flights RF#01 through RF#05 the two PSAPs were run in parallel (again, to study instrument precision), with one of them often measuring filtered air. For flights RF#06 through RF#19 one PSAP ("PSUW") measured the total aerosol and the other ("PSDC") usually measured only aerosol of dry aerodynamic diameter D<1micron. As with the nephelometer, we periodically switched PSDC to measuring total aerosol. These times are again indicated by one of the status flags. All of the measurements described so far were made at low (nearly always <45%) relative humidity. A separate measurement of the increase in 540nm integrated light scattering with relative humidity was made using two model M903 Radiance Research nepholometers (no reference) which were modified to use a 80nm wide bandpass filter (Corion CA-550). One of the Radiance nephelometers ("RRDRY") was run at low (<45%) RH and the other ("RRWET") at ~85% RH. By assuming an exponential fit to the increase in light scattering with RH (Kasten, 1969), f(RH), we were able to use this two-point data to determine f(RH) and thus predict light scattering at ambient relative humidity. Finally, a Condensation Paritcle Counter (TSI, Inc. 3010) measured total particle counts, as sampled from the nephelometers. Please note that the CPC data were used for quality- control only. These data have not been carefully calibrated or otherwise corrected for errors. Other particle-counting instruments on board the C-130 should be used for a determination of total particle count. 3.0 DATA COLLECTION AND PROCESSING Data were collected at between 0.2 and 2 sec resolution, depending on the instrument. However, While reported at 1 Hz, light absorption measurements represent 5-second averages. Similarly, the humidified scattering measurements by the Radiance Research nephlometers are filtered through a smoothing function with an approximately 20-second response time. In addition all of the nephelometers have some degree of built-in smoothing related to the time required to fully flush the instrument sample volume. The e-folding time constant associated with this effect is about 10-20 seconds. All times have been adjusted so that the start time of each data sample corresponds to when the aerosol was at the aircraft inlet. Processed data will be provided at 2 second, 10 second and 1 minute resolution. 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, hygroscopic growth factor for light scattering, and the lidar ratio). For the 2-second and 10-second data files light scattering values are smoothed over a window 38-seconds wide before calculating the Angstrom exponents. Similarly, light absorption is smoothed over a window 36-seconds wide before calculating single scatter albedo, fine mode fraction of light absorption and the lidar ratio. The smoothed data are used *only* to calculate these intensive parameters; in all cases, the reported extensive parameters (light scattering and light absorption) are un-smoothed. For all parameters, the missing/bad value code is "NaN". 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 during certain events, such as during filtered air measurements. DERIVATION OF MEAN VALUES: EXTENSIVE PARAMETERS Data from the TSI integrating nephelometers NTOT and NSUB are processed as follows: 1) Eight span gas (air and CO2) calibrations were made during the field campaign and were used to determine corrections to the TSI nephelometer gain and offset calibration coefficients. Calibration corrections were applied on a flight-by-flight basis. 2) 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). 3) Total and hemispheric backscatter were adjusted to ambient air density. 4) For NTOT only, light scattering at 532nm (ts532) was calculated using the following formulation, where gamma is derived from the Radiance Research nephelometers, as described below: tsG180RH = tsG*fRH where: fRH = ((100-RHn180)/(100-RHntot))^GAMMA ts532 = tsG180RH*(550/532)^[angstBG] (Note that RHn180 is usually very close to RHntot so fRH is near 1.0). 5) For NTOT only, light scattering at 550nm was calculated at ambient RH using the following formulation, where gamma is derived from the Radiance Research nephelometers, as described below: tsGambRH = tsG*fRH where: fRH = ((100-RHamb)/(100-RHntot)) ^GAMMA Data from the Radiance Research nephelometers, RRDRY and RRWET, are processed as follows: 1) The Radiance Research nephelometers' calibration coefficients were adjusted several times during the campaign using span gas measurements. Here we use in-flight filtered air measurements to adjust the nephelometers' calibration as needed to account for changes in calibration that occured between span gas calibrations. (Note that these adjustments were only significant for the first two flights). 1) Light scattering at 540nm is adjusted to ambient air density. 2) The value of gamma, used to calculate hygroscopic growth function, f(RH), for light scattering is calculated from the low (<45%) and high (~85%) RH light scattering as follows: GAMMA =ln((RRWETtsG)/(RRDRYtsG))/ln((100-RHrrdry)/(100-RHrrwet)) Notes: The angular sensitivity function for the Radiance Research nephelometers has not yet been measured, so no angular truncation correction factor has been applied to these data. Thus the light scattering values reported for RRDRY and RRWET do not encompass 0-180degree light scattering and are not comparable to total scatter values from the TSI nephelometer NTOT. Based on our initial calculations, error in the derived values of gamma and f(RH) should not be very large as there should not be a significant difference in the angular truncation corrections for RRDRY and RRWET. Also note that RHrrwet was measured both up- and down-stream of RRWET, not in the sample volume. RHrrwet is the average of these two RHs. Data from the 180-degree backscatter nephelometer N180 are processed as follows: 1) Eight span gas (air and CO2) calibrations were made during the field campaign and were used to periodically determine corrections to the 180-backscatter nephelometer gain and offset calibration coefficients. In-flight filtered air measurements were used between gas calibrations to determine changes in the calibration coefficients. Calibration corrections are applied on a flight-by-flight basis. 2) 180-degree backscatter, B180, is adjusted to ambient air density. Data from the Radiance Research Particle Soot Absorption Photometers (PSAPs), PSUW and PSDC, are processed as follows: 1) 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). 2) Light absorption, reported at standard temperature and pressure, are adjusted to ambient air density. 3) Light absorption at 532nm, abs532, is calculated from light absorption at 550nm, absG, as follows: abs532 = absG*(550/532) Data from the 3010 CPC (cpc3) are adjusted from the counts at instrument air density (assumed to be the same as that of NTOT) to ambient air density. The sum of total light scattering and light absorption yields light extinction. Total aerosol light extinction at 550nm, TOTextincG, is calculated from NTOTtsG and UWabsG. Total light extinction at 532nm, TOTextinc532, is calculated from NTOTts532 and UWabs532 (extinc532). Sub-micron light extinction, SUBextincG, is calculated from NSUBtsG and DCabsG, but only when both instruments were measuring only the sub-micron aerosol. INTENSIVE PARAMETERS The Angstrom exponent for (450,700nm), angstBR, and (450,550nm), angstBG, were calculated as follows: A_BR = -log(tsBs/tsRs)/log(450/700) A_BG = -log(tsBs/tsGs)/log(450/550) where tsBs, tsGs and tsRs are light scattering values that apply to 450, 550 and 700 nm, respectively and where these values have been smoothed by averaging over a 38-sec wide window. The fine mode fraction of light scattering is calculated whenever NSUB was measuring sub-micron aerosol (normal state): fmfscat = NSUBtsG/NTOTtsG The fine mode fraction of light absorption is calculated whenever PSDC was measuring sub-micron aerosol (normal state): fmfabs = DCabsGs/UWabsGs where DCabsGs and UWabsGs are light absorption values that have been smoothed by averaging over a 36-sec wide window. The single scatter albedo of the total aerosol, sub-micron aerosol, and super- micron aerosol are calculated as follows, where the latter two are only calculated when both NSUB and PSDC were measuring only sub-micron aerosol: TOTssa = NTOTtsG / (NTOTtsG + UWabsGs) SUBssa = NSUBtsG / (NSUBtsG + DCabsGs) SUPssa = SUPtsG / (SUPtsG + SUPabsGs) where SUPtsG = NTOTtsG-NSUBtsG and SUPabsG = UWabsGs-DCabsGs and DCabsGs and UWabsGs are light absorption values that have been smoothed by averaging over a 36-sec wide window. The lidar ratio of the total aerosol at 532nm is calculated as follows: LIDARRATIO = (NTOTts532 + UWabs532s)/B180 where UWabs532s is smoothed by averaging over a 36-sec wide window. DERIVATION OF UNCERTAINTIES: The 95% confidence interval uncertainty in the mean values are calculated for each extensive and intensive optical parameter, except for those derived from the Radiance Research nephelometers (i.e. RRDRYtsG RRWETtsG, gamma, and f(RH)). This is because the sources of measurement uncertainty for RRDRY and RRWET are not currently quantified. For now, we have fixed the uncertainty in gamma at 0.2 and we calculate the uncertainty in ambient-RH light scattering accordingly. Calculation of total uncertainty from multiple sources was made using standard propagation of errors under the assumptions that (i) each source of error is independent of the others such that they add quadratically and (ii) noise uncertainty decreases with the square-root of averaging time while all other sources of uncertainty do not change with averaging time. For the TSI integrating nephelometers, NSUB and NTOT, the following sources of uncertainty are accounted for: 1) instrument accuracy (Anderson et al., 1996) 2) instrument calibration uncertainty (Anderson and Ogren, 1998) 3) uncertainty in the angular truncation correction factors (Anderson and Ogren, 1998) 4) uncertainty due to instrumental noise (Anderson and Ogren, 1998) 5) for total scatter at ambient RH, uncertainty in the adjustment from low to ambient RH; calculated by allowing gamma to vary by 0.2. 6) for total and hemispheric backscatter at 532nm, uncertainty in the adjustment from scattering at 550nm to scattering at 532nm; set to half of the applied adjustment For the Particle Soot Absorption Photometers, PSUW and PSDC, the following sources of uncertainty are accounted for, as given by Bond et al., 1999: 1) instrument accuracy 2) instrument precision 3) uncertainty due to instrumental noise 4) uncertainty in the applied scattering correction 5) for light absorption at 532nm, uncertainty in the adjustment from absorption at 550nm to absorption at 532nm; set to the full magnitude of the applied adjustment. For the 180-degree backscatter nephelometer, N180, the following sources of uncertainty are accounted for, as given by Masonis et al., 2001: 1) instrument accuracy 2) instrument calibration uncertainty 3) uncertainty due to instrumental noise 4.0 DATA FORMAT For most variables, the average, standard deviation (std) and uncertainty of the mean (unc) are given. All values are tab-separated. For 2-sec resolution data files, the std is always NaN ("not a number"); for 10-sec resolutoin data, the standard deviation is calculated from 5 2-sec averages and for the 60-sec resolution data, it is calculated from 30 2-sec averages. Note that NaN is also the variable used to indicate missing data. For longer averaging times the std of the aerosol intensive parameters are set to NaN. (Variation of intensive parameters requires careful study to remove noise effects, which goes beyond the scope of this data release). All concentrations are referenced to ambient pressure and temperature (i.e. air density). All times refer to the start of the averaging period. All times have been adjusted so that the start time corresponds to when the aerosol was at the aircraft inlet. Thus the optical data should be coincident with the C-130 external probe data (i.e. temperature, pressure, RH, mixing ratio, etc.). The data are from the following instruments, as indicated: NTOT = TSI integrating nephelometer, which always measures the total aerosol (as admitted by the LTI inlet). Used to determine total integrated scattering (0- 360degrees) and hemispheric backscatter (90-270degrees) at 3 wavelengths. NSUB = TSI integrating nephelometer, which usually measures only aerosol of aerodynamic diameter D<1micron. Sometimes, NSUB is switched to measure the total aerosol (for comparison to NTOT). This is indicated in the "sysstat" flag. Used to determine total integrated scattering (0-360degrees) and hemispheric backscatter (90-270degrees) at 3 wavelengths. RRDRY = Radiance Research integrating nephelometer, which usually measures the total aerosol (as admitted by the LTI inlet). Used to determine total integrated scattering (0-360degrees) at 550nm wavelength and low RH. See "sysstat" flag to determine when RRDRY was measuring sub-micron aerosol only. RRWET = Radiance Research integrating nephelometer, which usually measures the total aerosol (as admitted by the LTI inlet). Used to determine total integrated scattering (0-360degrees) at 550nm wavelength and high (~85%) RH. See "sysstat" flag to determine when RRWET was measuring sub-micron aerosol only. N180 = Version of the TSI nephelometer that has been modified to measure 180- degree backscatter at 532nm wavelength. PSUW = Particle Soot Absorption Photometer (PSAP). Used to determine light absorption at 550nm wavelength. Used to measure the total aerosol for the whole campaign, except when it was measuring filtered air (as indicated by "psapqual" and "sysstat" flags). PSDC = Particle Soot Absorption Photometer (PSAP). Used to determine light absorption at 550nm wavelength. Used to measure either filtered air or the total aerosol for flights RF#01-RF#05. For RF#06-RF#19 this PSAP was sampling directly from the NSUB neph, so it was measuring sub-micron aerosol any time NSUB was measuring sub-micron aerosol. (Again, see the "psapqual" and "sysstat" flags). CPC3 = Condensation Particle Counter, model 3010. Measured total particle count. Used for quality control of data only. All "ambient" data (altitude, lat, lon, T, P, RH, mixing ratio) are as reported by the NCAR C-130 external probes *in-flight*. As of now (data version "c") we have not yet incorporated the post-campaign corrected data from NCAR. This means that at times you may see whacky things, like negative altitudes... Column numbers of each variable are given below. The units of each variable are given in square brackets. Note that the unit Mm^-1 is inverse mega-meters (i.e. scattering, absorption or extinction per 1E+6 meters travelled). The suffix .std indicates a standard deviation and .unc indicates an uncertainty: 1: DATE (yyyymmdd) [UTC] 2: TIME (hhmmss) [UTC] 3: DECIMAL JULIAN DAT [UTC] 4: ALTITUDE [meters] 5: LATITUDE [degrees North] 6: LONGITUDE [degrees East] 7-8: ambient pressure, Pambient & Pamb.std [mb] 9-10: ambient temperature, Tambient & Tamb.std [K] 11-12: ambient relative humidity, RHambient & RHamb.std [%] 13: PSAPQUAL (see below for full description) 14: SYSSTAT (see below for full description) 15-17: total scatter at 450nm; NTOTtsB, .std & .unc [Mm^-1] 18-20: total scatter at 550nm; NTOTtsG, .std & .unc [Mm^-1] 21-23: total scatter at 700nm; NTOTtsR, .std & .unc [Mm^-1] 24-26: hemispheric backscatter at 450nm; NTOTbsB, .std & .unc [Mm^-1] 27-29: hemispheric backscatter at 550nm; NTOTbsG, .std & .unc [Mm^-1] 30-32: hemispheric backscatter at 700nm; NTOTbsR, .std & .unc [Mm^-1] 33-35: total scatter at 532; NTOTts532, .std & .unc [Mm^-1] 36-38: hemispheric backscatter at 532nm; NTOTbs532, .std & .unc [Mm^-1] 39-41: total scatter at 450nm; NSUBtsB, .std & .unc [Mm^-1] 42-44: total scatter at 550nm; NSUBtsG, .std & .unc [Mm^-1] 45-47: total scatter at 700nm; NSUBtsR, .std & .unc [Mm^-1] 48-50: hemispheric backscatter at 450nm; NSUBbsB, .std & .unc [Mm^-1] 51-53: hemispheric backscatter at 550nm; NSUBbsG, .std & .unc [Mm^-1] 54-56: hemispheric backscatter at 700nm; NSUBtsR, .std & .unc [Mm^-1] 57-59: RRneph total scatter at 540nm & low RH; RRDRYtsG, .std & .unc [Mm^-1] 60-62: RRneph total scatter at 540nm & ~85% RH; RRWETtsG, .std & .unc [Mm^-1] 63-65: light absorption at 550nm; UWabsG, .std & .unc [Mm^-1] 66-68: light absorption at 550nm; DCabsG, .std & .unc [Mm^-1] 69-71: light absorption at 532nm; UWabs532, .std & .unc [Mm^-1] 72-74: 180-degree backscatter at 532nm B180, .std & .unc [Mm^-1] 75-77: total aerosol light extinction at 550nm; TOTextincG, .std & .unc [Mm^-1] 78-80: total aerosol light extinction at 532nm; TOTextinc532, .std & .unc [Mm^-1] 81-83: NTOT Angstrom exponent, 450 to 700nm; NTOTangstBR, .std & .unc 84-86: NTOT Angstrom exponent, 450 to 550nm; NTOTangstBG, .std & .unc 87-89: sub-micron aerosol light extinction at 550nm; SUBextincG, .std & .unc [Mm^-1] 90-92: NSUB Angstrom exponent, 450 to 700nm; NSUBangstBR, .std & .unc 93-95: NSUB Angstrom exponent, 450 to 550nm; NSUBangstBG, .std & .unc 96-98: NTOT total scatter at 550nm and ambient RH; NTOTtsGambRH, .std & .unc [Mm^-1] 99-101: total aerosol single scatter albedo at 550nm; TOTssa, .std & .unc 102-104: sub- 1micron aerosol single scatter albedo at 550nm; SUBssa, .std & .unc 105-107: super-1micron aerosol single scatter albedo at 550nm; SUPssa, .std & .unc 108-110: lidar ratio at 532nm; LIDAR RATIO, .std & .unc [sr] 111-113: fine mode fraction of light scattering at 550nm; FMFscat, .std & .unc 114-116: fine mode fraction of light absorption at 550nm; FMFabs, .std & .unc 117-118: exponential of hygroscopic growth function for light scattering, GAMMA & .unc 119: hygroscopic growth factor for light scattering for a change in RH of 40- 85%; fRH 120: NTOT pressure, Ptot [mb] 121: NTOT temperature, Ttot [K] 122: NTOT relative humidity, RHtot [%] 123: NSUB pressure (Psub) [mb] 124: NSUB temperature (Tsub) [K] 125: NSUB relative humidity (RHsub) [%] 126: RRDRY pressure (Pdry) [mb] 127: RRDRY temperature (Tdry) [K] 128: RRDRY relative humidity (RHdry) [%] 129: RRWET pressure (Pwet) [mb] 130: RRWET temperature (Twet) [K] 131: RRWET relative humidity upstream of sample (RHwetup) [%] 132: RRWET relative humidity downstream of sample (RHwetdown) [%] 133: n180 pressure (P180) [mb] 134: n180 temperature (T180) [K] 135: n180 relative humidity (RH180) [%] 136: C-130 cabin pressure (Pcabin) [mb] 137: C-130 report of ambient mixing ratio of water [g/kg] 138: cpc3 particle counts *diagnostic only* [#/cc] THE PSAPQUAL & SYSSTAT FLAGS: PSAPQUAL has five characters/flags that indicate the status of the two Particle Soot Absorption Photometers, PSUW and PSDC. SYSSTAT has four flags which give information on the nephelometers or our entire sampling system. In all cases we've set the flag to "1" when the system is working in it's normal/default state, to "0" when it is in an alternate state, and to "9" during either transitional states (such as between impactor states) or when the system function was sub-optimal. When a status flag is "9", data from the indicated instrument -- including derived intensive parameters -- are set to NaN. The one exception to this is that when the LTI was behaving sub-optimally we have, for now, simply flagged it as such; the data are not changed in any way. PSAPQUAL FLAGS (numbered left to right): 1 = PSUW measuring sample or filtered air (1=sample; 0=filtered air; 9=transitional) 2 = PSDC measuring sample or filtered air (1=sample; 0=filtered air; 9=transitional) 3 = PSUW filter being changed (1=sample; 9=filter being changed) 4 = PSDC filter being changed (1=sample; 9=filter being changed) 5 = PSDC sample source (1=sub-micron aerosol; 0=total aerosol; 9=transitional) SYSSTAT FLAGS (numbered left to right): 1 = RRDRY and RRWET sample source (1=total aerosol; 0=sub-micron aerosol; 9=transitional) 2 = NSUB impactor status (1=sub-micron aerosol; 0=total aerosol; 9=transitional) 3 = aircraft inlet (LTI) behavior optimal/not optimal (1=optimal; 0=not optimal... i.e. little-to-no turbulence; flow w/in 1.25% of isokinetic) 4 = entire system sample source (1=sample; 0=filtered air; 9=transitional) 5.0 DATA REMARKS The data shown here pertain to the aerosol that reaches our instruments, which may be different from the ambient aersol because of inlet and/or plumbing losses or enhancements. Losses or enhancements by the C-130 aircraft Low Turbulence Inlet (LTI) are not accounted for in these data. We are in the process of determining whether there was a significant loss of particles in the plumbing between the LTI and our instruments. With the exception of the Radiance Research nephelometer data, which is not yet corrected for angular truncation, we believe all data have the appropriate corrections/adjustments applied. At this time, these data are only being used to calculate gamma (from which f(RH) and total scatter at ambient RH are derived). The fact that the RRNEPH data are not corrected for angular truncation should not result in a significant bias in gamma. We are working on determining angular truncation correction factors for the Radiance Research nephelometers and future versions of our data may include corrected values of RRDRYtsG and RRWETtsG. >> Updates in the data set in going from version "c" (first release of data) to version "d": 1) An incorrect angular correction factor was being used to correct the integrating nephelometer data, resulting in a low bias in the derived blue and green light scattering values, NTOTtsB and NTOTtsG. Note that this also affected the parameters derived from these values (TOTextincG, TOTextinc532,NTOTangstBR,NTOTangstBG,TOTssa,SUPssa, LIDAR RATIO, & FMFscat). This problem has been fixed in version "d". >> Updates in the data set in going from version "d" to version "e": 1) GAMMA is used to adjust the total aerosol light scattering at the nephelometer RH to that at other relative humidities (ambient & 180-neph). GAMMA is derived from the Radiance Research nephelometer scattering values, RRDRYtsG and RRWETtsG. However,when the RR nephs are measuring sub-micron aerosol, the GAMMA we derive does not apply to the total aerosol. Thus, starting with version "e" of our data, we are using an interpolated value of GAMMA to calculate NTOTtsGambRH, NTOTts532, NTOTbs532, TOTextinc532, and LIDAR RATIO. A linear interpolation is used, with the endpoints being 30-second averages of GAMMA preceding and following the impactor switch. 2) The impactor used with the Radiance Research nephelometers to measure sub-micron scattering was improperly installed for flights RF#09 through RF#13. Therefore RRDRYtsG, RRWETtsG, GAMMA and f(RH) have been set to NaN during the times on these flights when we had switched to measuring only sub-micron aerosol with the RR nephs (as indicated by a "9" or "0" in the first SYSSTAT flag). 6.0 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. Doherty, S.J., T.L. Anderson and R.J. Charlson, "Measurement of the lidar ratio for atmospheric aerosols using a 180-degree backscatter nephelometer", Appl. Opt., 38, 1823-1832, 1999. Kasten, F., Visibility in the phase of pre-condensation, Tellus, 21, 631-635, 1969. Masonis, S.J., K. Franke, A. Ansmann, D. Mueller, D. Althausen, J.A. Ogren, A. Jefferson, P.J. Sheridan, An intercomparison of aerosol light extinction and 180-degree backscatter as derived using in-situ instruments and Raman lidar during the INDOEX field campaign, JGR, in press, 2001.