TITLE: UH Aerosol Sizing (OPC) AUTHORS: Antony Clarke, Steven Howell, Cameron McNaughton University of Hawaii, Department of Oceanography 1000 Pope Rd. Honolulu, HI 96822 USA showell@soest.hawaii.edu tclarke@soest.hawaii.edu http://www.soest.hawaii.edu/HIGEAR/ 1.0 DATA SET OVERVIEW: Introduction: This file describes aerosol particle size data obtained with an optical particle counter (OPC) aboard the NCAR C-130Q aircraft. The accompanying files provide size distributions in dN / d log D format from 0.12 to 12 micrometers at a resolution of 112 channels per decade. The OPC generally operated at reduced humidity compared with ambient conditions. While the distributions as given are adequate to show general features of the size distributions, people wishing to pursue quantitative work with these data should be aware that no corrections have been made for particle composition, relative humidity, or sampling efficiency. These issues are discussed below in more detail. In addition to normal (unheated) operation, sample air was periodically diverted through heated tubes to give some indication of the chemical composition of the particles. In the clean marine boundary layer, it has been demonstrated that particlulate volume lost after heating to 150 C corresponds well to measured sulfuric acid and heating to 300 C drives off ammonium sulfates [Clarke, 1991]. As the air sampled during ACE-Asia was rarely pristine, interpretation of the heated data is less straightforward. In addition to sulfuric acid, heating to 150 C will remove some nitrates and organic compounds, while heating to 300 C should remove most organic material as well as as ammonium sulfates. Refractory material that survives heating to 300 C is primarily dust, sea-salt, fly ash, and soot. Time period covered by the data: All research flights are included in this submission, though large fractions of RF02, RF09, and RF16 are missing due to OPC malfunctions. Physical location: Aboard the NCAR/NSF C-130Q aircraft. 2.0 INSTRUMENT DESCRIPTION: We used a modified LAS-X ASASP Optical Particle Counter. Essentially, the optics remain as manufactured, but the electronics have been replaced with a fast, wide dynamic range log amplifier and a 256 channel pulse height analyzer. The OPC was mounted on top of the front starboard rack, adjacent to the LTI. 20 liters/minute were taken from the 3-way splitter. 19 of that was simply exhausted as waste flow from a virtual impactor (which was never actually used while sampling). Before entering the OPC, a set of software-controlled valves determined whether the sample flowed through unheated or heated inlet tubing [Clarke, 1991]. There were two heated channels set at 150 and 300 C. Because OPC response is a function of composition and hence water content, changes in relative humidity affect sizing. To reduce this effect, sample air was diluted 50:50 with dessicated air. Specifications: Size range: 0.12 to 12 micrometers Sizing accuracy: Dependent on particle refractive index, shape, and diameter. Most likely about 5% up to 0.4 micrometers, 10% above 0.4. Large dust particles are often so aspherical that "diameter" is difficult to define, so sizes must be regarded as effective optical diameters. Sizing precision: <2% for particles of the same composition, shape, and orientation. Sensing angle: 35 to 145 degrees, but since it is an active cavity device, it is doubly sensitive from 60 to 120 degrees. Sensing wavelength: 632 nm (HeNe laser) Flow rate: 0.125 liters per minute +/- 5% Sampling period: 30 seconds 3.0 DATA COLLECTION AND PROCESSING: The OPC itelf reported counts every 3 seconds. During flight, samples were averaged over 30 second periods, separated by 3 seconds to eliminate artifacts due to valve switching. While flying level, sampling rotated between unheated, 150 C and 300 C channels. While ascending or descending, the heated channels were omitted to enhance time resolution. Description of derived parameters and processing techniques used: The OPC counts the number of particles in each of 256 voltage bins, which are proportional to the logarithm of the scattering intensity within the viewing angles of the optics. Calibrations with nearly monodisperse latex (0.1 to 3 micrometer) and glass (2 to 7 micrometer) spheres (Duke Scientific) were used to establish the relationship between voltage and diameter. A procedure similar to Clarke [1991] was used, but a cubic spline was used rather than piecewise polynomial fitting. This calibration procedure assumes that scattering intensity is monotonically related to particle diameter. Since the Mie scaattering intensity curves typically dip down between 0.4 and 0.6 micrometers and (more weakly) between 1.2 and 2 micrometers, sizing is ambiguous in these ranges. However, strongly absorbing aerosol such as we saw during ACE-Asia suppresses the downward wiggles in the curve, reducing this ambiguity. Due to the irregularities in Mie scattering, non-ideal characteristics of the log amplifier, and counting statistics, the calculated distributions are rather noisy. Therefore, the data presented here have been smoothed with a 29 point Gaussian window. Description of quality control procedures: Samples were edited to remove time periods when the laser power fell too low, the thermal channels were not at the correct temperature, and when obvious shattering of cloud particles occurred. Since the OPC got its air through the LTI, no data are reported when the LTI was not operating properly. There were 2 flights (RF03 and RF19) when the LTI data acquisition malfunctioned, but the LTI itself appeared to operate properly, meaning that LTI enhancements cannot be directly calculated. OPC data for those flights are included, but flagged as suspect. 4.0 DATA FORMAT: Data files are tab-separated ASCII text. A section describing the data format preceeds the data matrix. After the description is a 2-line header with column names on the first line and units on the second. Names for the particle size columns are the diameters in micrometers. (Strictly speaking, diameters are the geometric mean diameter of the size range included in the bin, but since the bins are narrower than the sizing errors, a precise definition is irrelevant.) Columns are as follows: 1) Start time of the sampling interval, YYYYMMDDhhmmss 2) End time of the sampling interval, YYYYMMDDhhmmss 3) Longitude (LONC) from RAF netCDF file, degrees 4) Latitude (LATC) from RAF netCDF file, degrees 5) Altitude (ALTX) from RAF netCDF file, meters 6) LTI flag, 2=proper operation, 1=LTI data failure, but it was apparently working properly, 0=improper functioning (shouldn't appear in the file, as I tried to edit them all out). 7) Inlet temperature flag, 1=unheated, 2=150 C, 3=300 C 8) Relative humidity of air entering the OPC, % 9) Ambient relative humidity from RAF netCDF file, % Following RAF recommendations, this was RHLA1 for RF04, 06, 08, 10, 13, and 16; RHLA for RF11; and RHUM for the rest of the flights. 10) Ambient pressure (PSXC) from RAF netCDF file, mbar 11) Ambient temperature (ATX) from RAF netCDF file, C 12-235) dN / d log D in each size range, # cm^{-3} at standard conditions (25 C, 1013 hPa). Note: common log (base 10), not natural log (base e). Data files are named RFXX_UH_OPC_vY.txt where XX is replaced by the flight number and Y is replaced by the version number. This is version 4. 5.0 DATA REMARKS: The dN / d log D convention used here allows different instruments to be compared directly and allows easy "visual integration" when semilog plots are used (log diameter vs. concentration). To obtain concentrations over a size range sum the appropriate channels and multiply by 1/112 = 0.0089286. As relative humidity changes, particles gain and lose water to a degree determined by composition. Since sample air was heated while accelerating to aircraft speed, heated or cooled by the cabin environment, and mixed with dessicated air before entering the OPC, sample RH was rarely the same as that outside the plane. Thus, the particles seen by the OPC and reported here are not at ambient diameters. A further complication arises because scattering intensity is a function of refractive index and particle morphology as well as diameter. Refractive index is determined by composition. The diameters included here must be interpreted as the diameters of latex spheres that scatter light with the same intensity as the sensed particles over the solid angles to which the OPC is sensitive. The upshot of all this is that wet particles (high RH, low refractive index) are undersized, small absorbing particles are oversized, large absorbing particles are undersized, and complex particles (typically dust) can be significantly oversized. (In contrast, aerodynamic sizing tends to underestimate sizes of complex particles). Particles entering the OPC passed through the LTI, 4 or 5 splitters, a valve, a tee, and roughly 3 meters of tubing of various diameters at flow rates ranging from 136 to 0.125 lpm. Each element changes the size distribution to some degree. The LTI enhances large particle concentrations to a degree that can be calculated. The rest of the flow path was designed to minimize losses, but unavoidably tends to lose particles, particularly large ones. LTI enhancement calculations have been done, but are not yet in a widely available, convenient format. We have measured particle losses as a function of size through much of the plumbing leading to the OPC, but that work is still ongoing. 6.0 REFERENCES: A. D. Clarke, A thermo-optic technique for in situ analysis of size-resolved aerosol physicochemistry, Atmos. Env. 25A:635-644, 1991. J. L. Hand, R. B. Ames, S. M. Kreidenweis, D. D. Day, and W. C. Malm, Estimates of particle hygroscopicity during the Southeastern Aerosol and Visibility Study, J. Air & Waste Manage. Assoc. 50:677-685, 2000 Porter, Clarke, Ferry and Pueschel, Aircraft Studies of Size- Dependent Aerosol Sampling Through Inlets, J. Geophys. Res., 97:3815-3824, 1992.