Ground based sampling of pyrogenic emissions from Big Elk Fire, Estes Park, CO

In order to fully understand the impact of pyrogenic emissions it is necessary to have a good understanding of the microphysics of clouds. Cloud formation and the aerosol emissions from wildfires are intimately connected. By understanding these interactions it is possible to investigate the role aerosols play in precipitation processes and the radiation budget. Of extreme importance are the aerosol particle size distributions produced as a result of wildfires and biomass burning. Using the Scanning Mobility Particle Sizer (SMPS) it was possible to sample pyrogenic emissions from the Big Elk Fire, Estes Park, CO. The SMPS system is transportable and was brought to a specific site to sample air downwind of the Big Elk Fire. This is the first study analyzing condensation nuclei size distributions obtained from Colorado wildfires as a result of ground based field work. By sampling air downwind of the fire it was possible to obtain both clean air and smoky air samples. High variability in wind speed and direction allowed for sampling of air containing various levels of CN. Smoke samples had consistently higher concentrations of particles, especially in the smallest size range detectible by the SMPS (approximately 0.012-0.025μm). These findings are consistent with other aerial based studies. Aerosol science is particularly interested in the size distributions of particles produced from such events due to their impact on cloud microphysics. By providing another dataset concerning particle size distribution and concentrations this study hopes to further support current trends in aerosol science regarding precipitation inhibition and alteration of the radiation budget. Introduction: Wildfires occurring in the state of Colorado are of increasing concern as both sources of massive property damage and, as we will deal with in this study, as a large source of aerosol particles. While there is no strict definition of smoke in current literature it is defined by Liousse et al., 1995, as an interactive atmospheric mix of gases, solid compounds, liquids and solutions of particles found in a plume. Recent studies suggest pyrogenic aerosol emissions from wildfires and biomass burning produce globally between 80-104 Tg yr (Kotchenruther, 1998: Reid et al., 1998) and is considered the second largest source of anthropogenic aerosol particles (Intergovernmental Panel on Climate Change (IPCC), 1995). The aerosols we will be dealing with are typically in the submicrometer size range they have significant local and regional effects and also may affect global atmospheric chemistry and the Earth’s climate (Reid, 1998). They may also affect the global radiation budget through changes in cloud absorptive capabilities as well as cloud albedo (Liousse, 1995). According to Andreae, 1998, smoke aerosols can influence climate by the “direct” effect, through the scattering of incident sunlight, and by the “indirect” effect through the modification of cloud properties. Modifications of particular importance are those that can lead to weaker hydrological cycles, increased drought and potentially increased wildfire activity. Increasing aerosols, through wildfire proliferation or biomass burning, causes an increase in the droplet number concentration of cloud condensation and ice nuclei (CCN, IN) (Ramanathan et al., 2001). Kaufman et al., 1997, believe smoke aerosols generated by fires and modified in the troposphere are effective cloud condensation nuclei. Aerosols containing large concentrations of small CCN nucleate many small cloud droplets, which coalesce very inefficiently into raindrops. The coalescence efficiency of cloud droplets into raindrops decreases drastically if the threshold effective radius of 14 μm (Rosenfeld, 1999) is not possible. Large emissions of smoke particles also increase the reflectance of clouds (Kaufman et al., 1997) by increasing the number of cloud droplets. Kaufman, 1993, shows that the presence of dense smoke can reduce cloud drop size from 15 to 9 μm. According to Ramanathan et al., 2001, one consequence of this is suppression of rain over polluted or smoke affected regions Wildfires further propagate a positive feedback through which the suppression of precipitation by aerosols prolongs their atmospheric residence time, further enhancing their impacts. The drier conditions due to the suppressed rainfall are conducive to raising more dust and smoke from burning of the drier vegetation (Ramanathan et al., 2001). Materials and Methods: The TSI Model 3934 Scanning Mobility Particle Sizer (SMPS) and Model 3010 Condensation Particle Counter (CPC) (Figure 1) were used to obtain measurements of condensation nuclei (CN) emitted from wildfires. Both instruments were produced by TSI incorporated and provided by the UCAR Cirres Lab. Particles are classified with an Electrostatic Classifier (Figure 2) and their concentration is measured with a CPC. The SMPS system also uses a Data Analysis Center, which includes a personal computer with custom softwear, to control individual instruments and perform data reduction (TSI manual, 1992). The system measures the number size distribution of particles using an electrical mobility detection technique. The SMPS uses a bipolar charger in the Electrostatic Classifier (Figure 2) to charge the particles to a known charge distribution. The particles are then classified according to their ability to traverse an electrical field, and counted with the Model 3010 CPC. The entire system is automated and uses a computer system for data analysis. The system can run in two modes: underpressure and overpressure depending on the type of data and method of sampling required. For this project the system was run in underpressure mode. This mode Figure 1: The Scanning Mobility Particle Sampler functions by drawing air through the system with a vacuum. The typical mode of operation for the SMPS system is underpressure mode using an impactor with a 0.0457 cm. diameter orifice. However, only an impactor with a 0.0508 cm. orifice was available. By using a 0.0508 cm. orifice we needed to calibrate the system for flowrates of 0.3 lpm rather than the standard flowrates of 0.4 lpm or greater for the 0.0456 cm. orifice. Due to these conditions we were only able to sample particles with diameters greater than 0.0143μm an less than 7.49μm. In order to run the SMPS software, the ratio of the Sheath Air to Polydisperse Aerosol flowrates on the Electrostatic Classifier is normally set to 10:1. The Sheath Air flowrate and the Excess Air flowrate must equal each other, as do the Polydisperse Aerosol flowrate and the Monodisperse Aerosol flowrate (see Figure 1 for valve location). Using a Monodisperse flowrate of 0.3 lpm, based on the selected impactor, both Sheath and Excess air must be 3.0 lmp. Output, in volts, was determined using the flowmeter curves included with the SMPS system accessories. See Table 1 for flowmeter set points.