TITLE: Twin Otter aerosol particle organic and elemental carbon (OCEC) concentrations. AUTHOR(S): Brian Mader Department of Environmental Science and Engineering Caltech MS 210-41 Pasadena CA 91125 626-395-4410 bmader@cheme.caltech.edu Richard Flagan Department Chemical Engineering Caltech MS 210-41 Pasadena CA 91125 626-395-4383 flagan@caltech.edu John Seinfeld Department Chemical Engineering Caltech MS 210-41 Pasadena CA 91125 626-395-4635 seinfeld@caltech.edu 1.0 DATA SET OVERVIEW Abstract Airborne levels of carbonaceous aerosols were measured using the Twin Otter aircraft during ACE-Asia. Samples were collected onboard a modified De Havilland DHC-6 Twin Otter aircraft operated by the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS). A total of 19 Research Flights were conducted between March 31 and May 1, 2001. The center of aircraft operations was located at the Marine Corps Air Station (MCAS) Iwakuni, Japan and the sampling area included portions of the Sea of Japan south and east of the Korean Peninsula, the East China Sea between China, Japan and Korea, and the Philippine Sea south of Japan. Ambient particles were collected using a newly developed honeycomb denuder sampler and organic (OC), elemental (EC) and carbonate (CC) carbon levels determined using a thermal-optical carbon analyzer. During some flights atmospheric layers could be identified as marine boundary, pollution- dominated or mineral dust-dominated. Angstrom exponent (a) values, calculated based on data from an onboard three-wavelength nephelometer, were used to discern the nature of the individual layers. Values of the Angstrom exponent for individual layers ranged from 0.2 to 2, corresponding to dust and pollution dominated layers, respectively. OC and EC concentrations below 3 km ranged from 0.58 to 29 (microgram-C m^-3 and 0.20 to 1.8 (microgram-C m^-3), respectively. On average EC levels were greater than those measured over Lake Michigan and similar to those in Pasadena, CA, only the highest OC levels were in the range measured in Pasadena, CA. Mixed layers of dust and pollution were found on some occasions. CC was detected in samples taken from layers in which a = 1.6, indicating that significant amounts of dust can be present even though a > 0.2. Values of the measured aerosol light absorption coefficient bap (Mm^-1) were correlated with the measured EC levels but significant scatter in the correlation indicates that parameters other than the mass of EC influence bap. The mass absorption coefficient (oap (m^2 g^-1) varied significantly between sampling events, the average value of 11 m^2 g^-1 (+/- 5.0) agrees well with previous published values. 2.0 INSTRUMENT DESCRIPTION Depending on the scientific goals of a mission, one or more flight profiles were used: Sampling was conducted over (1) level legs at a particular altitude, (2) spiral ascents and descents between altitudes of 50 m and 3000 m, and (3) level descents from 3000 m to 50 m. Ambient air was sampled using a dedicated inlet mounted to the nose of the aircraft. The inlet, designed to sample air isokinetically at an airspeed of 50 m s^-1, could be removed from the aircraft and cleaned between flights if necessary. A manifold used to separate the sample air among the three denuder samplers, while maintaining isokinetic flow conditions, included two impactors to remove cloud droplets from the sample air leading to the low-flow denuder samplers (Samplers A and B in Figure 1). A third impactor was a component of the high-volume particle trap impactor-denuder sampler (Sampler C in Figure 1). The compartment housing the denuder samplers was neither heated nor pressurized during flights. The temperature was monitored during flights using a temperature probe located in the samplers flow controller. The inlet and manifold were made entirely of aluminium, the impactors for samplers A and B were made of stainless steel, and the transfer lines connecting the manifold to the denuders samplers were aluminium (sample C) or copper (sampler A and B); no organic materials were in contact with the sample air throughout the entire system. Carbonaceous aerosol particles were collected using the denuder samplers described by Mader et al, [2001]. As shown in Figure 1, the sampling system consisted of three samplers: a pair of low-flow denuder samplers (Samplers A and B) to collect filter samples to be analyzed for OC, EC, and CC using a thermal- optical carbon analyzer, and a single, high-volume particle trap impactor- denuder sampler [Mader et al., 2001] (Sampler C) for collection of samples to be used for the determination of water-soluble organic carbon (WSOC) and individual organic compounds comprising the OC. The low-flow samplers operated at a flow rate of 16 Lpm and consisted of a XAD-4 coated honeycomb denuder placed upstream of a pair of either front and backup quartz fiber filters (QFFs) (4.7 cm diameter Tissuequartz QUO-UP 2500, Pall Gelman, Ann Arbor, MI) or a front QFF and backup carbon impregnated glass fiber filter (CIG). The high-volume particle trap impactor-denuder sampler was operated with a pair of front and backup QFFs (19.4 cm diameter), at a flow rate of 300 Lpm. Flow through each denuder sampler was controlled using individual volumetric flow controllers interfaced to an onboard computer; samplers could be turned on/off at any point during the flight. During air sampling, the volumetric flow, pressure, and temperature of the sample air downstream of the filters were monitored at one- minute intervals. The flow controllers had an accuracy of 0.5% and a precision of 1%. While in transit to the sampling site, there was no flow through the denuder samplers so as to prevent contamination of the samples. When the samplers were off, solenoid valves closed, isolating the low flow denuders (Samplers A and B in Figure 1) from the pumps, and preventing airflow from transporting gases and particles through the sampler. Samplers A and B could be operated in the two following configurations: (1) Samplers A and B1 configured with a denuder to remove gaseous OC, a filter (QFFf) to collect P-phase OC, EC and CC, and a backup filter (QFFb) to collect OC evaporated from particles collected on QFFf (Note that if the denuder does not remove 100% of G-phase OC, it is possible that these compounds can adsorb to QFFb). In this configuration two samples could be collected per flight; it was therefore possible to evaluate carbonaceous aerosol mass concentrations in different layers of the atmosphere, such as the marine boundary layer or a mineral dust layer. (2) Sampler A was configured with a denuder to remove gaseous OC, a filter (QFFf,a) to collect P- phase OC, EC, and CC and a backup filter (QFFb,a or CIGb,a) to collect OC evaporated from particles collected on QFFf,a (Note that if the denuder does not remove 100% of G-phase OC, it is possible that these compounds can adsorb to QFFb,a or CIGb,a). Sampler B2 was configured with a 2.0 mirometer pore diameter Teflon membrane filter (TMF) (Zefluor, Pall Gelman Sciences, Ann Arbor MI) to remove P-phase OC, EC and CC, a denuder to remove gaseous OC, and a filter (QFFb,b or CIGb,b) to adsorb gaseous semi-volatile OC not collected by the denuder. With this configuration, samplers A and B2 were run in parallel; it was possible to measure the OC, EC, and CC content of a sample (QFFf,a), the amount of OC evaporated from collected particles (QFFb,a or CIGb,a), and the ability of the denuder to remove gaseous OC (QFFb,b or CIGb,b). Prior to sampling, QFF were pre-cleaned by baking at 500 C in a muffle furnace. Since CIGs adsorb gaseous OC during storage in a freezer, CIGs were pre-cleaned immediately prior to sampling. CIGs were baked in a muffle furnace at 490 C for 16 h under an inert gas of either N2 or He. On a microgram-C cm^-2 basis, the field blank value for CIG on flight 15 was approx. 7 microgram-C cm^-2. This value is similar to that of 6 microgram-C cm^-2 calculated for Lewtas et al. [2001] using data given in that reference and equal to the value for CIG prepared in the laboratory at Caltech [Mader et al., 2001]. The field blank value for CIG on flight 19 was considerably higher, approx. 28 microgram-C cm^- 2, due possibly to the presence of high levels of VOC in the lab, or possibly due to a problem with the pre-cleaning process. If, while baking CIG in a muffle furnace, N2 leaks through an improperly sealed filter holder rather than flowing through the CIG filters, it has been observed in the laboratory that the blank levels of CIGs are high. TMFs were pre-cleaned by rinsing with 3-100 mL aliquots of acetone and 3-100 mL aliquots of dichloromethane (EM Science, Gibbstown, NJ) and then dried using ultra high purity N2. Immediately prior to sampling, filters were loaded into the respective samplers using stainless steel forceps. Sampling times depended on the particle mass concentrations measured at the sampling site and the overall goal of the particular Research Flight, and ranged from 32 to 223 min. When the aircraft returned from a flight, filters were immediately unloaded from the samplers and placed into individual aluminum-lined petri dishes. The aluminum liners had been pre-baked under the same conditions as the QFF. For each sampling event, blank QFFs and CIGs (only if CIG were used in the sampling event) were loaded into filter holders, removed, and stored with the other samples. The forceps and aluminum liners had been pre-cleaned by baking at 500 C in a muffle furnace for 12 h. Samples were stored in a freezer until analysis. All samples were analyzed for OC and EC within 48 h of sampling; CIG were analyzed within 8 h of sampling 3.0 DATA COLLECTION AND PROCESSING The OC, EC, and CC concentrations of the material deposited on the filters were determined using a thermo-optical OC/EC analyzer (Sunset Laboratories, Forest Grove, OR) [Birch and Cary, 1996]. Briefly, a 1.45 cm^2 punch of a QFF or CIG filter was loaded into the thermo-optical OC/EC analyzer. For QFF samples, OC and EC were determined as follows: OC was evolved under a stream of ultra-high purity He while heating the sample in four temperature steps of 1 min at 310 (C, 1 min at 450 C, 1 min at 575 C, and 1.5 min at 870 C. To evolve EC and pyrolized OC, the sample was heated under a mixture of 10% O2, 90% He in six temperature steps of 0.75 min at 550 C, 0.75 min at 625 C, 0.75 min at 700 C, 0.75 min at 775 C, and 0.75 min at 850 C, and 2.0 min at 900 C. For QFF samples, CC was determined as follows. After a 1.45 cm^2 punch of a QFF was analyzed for OC and EC, a second punch was taken from the same QFF. Using a disposable pipette, 2 drops of 6 M HCl was added to the QFF punch and the sample placed into the OC/EC analyzer. The sample was then heated using the same temperature/gas program used for the OC and EC analysis described previously. Based on experiments in which pure CaCO3 was spiked onto a QFF punch and the sample analyzed using the OC/EC analysis procedure previously described, it was observed that carbonate evolved during the fourth temperature step 870 C of the thermal/optical analysis. By comparing the thermograms of the acid-treated and untreated QFF punch, the presence of CC could be determined; if CC was present, a peak present in the fourth temperature step of the analysis of an untreated sample was not present in the acid treated sample. Using the OC/EC analyzer software, this peak could be integrated and the mass of CC determined. CIGs were analyzed as follows: OC was evolved under a stream of ultra-high purity He while heating the sample in five temperature steps of 1 min at 250 C, 1 min at 300 C, 1 min at 350 C, 1 min at 400 C and 0.5 min at 450 C. 4.0 DATA FORMAT: Data file structure is comma delimited ASCII and figure 1 is a .GIF image PI/DATA CONTACT = Mader Brian (Caltech); Flagan Richard (Caltech); Seinfeld John (Caltech) DATA COVERAGE = START: 04022001; STOP: 05012001 PLATFORM/SITE = Twin Otter SAMPLE COLLECTION INSTRUMENT = Caltech denuder samplers PARTICLE TRANSMISSION EFFICIENCY = 80% for 2.3 micrometer diameter dioctyl pthalate particles OCEC MEASUREMENT INSTRUMENT = Sunset Labs Analyzer OCEC MEASUREMENT METHOD = Published ACE-ASIA method LOCATION = mobile DATA VERSION = 1.0 (11 March 2002) Final REMARKS = California Institute of Technology ACE-Asia Date = yyyymmdd time = hh:mm:ss (UTC) Research Flight = Twin Otter research flight OC = organic carbon concentration (microgram carbon/m^3); if < precedes value then level is below the detection limit and the value given is the detection limit; it is likely the actual value is below this value (+/-) = standard deviation of OC measurement EC = elemental carbon concentration (microgram carbon/m^3); if < precedes value then level is below the detection limit and the value given is the detection limit; it is likely the actual value is below this value (+/-) = standard deviation of EC measurement CC= carbonate filter concentration (microgram carbon/ cm^2 filter) bap (Mm^-1) = particle absorption coefficient (mega meters^-1) (+/-) = standard deviation of bap measurement mass abs = mass absorption coefficient of EC (m^2/gm) a = Angstrom Exponent Average Latitude = self-explanatory (+/-) = standard deviation of latitude Average Longitude = self-explanatory (+/-) standard deviation of Longitude Average Altitude (m) = self-explanatory (+/-) = standard deviation of altitude sample time (UTC) = time when sample was collected in Greenwich standard time RH = relative humidity % RSD = percent relative standard deviation of RH T = temperature degrees Celsius % RSD = percent relative standard deviation of temperature bsp (mM^-1) = particle scattering coefficient for light of wavelength 550 nm (mega meters^-1) (+/-) = standard deviation of bsp btot (Mm^-1) = total extinction for light of wavelength 550 nm (mega meters ^-1) (+/-) =standard deviation of btot 5.0 DATA REMARKS Quality assurance of OC/EC analysis data The accuracy of the determination of OC was evaluated by analyzing filter samples spiked with a known mass of an organic compound. Sucrose was used as the standard compound since it is known to pyrolize during analysis and can thus be used to test the ability of the analyzer to correct for charring. The OC/EC analyzer was operated in the field during ACE-Asia. Standards were prepared as follows: a pre-baked 1.45 cm^2 QFF punch was spiked with a precisely measured volume (5 to 25 micro-Liters) of a sucrose solution of known concentration (0.5512 or 5.586 microgram-C micro-Liters^-1). The punch was then analyzed for OC and EC as described previously. A series of standards were analyzed; the range of filter loadings encompassed the range observed in samples collected during ACE-Asia. The mean percent difference between measured and true values is -2%. No EC was measured for the sucrose standards, indicating that the instrument properly corrected for the pyrolysis of this compound. Since no EC standard exists, it was not possible to determine the accuracy of the EC concentrations measured using this method. The precision of the OC and EC analysis method was determined by replicate analysis of samples having the same OC/EC filter loading. These samples were comprised of standards made using sucrose or filter samples obtained during ACE- Asia. At a given filter loading, from three to five replicate analyses were completed and the relative standard deviation calculated. The precision of the OC and EC measurements were a function of the filter's carbon loading. An inter-laboratory comparison of OC and EC measurements was conducted among eight laboratories operating OC/EC analyzers manufactured by Sunset Laboratories. Blind sub-samples from four filters were sent to each participant; the samples were analyzed while the OC/EC analyzers were operating in the United States, China and in the field during ACE-Asia. The results of the intercomparison are described by Schauer et al. [2002]. Briefly, the precision of the measurements was a function of the filter's carbon loading; OC and EC filter loadings were within 4-13% of the consensus values. The detection limit for OC and EC was defined as the lowest ambient mass concentration (microgram-C m^-3) of OC and EC that could be measured. It was determined as follows: A 4.7 cm QFF was pre-cleaned and stored in a petri dish as discussed previously. Prior to the sampling mission or on days without a sampling mission, the pre-cleaned filter was loaded into denuder sampler A or B present in the nose of the aircraft. No air was drawn through the sampler (i.e. the inlet was capped and the solenoid valves downstream of the filter closed). After 15 min to 4 hours, the filter was removed from the sampler and handled identically to a field sample. For a given sampling event the method detection limit (microgram-C m^-3) is, MDL= M field blank / V sample where M field blank is the mass of carbon (microgram-C) measured on the field blank filter and V sample the volume of air that was sampled (m^3). Thus the method detection limit decreases as the air sample volume increases. If the measured OC or EC concentration did not exceed twice the MDL; the concentration was reported as less than the detection limit. Other data notes: In the data table if the symbol < precedes value then level is below the detection limit and the value given is the detection limit; it is likely the actual value is below this value CC is reported as the filter concentration (microgram-C cm^2 filter) since the transmission efficiency of particles greater than 2.3 micrometers diameter was not measured. No OC, EC or CC measurements were made on flights 9 and 10 due to a pump malfunction 6.0 REFERENCES: Birch, M. E. and R. A. Cary, Elemental carbon-based method for monitoring occupational exposures to particulate diesel exhaust, Aerosol Sci. Technol., 25, 221-241, 1996. Lewtas, J., Y. Pang, D. Booth, S. Reimer, D. J. Eatough and L. A. Gundel, Comparison of sampling methods for semi-volatile organic carbon associated with PM2.5, Aerosol Sci. Technol., 34, 9-22, 2001. Mader, B. T., R. C. Flagan and J. H. Seinfeld, Sampling atmospheric carbonaceous aerosols using a particle trap impactor/denuder sampler, Environ. Sci. Technol., 35, 4857-4867, 2001. Schauer, J. J., B. T. Mader, J. T. Deminter, G. Heidemann, M. S. Bae, J. H. Seinfeld, R. C. Flagan, R. A. Cary, D. Smith, B. J. Huebert, T. Bertram, S. Howell, P. Quinn, T. Bates, B. Turpin, H. J. Limp, J. Yu and C. H. Yang, ACE- Asia intercomparison of a thermal-optical method for the determination of particle-phase organic and elemental carbon., Submitted for publication, 2002.