Lidar and Balloon-Borne Cascade Impactor Measurements of Aerosols: A Case Study

Abstract

Aerosol size distributions, elemental components, complex refractive indices, extinction profiles and extinctionto-backscatter ratios have been measured and inferred from balloon-borne cascade impactor and lidar observations made during a cooperative joint experiment conducted during the period 4-10 April, 1980 in Tucson, AZ. Size distributions obtained from quartz crystal microbalance (QCM) cascade impactor measurements at different heights (1 to 1000 m) and times over a period of several days were fairly similar in form, being clearly bimodal in their mass distributions with the coarse particle mode being dominant. Electron microscope and energy dispersive X-ray analyses of particles deposited on the QCM stages over the particle radii range -0.5-4.0 ym revealed that the particle samples were elementally dominated by both sulfur and crustal type (Al, Ca, Mg and Si) elements. Complex refractive index estimates for a wavelength of 649 nm were obtained by comparing the lidar inferred aerosol extinction-to-backscatter ratios with theoretically computed values calculated for the impactor-derived size distributions. The real part of the index was estimated to be 1.45 for most cases, while the estimates for the imaginary part ranged between 0.000 and 0.01. Aerosol extinction coefficients calculated for the impactor-derived size distributions were found to be somewhat smaller but in fair agreement with the extinction coefficients retrieved from the lidar measurements.

Assessment of Aerosol Extinction to Backscatter Ratio Measurements Made at 694.3 nm in Tucson, Arizona

“…A Junge distribution of slope v = 3 is also included because Junge-type distributions with slopes bounded between -2 < v < 3 are also frequently obtained from inversions of Tucson spectral optical depth data . The final distribution included in Figure 3, referred to as the mean balloon impactor distribution, is the average of several balloon-borne cascade impactor measurements made in Tucson during April 1980, as previously reported by Reagan et al (1984). With the exception of the v = 3 Junge distribution, they all display a noticeable peaking in the large or coarse particle mode.…”

“…Intercomparative experiments involving airborne direct aerosol measurements as well as crosschecks with other remote sensing measurements (Reagan et al, 1977(Reagan et al, ,1984Spinhrne et al, 1980) have demonstrated that the slant-path lidar technique can be employed with good success. In addition, the solution procedure produces standard deviation estimates for the retrieved values of Sa, P,(r), and a,(r).…”

“…In addition, although only a limited set of size distributions was considered in the analyses, these distributions do cover a rather wide range of distribution shapes that are thought to be representative of atmospheric aerosols for at least the locale where the lidar measurements were made. (Reagan et al, 1977(Reagan et al, , 1984 have indicated an abundance of sulfur for particles with radii as large as -1.0 pm.…”

“…Analyses of particle samples collected near Tucson by both aircraft (Reagan et al, 1977) and a balloon-borne cascade impactor (Reagan et al, 1984) have revealed ME abundance of sulfur for partides ' iyTith radii as large as -1.0 pm. Larger imaginary refractive index values in the 0.005 to 0.01 range do not correspond to any spec~fic substance commonly associated with atmospheric aerosols, but such values may possibly result from small amounts of absorptive material such as carbon mixing with otherwise very weakly absorbing particles (e.g., Lindberg and Gillespie, 1977;Ackerman and Toon, 1981).…”

“…This is basically the refractive index range that remains after excluding the index regions that appear to best characterize the h g h and low values of S,. T h s resulting range also includes the average refractive index value of 1.45-0.0041' determined by Reagan et al (1984) from the combined lidar and balloon-borne cascade impactor measurements made near Tucson in April 1980.…”