As part of a research project to investigate sources, sinks and climate influence of aerosols we carried out measurements of the aerosol size (diameter), area and volume distributions from a summit location (2.8km altitude) in the Catalina Mountains in southern Arizona. We find the air to be very clean with respect to aerosol concentration, in fact at certain times nearly as clean as the air at the Mauna Loa Observatory in the central Pacific Ocean. Chemical reactions involving aerosol surface, optical extinction and scattering are controlled by a sub micron and quite volatile (with respect to vapor pressure) "accumulation mode". We present evidence suggesting that the gas to particle conversion rate in this mid-tropospheric locale is on the order of 10-19 g cm-3 s-1.
INTRODUCTION
There is considerable interest in the chemical properties and climatic influence of aerosols. Here we report on the findings from Mount Lemmon in the Catalina mountains in Arizona. Twomey had earlier made measurements of particle concentration, cloud nucleus concentration, particulate scattering and optical absorption from the same location. He found (Twomey, 1983) that the scattering and absorption coefficients were less than the values adopted by many modellers or promulgated in documents like the Global Atmospheric Resarch Program's Report Aerosols and Climate (1978) and less than generally used in models made of the background continental aerosol. Our measurements of the size distribution spectrum of the aerosols are consistent with Twomey's proposal that Mt. Lemmon is a particularly clean location for performing background atmospheric studies because it is frequently in the free troposphere undisturbed by cloud. In fact the air is only about twice as "dirty" as that at the Mauna Loa Observatory at a similar altitude, but in the Central Pacific.
THE EXPERIMENT
Our initial experiment's objective was to confirm Twomey's earlier findings of the locale's cleanliness, and to compare the site to the Mauna Loa Observatory and try to obtain some insight to the sources of the aerosol at such a continental background location.
Mt. Lemmon is in the Santa Çatalina Mountain range, which runs east west in southern Arizona. The summit is at an elevation of 2791m above sea level. The valley flood plains are approximately 2100 meters below the site and though they are populated with significant aerosol sources (Tucson, Phoenix, San Manuel copper smelter, etc.), they do not constitute a threat to the air cleanliness high in the Catalina Mountains, provided times are selected for experimentation during stable atmospheric conditions. All measurements discussed here were carried out in the winter months on days when the atmosphere had developed overnight surface-based temperature inversions. The aerosol concentrations (in cm-3) in Tucson were usually several tens of thousands, and decreased rapidly with increasing altitude. Additional information about the mixing depth for Mt. Lemmon is given in Twomey, 1983. We measured the concentrations on the road leading up to the observatory and found that the aerosol concentrations were low (typically under a thousand cm-3) and constant above the inversion. The measurements reported here were carried out in the morning hours before solar heating could create convection.
We measured the aerosol size probability spectrum with two types of instruments: an optical particle counter sizing individual aerosols on the basis of the intensity of scattered laser light (Particle Measurement System Inc., Boulder, Co., model LASX) and an instrument separating aerosols on the basis of their electrical mobility and detecting them with a condensation nucleus counter (TSI Inc., Minneapolis, Mn.,, Differential Mobility Particle Sizer, using alcohol counter model 3021). The mobility classifier used flow rates recommended by the manufacturer. Experiments were made to evaluate and calibrate the instruments using polystyrene spheres. The PSL experiments were done on both (electrical mobility and optical) instruments to validate the merging technique. Account was taken of the possible difference in index of refraction, hence scattered light intensity, of the polystyrene spheres and the atmospheric aerosols. Our calculations from the Mie scattering assumed the aerosols were ammonium sulfate.
The aerosol size probability spectrum was derived over a two and one half order of magnitude range (diameter interval 0.01 to 3.0 microns), the lower limit being set by the difficulty one has in charging particles who's electrical potential is greater than kT (Reischl et al., 1995; Rich et al., 1959) and the upper limit being determined by particles slipping out of the airstream in the intake plumbing of the particle counter. Over this size range, the differential size probability spectrum, dn/d(log(r)) varied by as much as 6 orders of magnitude!
The two different measuring techniques (optical scattering and mobility classification) agreed with each other in the overlapping particle diameter range (from about 0.15 to 0.30 m to about ten percent. In the graphs we show, we adopted the mean value sensed by the two instruments in this overlapping particle diameter range.
The DMPS measures electrical equivalent (mobility) diameter and the LASX measures optical equivalent diameter. These may be different (Hogan et al., 1993), but if so, the differences seem to be fairly minor (less than ten percent) at least for the overlapping sub micron size range. Apparently the particles sensed at Mt. Lemmon are spherical or nearly spherical in the sub micron range. It would be expected that the larger, super micron, particles since they are frequently composed of crustal material, may have more complex, less spherical shapes. We have treated them as being spherical.
AEROSOL NUMBER, AREA AND VOLUME SIZE DISTRIBUTIONS
The aerosol spectrum dn/dlogr at Mount Lemmon was unimodal . This is vastly different than what is frequently observed in the marine boundary layer (Hoppel et al, 1985). Our group has also found bimodality of aerosols in the marine boundary layer. The bimodality of aerosol within the marine boundary layer is related to cloud-aerosol interaction in the marine mixing layer. A lack of two modes at Mount Lemmon is consistent with the hypothesis of new particle production in the free troposphere and/or reduced interaction between the free tropospheric aerosol with clouds at this location. Cloudiness is indeed low in this geographic region which is frequently under the influence of the subtropical eastern Pacific anticyclone. We show the geometric mean of the aerosol size distribution measured at Mount Lemmon in Figure 1, along with bi-modal aerosol distributions found by Hoppel in the tropical Atlantic and Pacific and for polluted air advecting off the east coast of the United States.
Figure 1
When the size radius probability spectrum is converted to aerosol volume probability spectrum, dV/d logr, where two distinct modes always appeared, as shown in Figure 2. Whitby (1978) found from his experimental work on aerosols that multimodal volume distributions are common for urban aerosols. He interprets the mode at diameter approximately 0.3 micron to be removal-resistant material building up in the atmosphere. This "accumulation mode" aerosol lies in a particle size range where neither Brownian diffusion or inertial slippage across aerodynamic streamlines operates very efficiently. The diameter of the "accumulation mode" of aerosols found at Mount Lemmon is similar in magnitude to that found by Whitby in his experiment on urban aerosol. However Greenfield first demonstrated this for tropospheric aerosol, e. g., the "Greenfield Gap" (Greenfield, 1957).
Figure 2
The bimodal aerosol volume probability spectrum shown in Figure 2 is, as is common practice in the literature, plotted using a logarithmic ordinate; the bimodal behavior is much more apparent if the plot is exhibited using a linear scale for dV/d logr, as shown in Figure 3.
Figure 3
Clearly the "giant" volume aerosol mode reaches its maximum at some diameter larger than we were able to measure and probably contains the majority of aerosol mass. This is important to properly interpret chemical measurements which might be made on such an aerosol system: inferences about the chemical composition of bulk aerosol collected on filters would probably be dominated by the larger of the two modes.
The surface area of the aerosol, which is relevant for many chemical reactions with trace gases, on the other hand, is dominated by the smaller of the two modes. In Figure 4 the area probability distribution dA/d logr is shown for aerosols at Mt. Lemmon and, for comparison, distributions illustrating intense smog, typical urban air pollution and for what had been suggested by Slinn (personal communication) for the continental background aerosol. Note that the aerosol system at Mt. Lemmon is about an order of magnitude less than what had been suggested to be representative for the clean troposphere.
Figure 4
CHEMICAL COMPOSITION
Though we didn't collect particles for actual chemical analysis, we were able to infer that the composition of the aerosol in the two modes differed. In passing the aerosol through a one meter tube heated to approximately 300 C, the smaller of the two modes disappeared, while the larger of the two survived the elevated temperature environment. The smaller of the two modes consist of volatile substances, perhaps mixtures of sulfuric acid droplets, or ammonium sulfate or bisulfate, all of which are known to break down under the temperature-time conditions we used (Clarke, et al, 1985 ). The large mode is refractory material, perhaps crustal dust.
COAGULATIVE MASS FLUX
We calculated the coagulative mass flux passing through the aerosol size distribution. This is (Twomey, 1977),
where n(v) is the differential number distribution of aerosols in terms of aerosol volume and K(n,u) is the coagulation kernel. If there are n(u)du particles between u and u + du, then for each of these there is a probability K(u,v)n(v)dv of coagulating with v particles in unit time so the rate of occurrence of encounters between u particles and v particles is K(u,v)n(u)n(v)dxdy per unit volume per unit time. Equation 1 is the coagulative mass flux through a volume u and is calculated by evaluating the rate of transfer of particles less than u into sizes greater than u.
The single-peaked mass flux (figure 5) suggests that mass is being systematically transferred from smaller to larger aerosols by coagulation at a rate 3x10-19 g cm-3s-1. This implies the existence of a source function for aerosols less than about two hundredths of a micron diameter and a systematic sink for aerosols larger than about 0.2 micron diameter. Active particle productions are indicated below a few hundredths of a micron diameter and non-coagulative removal processes are indicated for particles larger than a few tenths of a micron diameter. Between these sizes
coagulative mass flux is relatively constant.
Figure 5
DISCUSSION
The "magnitude" of the two volume modes shown in figures 2 and 3 varied seemingly independent of each other from day to day (correlation coefficient = 0.15) suggesting, along with the differing volatility's, that the modes have different origins.
On days when the larger of the two particles modes was enhanced, a distinct and small visible aureole could be seen around the sun. It appeared to be several degrees in angular diameter and was readily apparent when the direct disk of the sun was blocked out. We surmise that this "small" aureole is due to the enhanced numbers of "large", supermicron particles, which would have smaller Fraunhofer diffraction pattern: e. g., they scatter more light in the near-forward direction.
Though the aerosol mass is dominated by the giant particle mode (seeming to be a "few" micrograms m-3, compared to only about 0.5 microgram m-3 for the accumulation mode), optical scattering and effects such as heterogeneous chemical reactivity on the aerosol's surface are controlled by the smaller, so-called accumulation mode aerosol at the Mount Lemmon site.
We attempted to relate variations in aerosols with meteorological variables: air temperature, height of the 700 mb level, wind direction and speed, dew point depression and features on the 700 mb synoptic maps, but found no significant correlations.
As is apparent from figure 4, the atmosphere at Mt. Lemmon is indeed very clean, in agreement with Twomey's findings, and also in agreement with comments about the low optical turbidity of Mt. Lemmon by Dunkleman and Scolnik (19 59), who carried out measurements of the solar spectral intensity in the ultraviolet region from the same site. In fact the aerosol optical depth calculated for the Mt. Lemmon aerosol shown in Figure 1 and for an assumed homogeneous aerosol thickness of 2 km agrees with that measured by Dunkleman and Scolnick.
GAS TO PARTICLE CONVERSION RATE
McMurry and Friedlander (1979) derived the following expression for the relationship between aerosol surface area, A, (cm2 cm-3) , and the rate of gas-to-aerosol mass conversion , F (g cm-3 s-1), and the aging time t, (hrs), for a self preserving aerosol:
In Table 1 we list the flux F for different values of aging time. We see that the rate of transfer of mass through the size distribution from small to large sizes by coagulation (3x10-19 g cm-3 s-1) is comparable to the calculations for self preserving sizes for aging times of approximately one day.
TABLE 1
Aobserved=13.9 micron2 cm-3
Aging time (hr) F (rate of aerosol mass creation) g cm-3s-1
1 7.8x10-19
24 2.4x10-19
100 1.7x10-19
Estimated mass flux passing through distribution
by coagulation = 3x10-19 g cm-3s-1
The estimates of gas to particle mass creation is close to that deduced by Barrie and Hoff (1984) for particles converting from gases during the spring sunrise over the Arctic. They found the conversion of sulfur dioxide to sulfate particles to be 0.063 %/hr. For their measured average SO2 concentration of 3.1 micrograms m-3, this rate of gas-to-particle conversion corresponds to 4x10-19 g m-3s-1, or similar to that deduced from the coagulative mass flux passing through the aerosol spectrum at Mt. Lemmon.
CONCLUSIONS
The aerosol system in the mid-troposphere of the region around the southwestern United States is very clean. A similar finding was made by studying the concentration of cloud concentration nuclei at this site (Philippiun and Betterton, 1995). Though the majority of the mass is in a large super micron aerosol, being probably dust, chemical reactivity , optical scattering and absorption are dominated by a volatile sub micron accumulation mode. Geometric means for the accumulation mode are : number 550 cm-3, Volume 0.33 cubic microns m-3, and geometric mean radius=0.12 microns.
New particles are evidently being created by gas-to-particle conversions at small sizes and coagulating up to larger sizes, around a micron, where they then get removed by some kind of external processes, perhaps by serving as condensaton nuclei in precipitating clouds. The rate of particle production is order ~10-19 g-1 cm-3 s-1.
We propose that the crustal dust seen at Mt. Lemmon might have originated in the Eurasian deserts. When these events were observed, the airflow had come from the west in subsiding motions, traveling around the Pacific High pressure system found off the coast of California. Those this seems surprising, since the observatory is in a desert region itself, it should be investigated as a possible source in future studies.
ACKNOWLEDGMENT
The author thanks Professors Sean Twomey and Phil Krider at the Institute of Atmospheric Physics and the Physics Department of the University of Arizona for granting permission for me to use the cosmic ray monitoring station on Mt. Lemmon for making measurements. I thank Dr. Austin Hogan for helpful editorial assistance. The analysis portion of this work was supported by NASA grant 2818-AERO92-0049.
BIBLIOGRAPHY
Barrie, L. A. , and R. M. Hoff, The oxidation rate and residence time of sulfur dioxide in the Arctic atmosphere, Atmos. Environ. 18, 2711-2722, 1984
Clarke, A., D. , Atmospheric nuclei in the Pacific mid-troposphere: their nature, concentration and evolution, J. of Geophys. Res., 98, 20,663-20,647, 1993
Dunkleman, L., and R. Scolnik, J. of Appl. Optics, 49, 356, 1959
Greenfield, S. M., Rain scavenging of radioactive particulate matter from the atmosphere., J. Meteor., 14, 115-125, 1957.
Hogan, A. W., and A. J. Gow, Particle transport to the snow surface at the South Pole: the beginning of a tropospheric history, Tellus, 45b, 188-207, 1993.
Hoppel, W. A., G. M. Frick and R. E. Larson, Effect of non precipitating clouds on the aerosol size distribution in the marine boundary layer, Geophy. Res. Lett., Dec. , 1985
McMurry, P. H., and S. K. Friedlander, New particles formulation in the presence of an aerosol, Atmos. Environ., 13, 1635-1651, 1979
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FIGURE CAPTIONS
Figure 1: The size distribution for aerosols. Solid heavy line is geometric mean of measurements made in the Catalina Mountains (at Mt. Lemmon). For comparison, distributions are shown for aerosols advecting off the East Coast of the U. S. and in the tropical Atlantic and Pacific, the later curves taken from Hoppel et al, 1985). Tic marks are for "2" and "5", in the log scales running from 1 to 10 and similarily for the other decades.
Figure 2: The volume distribution for aerosols at the Mt. Lemmon site. The heavy solid line is the geometric mean distribution; the thin lines are the lower and upper observed distributions. Note the two modes. Note that we have used a certain amount of "artistic license" in drawing what the large mode might look like above 3 microns diameter, which was the largest size actually measured.
Figure 3: The volume distribution for aerosols at the Mt. Lemmon site plotted with a linear scale ordinate to emphasize the bimodality.
Figure 4: The area probability density function for aerosols, dA/dlog R for aerosols at Mt. Lemmon. Distributions for intense smog, typical urban air pollution and continental background distributions estimated by G. Slinn are shown for comparison. Note the cleanliness of the Mt. Lemmon air.
Figure 5: The computed coagulative mass transfer flux passing through the aerosol size distributions.