3506B24 Final Report
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- RM Young 05305AQ 0-360 deg
- 5.1.2 Ozone
| Figure 10: Schematic of OLC site upgrades: platform, fence, electrical wiring (left), and photograph of the platform (from NE) with sampling equipment (right) taken on 1/27/2003.
Table 4: The aerosol sampling strategy allows the capture of multiple burn events per week due to a most efficient use of limited sampling media.
The PM2.5 sampling was accomplished by use of four PCM that employed diffusion tubes (denuders) and filter packs as illustrated schematically in Table 5, each operating 3 channels simultaneously. Each of the sampling media (denuders and filters) underwent special treatment before and after sampling, including the mass equilibration of Teflon filters, the pre-baking and coating of quartz, glass and cellulose fiber filters following Standard Operating Procedures (SOP) specially developed and now routinely applied to the PCM sampling. Only a few issues covered in the SOP of the gravimetric mass determination, IC and TOT operations will be presented in the analytical section below. Prior to collection of PM2.5 on filter media, important inorganic and organic gaseous aerosol species such as NH3, SO2 and HNO3 , as well as semi-volatile polycyclic aromatic compounds (PAH), pesticides, and halogenated species are effectively removed from the aerosol sample stream by gas diffusion and adsorption to specially coated denuder walls. The main species quantified and reported are the gaseous NH3, HCl, HONO, HNO3, SO2, and the light organic acids (LOA) HCOOH, CH3COOH, and (COOH)2. For fine PM composition, the particle phase concentrations of sodium (Na+), potassium (K+), calcium (Ca2+), ammonium (NH4+), chloride (Cl-), nitrate (NO3-), sulfate (SO4=), formate (HCOO-), acetate (CH3COO-), and oxalate (C2O4=), as well as elemental and organic carbon (EC, OC) are reported. Table 5: PCM channel configurations with Ch …channel number, D …denuder, F …filter, SC …sodium carbonate, PA …phosphorous acid, T …Teflon, PPA …PA-coated paper (cellulose), GSC … SC-coated glass fiber, Q … quartz fiber, XQ …xad4-coated quartz fiber, CM …carbon monolith. The flow direction is from left to right. The grey-shaded components are permanent, while all others are being analyzed for aerosol constituents (gas- and particle-phase, incl. semi-volatiles) in each sample described later. Ch D1 D2 D3 F1 F2 F3 Flow Direction =================================== 1 SC PA T PPA 2 SC SC PA T GSC GSC 3 CM Q XQ The XAD-4, a porous macroreticular, non-polar, polystyrene-divinyl-benzene resin, was selected as the sorbent here because of its high surface area (725 m2g-1) for adsorption of a wide range of SVOC, and because of previous extensive development and validation work for the IOVPS and IOGAPS [Gundel et al. 1995, Lane and Gundel, 1996; Gundel and Lane 1998; Lane, 1999; Stevens et al. 1993 and Pinto et al. 1998]. Channel 3 deploys a carbon monolith CM (NovacarbTM, Mast Carbon Ltd., UK) extruded from phenolic resin to ~10 cm long, ~3 cm dia. cylinder with <200 channels, controlled micro-porosity of ~1 nm, and a specific surface area of ~1,000 m2/g. Thankfully, the USAIC at Fort Benning helped cover the expenses associated with this effort. Two High-Volume Samplers (HVS, labeled E and F) for the detection and quantification of particle-phase organic compounds (POC) have been installed and operated on the platform next to the four PCM (labeled A, B, C, and D). Thanks to the valuable collaboration with OLC personnel, a sampling schedule could be set to capture the various stages of the burn, i.e. when the prescribed burn unit on the base was mostly flaming versus mostly smoldering, in comparison with and contrast to the unperturbed conditions 24 h before the burn, characterized as background. In order to be able to capture more than one burn event per week, the PCM and the HVS generally operated according to the schedule illustrated in Table 4, with minor alterations. The sample media of the HVS and PCM were exchanged before 1200 and at 2200 on a designated burn day, and again at 800 the following morning, in order to potentially capture the likely impacts by the burn plumes late nights and early mornings, due to katabatic drainage flows developing during clear calm nights as mentioned above. Measurements of the trace gas species O3, CO, NO, and NOy, as well as PM2.5 mass concentration (TEOM) and meteorological parameters were made continuously. Instruments operated inside the air-conditioned 3 x 4 m shelter. Meteorological sensors, gas sampling and PM2.5 cyclone inlets were mounted to a 8 m tall meteorological triangular Al tower. The meteorological parameters measured, their units, the sensor height in m above ground (mag), the sensors’ specifications and accuracies are listed in Table 6. Table 6: Most important meteorological parameters measured at OLC.
The sample gas for the measurement of ambient levels of NO, NOy, O3, and CO, was drawn through Teflon (PFA) tubing with inlets mounted on top of the tower, at about 6 mag. All analog signals were acquired via 24 differential inputs at 14-bit resolution and 1 Hz by an ESC data logger model 8816 utilizing a separate module for meteorological data inputs. The 1 Hz data were internally averaged and stored as 1 min averages of the unconverted analog Vdc signals. The 1 min data were converted to physical units and reduced to 30 min averages on a monthly basis undergoing routine QA/QC procedures as outlined below. The reported time is Eastern Standard Time (EST). The O3, and CO analyzers inside the shelter shared a 9 m long, 3 mm ID PFA sample tube through which ambient air was drawn down from the tower at an average flow rate of 3 slm (liters per min @STP). The average residence time of the sample gas in the line was 1.3 s. The quality of the measured data is assessed below in terms of response time, detection limit, precision and accuracy. Values for detection limit and precision are expressed at 95 % confidence level assuming normal distribution of zero and span check signals acquired during the measurement period. 5.1.2 OzoneO3 was measured using a pressure and temperature compensated commercial UV absorption instrument (model TEI 49-C, Thermo Environmental Instruments, Inc., Franklin, MA), being absolutely calibrated by the known absorption coefficient of O3 at 254 nm. The signal was generated by the difference of frequently alternating measure and reference (zero) cycles, i.e. full transfer of O3 through versus complete removal of O3 from the flow system. The linearity and precision of the analyzer was checked on average once every 23 hours. Precision check mixing ratios of 0, 40, 80, 120, and 160 ppbv were provided by a reference standard within a multi-gas calibrator (model TEI 146C). The calibrator was supplied with O3-free (zero) air from a cartridge of activated carbon that effectively removed O3 from the ambient air. Each precision check resulted in a 5 point linear regression. Assuming normal distribution of the regressions’ intercepts, the O3 analyzer’s detection limit was 0.5 ppbv; whereas the slope of the linear regression yielded ±5 % precision. The accuracy is estimated to be the same. Download 11.49 Mb. Share with your friends:
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