USE OF THE ELECTROSTATIC CLASSIFICATION METHOD TO INVESTIGATE THE SIZE DISTRIBUTION OF AEROSOLS NEAR HURRICANE ERIKA P1.13 Natasha Greene Program in Atmospheric Sciences, Howard University, Washington, D. C. Vernon Morris, Ph. D. Department of Chemistry and Program in Atmospheric Sciences, Howard University, Washington, D. C. Arthur Aikin, Ph. D., Walter Hoegy, Ph. D., and Don Silbert NASA – Goddard Space Flight Center, Greenbelt, MD Abstract. We present measurements of aerosols acquired with a differential mobility analyzer – condensation particle counter (DMA-CPC) system aboard a P-3 flight from Miami, FL to Bermuda during Hurricane Erika in 1997. We have analyzed particle number densities as a function of particle diameter, wind speed, and distance from the eye of the hurricane. Our results indicate that particles with diameters larger than .025 µm are scavenged efficiently within the hurricane, resulting in a mono modal distribution near the eye. Introduction The primary objective of this work is to provide an assessment of the local (or regional) effect of Tropospheric aerosol global distributions and hurricanes on the size and number distributions of properties are poorly known due in part to their short tropospheric aerosols. A secondary objective is to lifetimes and reactivity. Airborne particle sampling offers demonstrate that the EC-CPC system used in this the unique advantage of gaining in situ, highly resolved investigation is suitable for future in situ aircraft studies temporal and spatial aerosol distributions (Flagan et al., in the vicinity of a hurricane. Measurements of aerosol 1996; Daum and Springston, 1993). Russell et al. concentrations have been used to quantify the effects of (1995) reported that observing the dynamics of wind speed and convective processes of Hurricane atmospheric aerosols where nucleation is occurring Erika on tropospheric aerosol distributions. requires near real-time measurement of the particle size The upward and downward convection distribution under study. In marine air, or over water processes within the hurricane can cause a measurable surfaces, there are fewer small aerosols present, unless and characteristic change in the aerosol distribution. some perturbation, such as a tropical cyclone or The variation in particle distribution is also influenced by hurricane, affects the distribution of these particles. the rainband distribution of the hurricane via the Findings discussed in Russell et al. (1996) states that scavenging of larger particles. These factors can aerosols show low-concentration bimodal distributions effectively be determined through airborne, in situ in cleaner air masses and higher-concentration single- particle measurements. mode distributions in air masses with apparent recent continental influence. However, studies of aerosols Aircraft Measurements depend heavily on interpretations of observations and measurements to provide guidelines for the Hurricane Erika occurred from September 3rd development of models and for attaining an to September 15th of 1997 in the Atlantic Ocean. The understanding of the workings of aerosol cycles flight track of the P-3 aircraft is shown in Figure 1, (Prospero et al., 1983). A leading force in the hurricane extending from the base of Miami, Florida to Bermuda research field is the Convection And Moisture on September 10th, 1997. Experiment (CAMEX), whose using remote sensing During the day of the flight, the wind speed of instrumentation to yield high spatial and temporal Hurricane Erika (HE) peaked at 110 knots and was information of hurricane structure, dynamics, and moving north-northeast of Bermuda. The force winds motion. This paper describes the analysis of aerosol extended outwards up to 145 miles from the center and data acquired in the presence of a hurricane. An the tropical storm winds extended outward up to 290 airborne, in situ system of an electrostatic classifier miles. Aerosol measurements, with diameters from .01 (EC), differential mobility analyzer (DMA), and to 1.0 µm, were obtained at altitudes up to 21,000 feet. condensation particle counter (CPC) was used to obtain The instrumentation utilized for the concurrent measurements at high temporal and spatial measurements was an electrostatic classifier (EC), resolutions in order to assess some of the composed of a TSI model 3071 differential mobility instantaneous effects of hurricanes on tropospheric analyzer (DMA), and a TSI model 3010 condensation aerosols. particle counter (CPC). The Flagan and Russell research team used a similar mechanism during their * Corresponding author address: Natasha A. Greene, Howard University, Program in Atmospheric Sciences, Washin gton, DC 20059; email: [email protected] 1 study of mixing effects in accelerated DMA-CPC path. The data bank for this study includes over 200 measurements (Russell et al., 1995). Atmospheric locations representing the evolution of the aerosols over aerosols were collected by the common inlet system the course of the flight. Three example plots are shown through one window of the P-3 aircraft every one and a in Figures 2-4. half minutes. The sample inlet enabled collected The local maxima in the size distribution plots particles to enter the classifier, and sequentially pass were. In the following discussion, a mode is defined as through the DMA, and into the CPC. The inlet system a local maxima in the particle size distribution of the was positioned forward of the engines. Once the atmospheric aerosol (Morawska, 1999). Quantitatively, aerosol inlet tubing reached inside of the aircraft, a this point should exceed its surrounding points by a smaller tube of 1.5 inches was inserted in the center of concentration great enough to be characterized as a the larger inlet (2.0 inches) in order to transfer the peak in the distribution. The aerosol plots displayed particles to the impactor. complex multimodal distributions everywhere, except Although, electrostatic classifiers are standard within the hurricane. instrumentation for laboratory measurement of The trend line plotted along the aerosol data is aerosols, several modifications to the commercial a moving average for every three readings. It is system were necessary in order to use the EC for denoted in the legend as 3 per. Mov. Avg.. Additionally, research flights. In this study, the modifications made the points or peaks that were found to be significant were the flow rates of the inner and outer cylinders of have been marked and numbered accordingly, as the DMA, the scanning electrode voltage used, and the demonstrated on each of the plots. resolution of the DMA. The aerosols pass into the EC Aerosol Concentration versus Diameter Size: File a970910.s10 system through an impactor, which discriminates Concentration: 3 per. Mov. Avg. (Conc entration:) aerosols above/below 1 µm or with a resolution of .01 to 800 µ 1.0 m. The nominal flow through the impactor is 700 regulated at 0.3 L/min. After the impactor, aerosols enter a Kr-85 bipolar charger, set at 2 mC where high 600 1 concentrations of both positive and negative ions are 500 distributed to the aerosols. Once the particles reach an 4 2 equilibrium charge distribution, they travel to the outer 400 3 dN / d log (Dp) log d / dN cylinder of the differential mobility analyzer (DMA). 300 There are two concentric cylindrical electrodes within the DMA. A 3 L/min flow of sheath air through the inner 200 cylinder of the DMA is maintained while a 0.3 L/min 100 sample flow is maintained in the outer cylinder, without 0 any mixing of the two streams; the sheath air and the 0.010183 0.0210298 0.0434305 0.0896925 0.185232 0.307744 0.63555 sample aerosol flow. The electric field in the DMA is Diameter Size (microns) generated by the interaction of the negatively charged Figure 2. Aerosol Concentration versus Diameter Size electric rod, scanned from 20 to 10,000 negative volts, (25.89N, -79.7 E, 2.75 km altitude) centered in the inner cylinder of the DMA and the Aerosol Concentration versus Diameter Size: File b970910.s03 grounded outer cylinder of the DMA. Since the Concentration: 3 per. Mov. Avg. (Concentration:) negatively charged rod is in the center, the positively 2000 charged particles are attracted through the particle-free sheath air and radially flowed to the rod. As the 1800 electrical mobility of the particle decreases, particles of 1600 2 a narrow, predictable mobility range are released 1400 through the center electrode and directed into the 1 condensation particle counter (CPC). Therefore, the 1200 modifications were necessary in order to achieve the 1000 specified diameter band of particles due to the fact that (Dp) d log / dN 800 3 the resolution of the released particles through the 600 center electrode of the DMA is a factor of the flow rates 400 and electric field produced within the EC. Further 4 discussion of this mechanism can be found in Knutson 200 and Whitby (1975). 0 The CPC measures the concentration of the 0.010183 0.0210298 0.0434305 0.0896925 0.185232 0.307744 0.63555 aerosols coming from the DMA. In this study, it was Diameter Size (microns) configured to measure in the .01 to 1.0 µm diameter size range. Figure 3. Aerosol Concentration versus Diameter Size (26.18N, -78.1 E, 4.80 km altitude) Results and Discussion The aerosol number density and size distribution was analyzed continuously along the flight 2 Aerosol Concentration vs Diameter for Figures 2-6 Aerosol Concentration versus Diameter Size: File b970910.s39 3000 Concentration: 3 per. Mov. Avg. (Concentration:) 2000 1800 2500 1 25.89N,- 79.7E 1600 26.18N,- 78.1E 30.66N,- 62.9E 2000 1400 32.47N,- 58.8E 33.44N,- 58.5E 1200 p) D ( 1500g 1000 dN / d lo dN / d log (Dp) / d dN 800 1000 600 400 500 200 0 0 0.010183 0.0210298 0.0434305 0.0896925 0.185232 0.307744 0.63555 0.010183 0.0210298 0.0434305 0.0896925 0.185232 0.307744 0.63555 Diameter Size (microns) Diameter (um) Figure 4.