On the variation of cloud condensation nuclei in association with cloud systems at a mountain-top location

Atmospheric Research, 31 ( 1 9 9 4 ) 13-39

13

Elsevier Science B.V., A m s t e r d a m

On the variation of cloud condensation nuclei in association with cloud systems at a mountain-top location 1 T.P. D e F e l i c e ~ a n d V.K. S a x e n a b •Department of Geosciences, Universityof Wisconsin-Milwaukee, Milwaukee, WI 53201, USA bDepartment of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695-820& USA ( Received January 19, 1993; revised and accepted March 16, 1993 )

ABSTRACT The measurements of cloud condensation nuclei, CCN, concentration (both in cloud-free air mass and within clouds-interstitial CCN), cloud droplet size distribution and liquid water content are reported for three cloud episodes observed in Mount Mitchell (38°44'05"N, 82 ° 17' 15"W, 2038 m MSL--highest peak in eastern U.S.A.) State Park, North Carolina. The comprehensive database is analyzed to understand the temporal variation of the interstitial CCN during air mass transport and mixing and to understand relationship between CCN concentration, aqueous phase chemical composition, and relative humidity during the evaporative stages of a cloud. Large variations in the interstitial CCN concentration ( > 2 0 0 c m - 3 h - I ) were found to be related to the sampling near the boundary of different air masses. Large temporal interstitial CCN variations that occur near the end of an event are likely due to evaporation. More subtle changes in the CCN concentration ( < 200 c m - 3 h - m) were associated with air mass transport. The measured CCN concentrations made in association with the dissipative stage of a cloud event appear to be a function of air mass chemistry and the instantaneous value of relative humidity. RI~SUMI~ On pr~sente des rrsultats de mesures de la concentration en noyaux de condensation nuageuse, N C N (en air clair ou en air nuageux), de la distribution dimensionelle des gouttelettes de nuage et du contenu en eau liquide rralisrcs au cours de trois 6pisodes nuageux au Mont Mitchell en Caroline du Nord (38 ° 44' 05 "N, 82 ° 17' 15 "W, 2038 m - - l e pic le plus 61ev6 de l'Est des Etats-Unis d'Amrrique). Les donn~es permettent de suivre les variations temporelles des N C N en air nuageux pendant le transport et le mrlange de la masse d'air, et de comprendre les relations entre la concentration en NCN, la composition chimique de la phase aqueuse, et l'humidit6 relative lors de l'rvaporation du nuage. On trouve que les fortes variations observres dans la concentration en N C N en air nuageux ( > 200 cm-3 h - 1) s¢ produisent ~ la limite de diffrrentes masses d'air. Les fortes variations temporelles de NCN en air nuageux que r o n observe vers la fin d'un 6pisode sont probablement dues ~ l'rvaporation. De plus faibles variations dans la concentration en N C N ( < 200 cm -3 h-~ ) sont associres au transport ~Parts o f this text h a v e b e e n presented at the t h i r t e e n t h I n t e r n a t i o n a l Conf. o n Nucl. a n d Atmos. Aerosols, Salt Lake City, UT, August, 1992.

0 1 6 9 - 8 0 9 5 / 9 4 / $ 0 7 . 0 0 © 1994 Elsevier Science B.Y. All rights reserved. SSDIO 1 6 9 - 8 0 9 5 (93)E0025-T

14

T.P. DEFELICE AND V.IC SAXENA

de la m a s s e d'air. Les c o n c e n t r a t i o n s en N C N mesur~es en p h a s e dissipative d u n u a s e sont en relation avec la c h i m i e de la m a s s e d ' a i r et avec la valeur i n s t a n t a n 6 e de l ' h u m i d i t 6 relative.

INTRODUCTION

The cloud condensation nuclei, CCN, concentration (both in cloud free and within clouds-interstitial CCN), the cloud droplet size distribution and the water content were measured in Mt. Mitchell State Park, NC during the summer (mid-May through mid-September) of 1988, in conjunction with the on going measurements of the Mountain Cloud Chemistry Project. The inter-relationship between interstitial CCN, cloud droplet spectra and the water content is beneficial since it will lead to: (a) the formulation of Eulerian relationships of the C, k parameters, the CCN concentration and of the information that may be derived from these, (b) a better understanding of the processes involved in the creation, maintenance and/or dissipation of a cloud, and ultimately (c) an indirect assessment of the contribution to the cloud albedo that results from the change in the interstitial CCN. CCN measurements would also prove valuable in investigations related to the effect that they might have on climate (e.g. Charlson et al., 1987). Ghan et al. (1990) calculate that a factor of 4 increase in the CCN concentration in stratus clouds over the oceans would cause the worldwide albedo to increase by 2% cancelling the CO2 effect. The existence of a Eulerian relationship between CCN measurements, large (e.g. change in air-mass) and small (e.g. evaporation) scale meteorology was qualitatively suggested by Radke and Hobbs (1969). Hudson (1984) used interstitial CCN measurements to derive information on the effect that mixing has on the evolution of droplet spectra. A number of laboratory and field studies (e.g. Twomey and McMaster, 1955; Radke and Hegg, 1972; Hegg et al., 1990; Radke and Hobbs, 1991 ) have reported an enhancement (relative to the one upon which the droplets were formed) in the number of CCN found in the vicinity of cloud droplets in subsaturated environs. Lodge and Baer (1954) and Mitra et al. ( 1992 ) have found no such enhancement. However, Mitra et al. did not conduct experiments with the relative humidity below that required for crystallization of their CCN. Twomey and McMaster ( 1955 ) reported their droplets to be well ventilated, initially 1 to 10 #m in radius, and to crystallize around 70% relative humidity. They estimated the number of generated particles to be on the order of several hundred or more, with masses in the range of 10-14 to l0 -18 g. Twomey and McMaster also noted that a single salt particle of mass 10 -1° g could produce several hundreds or more nuclei without undergoing an appreciable reduction in size. The measurements reported herein: ( 1 ) quantify those of Radke and Hobbs ( 1969 ) and show that the temporal variation of the interstitial CCN may be

VARIATIONOF CLOUD CONDENSATION NUCLEI

15

used to obtain information concerning air-mass transport and mixing and (2) suggest a relationship between CCN concentration, aqueous phase chemical composition, and the relative humidity near areas of, or during periods of evaporation. The purpose herein is to provide examples of the former relationship and to point out the likelihood of our results contributing to the latter. SITE D E S C R I P T I O N A N D I N S T R U M E N T A T I O N

The site in the Mt. Mitchell State Park, NC is located on Mt. Gibbs ( ~, 3.2 km southwest of Mt. Mitchell), which is located in the westernmost portion of the state, and consists of a 17.1 m walk up tower equipped with temperature, pressure, wind speed, wind direction and humidity instruments near its top. A carriage, positioned on the tower's northern face, carries the Atmospheric Science Research Center (ASRC) passive teflon string cloudwater collector (e.g. DeFelice and Saxena, 1990) and the Particle Measuring Systems Forward Scattering Spectrometer Probe (FSSP) from the ground to as high as a couple of meters above the top of the tower (see Saxena et al., 1989; DeFelice and Saxena, 1991 for details). An instrument shed at the base of the tower houses the gaseous instruments which are hooked into a sampling manifold that extends to the top of the tree canopy (approximately 12.1 m below the top of the tower). The CCN-Spectrometer (Fukuta and Saxena, 1979a, b) was operated from a tool shed located about 8.0 m to the northeast of the tower. Its inlet is an inverted funnel (0.1 m diameter) that extends 0.5 m out from and whose opening is 2 m above the top of the shed. The opening of this funnel is oriented perpendicular to the airflow past it. This does not allow for the sampling of cloud droplets and the CCN they might contain since the system airflow is not strong enough to overcome their inertia. For example, taking the worse case scenario of an average windspeed of 2 m s- 1and a 1/zm diameter droplet, the time this droplet is in the vicinity of the inlet would be 0.05 s. The velocity into the inlet ~ 0.12 m s -~. Ignoring the terminal velocity, the 1/an droplet will travel 0.1 m distance, in the direction of the flow. This droplet moves vertically toward the inlet by only ~ 0.003 m during this time. Consequently, it is unlikely that any cloud droplets were sampled. Studies such as those by Saxena and Fowler ( 1973 ) and Katz and Mirabel (1975) lead to an instrument, termed the CCN-Spectrometer (Fukuta and Saxena, 1979a, b), that yields a continuous measure of the number of aerosols activated over the entire range of atmospheric supersaturations in any instant, and the modem era (1979-present) of activated aerosol measurement began. This instrument is the first to provide the ability of tackling the problems associated with the role of the physical-chemical characteristics of the aerosols on the formation of clouds on the order of the lifetime of a cloud. Presently, there are three CCN spectrometers reported in the literature (Fukuta and Saxena, 1979a, b; Radke et al., 1981; Hudson, 1989) for measuring

16

T.P. DEFELICE AND V,K. SAXENA

the activation spectrum of cloud condensation nuclei. The dichotomy between CCN and CN (condensation nuclei ) is conventionally made using the capability of the former to form cloud droplets at supersaturations (with respect to water) comparable to those encountered in natural cloud and fog formation. Consequently all spectrometers produce and sustain cloud supersaturations based on the design of the Twomey-Type ( 1963 ) chamber. There are differences in how the supersaturations are produced and how the activated CCN are recorded. Such differences should be considered in determining which spectrometer one should use for achieving desired objectives. The following briefly outlines the differences between the Hudson Spectrometer, and that of Fukuta and Saxena:

Hudson's CCN Spectrometer - The supersaturations are produced in ascending order along the sample flow. The normal operating supersaturation range is 0.01% to 1% and can be narrowed, moved upward or downward by changing the temperatures of the top and bottom plates. - The CCN spectrum is not directly measured, it is instead derived from the final droplet size distribution. The underlying assumption is that the droplet size resulting from the diffusional growth is proportional to the dry particle radius. This will hold good only if the condensation coefficient is constant during the growth process. Experimental evidence (Hagen et al., 1989) shows it otherwise, namely, the condensation coefficient decreasing with time elapsed after the onset of cloud formation. This is a major drawback of the Hudson CCN Spectrometer. - The deduced CCN spectrum is dependent upon the droplet concentration within the cloud chamber due to competition for available water vapor (vapor depletion effect). - The calibration of the recording device depends upon the CCN concentration in the test sample. According to Hudson (1989), "the most accurate data is obtained when the applied calibration has similar concentrations to those encountered" in the test sample. Marine aerosols usually contain CCN active at low supersaturations (less than 0.2%). To obtain reliable measurements for such aerosols, Hudson (1989) suggests that "in the presence of high concentrations of low Sc particles, the spectrometer requires calibration with a bimodal aerosol". - The following are required to calibrate the Hudson CCN Spectrometer: (a) An aerosol of known composition and size (an aerosol atomizer), (b) electrostatic classifier, and (c) a condensation nuclei counter.

The Fukuta-Saxena CCN Spectrometer - Cloud supersaturations are produced perpendicular to the sample flow. The

VARIATIONOF CLOUDCONDENSATIONNUCLEI

17

normal operating supersaturations range is 0.1% to 2% which can be narrowed, moved up or downward as in the Hudson's CCN Spectrometer. - In contrast to the Hudson's device, the CCN spectrum is directly measured. All activated CCN at each supersaturation grow to at least 10/an size through diffusional condensation and are counted photoelectronically. This method of dichotomizing between CCN and CN is very well established (since Twomey, 1963) for CCN counting. The variation in condensation coefficient does not affect the operation of the Fukuta-Saxena CCN-Spectrometer. - The measured CCN spectrum is independent of the droplet concentration within the cloud chamber because the available water vapor for the droplet growth is so adjusted that these do not grow much beyond 1.0/an thus minimizing the competition for available water vapor. - The activated CCN are registered regardless of their concentration in the test sample. The sample flow rate and the moisture supply are adjusted before embarking upon the measurements. None of the items required for calibrating the Hudson's device is needed for measuring the CCN spectrum using the Fukuta-Saxena CCN-Spectrometer. The CCN counting is done by a particle analyzer with a built-in calibration check. -

METHODOLOGY

The CCN, and other, measurements in this study were made during and between cloud episodes, and were periodically calibrated. Table 1 highlights the error involved in making the CCN measurements during the non-steady state portion of evaporation at the end of three events sampled during the summer of 1988, namely, June 24, June 30 and July 22. The significance of measuring CCN in cloudy versus non-cloudy air is conceptualized in Fig. 1a. A non-cloudy air-mass traversing the sampling inlet, subsequently experiences a supersaturation OfSx, resulting in the activation ofn~ CCN. Similarly, TABLE 1 Maximum error, AN, for CCN concentration measurements (at 0.92% supersaturation) during the evaporative stages of the June 24, June 30 and July 22, 1988 events Event June 24 June 30 July 22

N

k

ztS a

AN

(cm -~)

(%)

(cm =3)

(cm -3)

3570 432 180

0.8 0.9 1.9

3.8 3.8 3.8

109 15 13

"Based on Fukuta and Saxena (1979a) error for using linear vapor density profile in the CCN-Spectrometer plus the error due to reading offthe position of the sampling inlet (both at S--0.92%).

18

T.P. D E F E L I C E A N D V.K. S A X E N A

NON-CLOUDY

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Fig. 1. (a). The conceptual difference of measuring cloud condensation nuclei in cloudy versus non-cloudy air. A visualization of the sampling scenario for interstitial CCN during the course of a cloud event. (b) The region between the sudden change from Nm to Nc is the region of the boundary between the different air-masses.

if this air-mass experienced a supersaturation of $2, then n2 particles will become activated as CCN. The relationship between S and n continues as

n = C S k,

(1)

where C and k are the parameters of the distribution. Now consider an airmass in which a cloud forms prior to its passage of the sample inlet for the CCN-Spectrometer. Given that the cloud has just formed, then one CCN for

VARIATIONOF CLOUDCONDENSATIONNUCLEI

19

every cloud droplet may be assumed, and suppose nl cloud droplets were observed. This implies that the critical supersaturation (So) of the nuclei equals $1 (which is termed the effective supersaturation, Seff; e.g. Hudson, 1984). Those particles with critical supersaturations less than S~r will be incorporated into the cloud droplets, those with a critical supersaturation greater than Sefr are known as interstitial CCN. No cloud droplets were sampled during the CCN measurements. Consequently, our in-cloud measurements represent interstitial CCN. Cloudwater samples were obtained on an hourly basis, and each had their pH measured immediately upon collection. They were preserved for later analysis of their ionic composition following EPA protocols. The ionic composition of the cloudwater at our site during our sampling is well documented (e.g. see Saxena et al., 1989; Saxena and Lin, 1990, 1991 ) and is primarily H+, SO7, NO~" and NH~ respectively. The meteorological and microphysical data were continuously measured, and periodically checked to ensure their accuracy. Table 2 contains the record of calibration for the FSSP during the entire field study. TABLE2

Calibration date and method(s) used to perform the calibration of the FSSP. Where the calibration took place is also listed. The maximum diameter is 32/an Date

Method

Location

1988

May 9

Beads, beam ali tmment device (bad) + (Depth of field, DFffi 3.00 mm; Beam Diameter, BDffiO.219 ram) Calibration results using beads (size range 1 )

NCSU Lab

2-4 4-6 6-8 8-10 10-12 12-14 14-16 16-18 18-20 20-22 22-24 24-26 26-28 28-30 303-9/an × 10-15 X

15-25 25-35 June 23 June 29 July 7

× Beads, bad (DF= 1.89 mm; BD--0.234 mm)

Beads

Field Field

Beads, bad ( D F = 1.91 ram; BDffi0.275 mm) NCSU Lab Calibration results using beads (size range 1 ) 2-4 4-6 6-8 8-10 10-12 12-14 14-16 16-18 18-20 20-22 22-24 24-26 26-28 28-30 303-9/zm X 10-15 X 15-25 X 25-35 x

(instrument sat in lab until next field event) July 21, 22 Quick check with beads in the field-Calibration is okay. July 25 Quick check with beads in the NCSU lab-Calibration is still okay. *PMS-Particle Measuring Systems, Colorado; " × " indicates signal peak produced by the particular bead sizes; + Scale Model Builders, Ontario, Can.

20

T.P. DEFELICEAND V.K. SAXENA

Our measurements (physical-including CCN, chemical and meteorological) have collectivelylead to the following conceptualized physical and chemical model of a cloud system (Fig. I b ). A cloud forms in air-mass "m" (cloud free CCN concentration of Nm ) as it encounters mountainous topography moving toward the right (in the figure) yielding an interstitial CCN concentration of Nm. These CCN have a chemical composition of Xm. The sampling continues until the interstitial CCN concentration "suddenly" changes to Arc (N'c is the concentration of air-mass "c" without clouds), Xc. (Note that the value of Xm does not necessarily equal that of Xc. ) The CCN concentration becomes Arein the same time period as the change from Nm to Nc, or quicker, as the cloud dissipates. In addition, the chemical composition of the CCN at Areremains ~ Xc. Nonetheless, the magnitude of the Arcto ATechange is usually greater than that for Nm to Nc. The region between the sudden change from Nm to Arc is the region of the boundary between the different air-masses and N~ is due to evaporation. RESULTS AND DISCUSSION

(a) Measured CCN spectra The CCN concentration (S= 0.92%) measurements (including those made during cloudy periods) ranged between < 100 and 3600 cm -3, and are consistent with other studies (e.g. Hobbs et al., 1985). The <_100 cm -3 CCN concentrations were also measured in the Olympic Mountains (e.g. Radke and Hobbs, 1969) and have been observed to follow widespread rain (Twomey, 1959). CCN concentrations <200 cm -3 (e.g. Jiusto, 1967; Twomey and Wojciechowski, 1969) are typical of maritime air-masses. Unpolluted continental air-mass concentrations are usually between 200 and 2000 cm-3, while polluted continental air-masses have concentrations above 2000 cm -3 (e.g. Twomey, 1959, 1977; Twomey and Wojciechowski, 1969 ). Figure 2 shows the representative CCN spectra before and after three cloud events during the 1988 field season, namely June 24 (a), June 30 (b), July 22 (c). The June 30 event is not likely accompanied by a change in air-mass, while the remaining two events (namely, June 24 and July 22) are. The analysis of the temporal interstitial CCN data of the cloud events listed in Fig. 2 indicates three noteworthy results: ( 1 ) the conceptual model shown in Fig. lb (with concurrent physical and chemical measurements). (2) the existence of a threshold magnitude of the time rate of change in the total interstitial CCN concentration to be 200 cm-3 h-~, associated with sampling near the boundary of different air-masses (i.e., that associated with the passage of a surface front or upper level disturbance) or air-parcels and evaporation. Temporal changes in total interstitial CCN concentration less than the threshold are associated with synoptic scale or greater processes. And (3) an

VARIATION OF CLOUD CONDENSATION NUCLEI

21

apparent relationship between the CCN concentration, relative humidity, and chemical composition of droplets during periods of evaporation. These resuits will be discussed in conjunction with the following well documented events, namely, June 24-25, June 30 and July 22. Case studies I: June 24-25, 1988 The June 24-25, 1988 event began at 1630 EST with thunderstorms that delayed sampling until 1926 EST and continued for another 9.5 h as a surface high (that extended itself into the Great Lakes, the mid-west and the midAtlantic states 12 h prior to this event) became oriented southwest to northeast by 0700 EST June 25 due to an eastward moving southern Canadian frontal system. The 85 kPa level, the closest standard vertical level to our site ( ~ 81 kPa), shows the passage of an upper level disturbance around 0300 EST. Figure 3 shows that the 85 kPa 24-48 h back trajectories [ending 0700 EST June 24 (A) and 0700 EST June 25 (B)] shifted from WNW to NW flow during the course of this event. Figure 4a shows the temporal variation of the total droplet concentration, Nd, and the total (in-cloud or non-cloudy) cloud condensation nuclei concentration (based on a supersaturation of 0.92%). The Nd varied between 75 and 500 cm -3, while the total CCN concentration ranged between 60 and 3570 cm -3. A period of light drizzle was observed between 0103-0203 EST and explains the latter peak in the droplet concentration. The total droplet and interstitial CCN concentration values show a significant change after the 2 hour period ending 0203 EST compared to before it, excluding the likelihood of evaporation during the last part of the event. This change is also evident in the concurrent site wind direction measurements (top of Fig. 4a). The wind direction begins to shift westward toward north around 0100 EST. The 0.25 h averaged pressure dropped by 0.1 kPa (P) between 0145 and 0200 EST. This is despite a tendency for the site pressure to rise slightly during this period. The segment of the interstitial CCN curve between the open boxes (0218-0309 EST) represents a time rate of change in the magnitude of the CCN concentration of 208 cm -3 h-~. The temporal CCN measurements reported by Radke and Hobbs ( 1969 ) indicate an unquantified change in CCN concentrations under dynamically similar meteorological situations. The open oval denotes the onset of evaporation. The 0.25 h averaged pressure rose by 0.1 kPa (circle P) between 0645 and 0700 EST. Figure 4b shows the primary ions, excluding H +, present in the hourly collected cloudwater. The ionic composition of the cloudwater at our site during our sampling campaigns is well documented (e.g. see Saxena et al., 1989; Saxena and Lin, 1990, 1991 ) and is primarily H +, SO~, NO~- and NH~ respectively. The early event maximum, especially in the case of sulfate, may be the result of additional sulfate

22

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VARIATION OF CLOUD CONDENSATION NUCLEI

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Supersaturation (%) Fig. 2. Representative CCN spectra before and after the sampling of three cloud events in 1988, namely June 24 (a), June 30 (b), July 22 (c).

Fig. 3. The National Weather Service 850 mb map for 0700 EST June 25. The 24-48 h 850 mb back trajectories ending 0700 EST June 24, (A), and 0700 EST June 25, (B), are shown by the arrows.

24

T.P. DEFELICE A N D V.K. SAXENA

sw

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Modal Radius (~tm)

VARIATIONOF CLOUDCONDENSATIONNUCLEI

25

production caused by the scavenging of the 100 ppb ozone concentrations present at the beginning of this event (DeFelice and Saxena, 1991 ). The minimum in the concentration of these ions is likely due to the presence of precipitation, and the subsequent rebound could be indicative of a new source of CCN. Evaporation may also have enhanced this rebound. The relative humidity at the end of this event is estimated to be 59% (based on NWS 12Z June 25, 1988 weather charts). Note too that the composition of the primary cloudwater ions reversed order with respect to their abundances after the precipitation period. The comparison of Fig. 4a, b shows some misalignment among the presented parameters. This is due to the different time scales over which the data portrayed in these figures were obtained. Figure 4c shows the dN (dLog r) - ~ versus r droplet spectra obtained during each of the periods denoted by A, B', B, B (period of drizzle ), C, D in Fig. 4a separated into noncloudy and precipitating (i.e., the B's) spectra. The droplet spectra before and after the period of precipitation appear to differ slightly. In particular, the spectra at A (which is representative of the period around 2118 EST) implies the existence of a bimodal cloud with a greater number of droplets than the other two cloud only spectra obtained after the period of precipitation. This "bimodalness" could be due to entrainment of relatively cloud free air from the local canopy, or to the arrival of the early fringes of a new cloud cell. A visual observation made by one of the authors (TPD) revealed that the sampling platform was in and out of cloud (the period out of cloud was less than the period in-cloud and did not exceed 8 minutes at any one time) between 2100 and 2130 EST. The 2118 EST measured CCN spectra had two different C and k parameter sets. Saxena (1980) suggested that they might be due to making measurements in polluted air, or to sampling of non-steady state circumstances (e.g. at the boundary of two different air-masses, evaporation). Hudson ( 1984 ) successfully used the effective supersaturation to derive information on the effect of dry air entrainment on the droplet spectra. The temporal variation of the effective supersaturation, Self, is shown in Fig. 5a with the corresponding CCN spectra given in Fig. 5b, and droplet spectra in Fig. 4. The effective supersaturation was obtained based on Hudson (1984). Figure 5a shows the sampled cloud to essentially have the same Selfthrough Fig. 4. (a) The temporal variations of cloud droplet concentration, Nd, and the cloud condensation nuclei (CCN) concentration, and (b) the predominant cloudwatcr ions, excluding H +, namely, SO~, NO~" and NH + , during the June 24-25, 1988 event. The open boxes indicate the period during which the magnitude of the time rate of change of the CCN concentration is > 200 c m - 3 h - ~. The oval represents the same as the open squares, except where evaporation is suspected. The P denotes a 0.1 kPa fall in the 0.25 h average atmospheric pressure. The circled P denotes a 0.1 kPa rise in the 0.25 h average atmospheric pressure. (c). The dN (dLog r) -~ versus r droplet spectra obtained during each of the periods denoted by the upper case A - D letters in (a) grouped with respect to cloud only and precipitation and cloud (i.e., the B's).

26

T.P. DEFELICEAND V.K. SAXENA

(a) LEGEND

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!

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0.80

S u p e r s a t u r a t i o n (%) Fig. 5. ( a ) T h e t e m p o r a l v a r i a t i o n o f the effective s u p e r s a t u r a t i o n a n d ( b ) the c o r r e s p o n d i n g C C N s p e c t r a a s s o c i a t e d w i t h the J u n e 2 4 - 2 5 episode. T h e effective s u p e r s a t u r a t i o n w a s determ i n e d a c c o r d i n g to H u d s o n ( 1 9 8 4 ) .

2330 EST. This suggests that the composition of the interstitial CCN sampled (Twomey, 1977), or the dynamics associated with this system, during this period remained unchanged, and that sampling was conducted within the same air-mass. This is expected according to the sampling scenario illustrated in Fig. lb. The dashed line in Fig. 5a represents the period of thunderstorms. The Seff increases to ~0.76% as the cloud becomes bimodal, (i.e. ] in Fig. 5a), and reaches to a maximum shortly after the onset of the mature cloud period, The subsequent decrease may be due to the precipitation. By the end of the open boxed segment (namely, the end of the >200 CCN cm -3 h -~ sector in Fig. 4a), i.e., 0308 EST, the effective supersaturation is 0.63%. This value is significantly lower than the initial value of ~ 0.74%. Shortly hereafter

27

VARIATION OF C L O U D C O N D E N S A T I O N N U C L E I

10 3

•

-

•

r

•

"-'-D'--

-"--O-'-

~- 10 2

-

•

i

"

"

-

i

"

"

"

i

•

Before (S=0.92%) After (S=0.92%) Before (S=.35%) After (S=.35%)

ea C

Z•~

lo ~

/"

I0 °

10" I

19

-

21

22

23

00

01

02

0"4

05

Time (hr, Est)

Fig. 6. The temporal variation of the number of interstitial CCN (active at 0.92% and at 0.35% ) was normalized to the respective values of N given by the representative spectra (Fig. 2 ) before and after the June 24 event.

the cloud dissipates (i.e. 4 in Fig. 5a) producing a large number of particles whose critical radius is relatively larger corresponding to an Seff of ~ 0.02%, and rebounds to 0.4% as a cloud remnant passed by before yielding to the non-cloudy sampling period. The values of Safjust at the end of the event are generally lower than that at its start. The temporal variation of the effective supersaturation supports the indications of the other data for this event. The number of CCN measured at the end of the event (i.e. 0437 EST) must be addressed as to whether it is physically meaningful, or not. There is a possibility that it is due to a random or systematic error. The CCN concentration, N, was 3570 cm -3 at S=0.92%. The maximum error in Nis given (assuming N is defined as N = C S k, with C and k constants of the distribution) by: IAN I = N{ Ik (AS S - ~) I}, where I I denotes the magnitude of the quantity that is within it, and AS is defined as the difference between the respective measured quantity and its most likely value (ideally its "true" value). The maximum error in N, IANI, is 109 c m - 3 (Table 1 ). Even if I~NI = 1785 c m - 3 , the interstitial value of N = 430 cm-3, a droplet concentration Nd= 220 cm-3 and 1 CCN per evaporating droplet, then one would expect to measure a maximum CCN concentration (at S = 0.92%) of ~ 2400 c m - 3 . This still does not account for the ~ 1000 cm-3 of the measured CCN, and suggests a consistency with the results of Twomey and McMaster (1955). The value of

28

T.P. DEFELICEAND V.K.SAXENA

N = 3 5 7 0 cm -3 is consequently believed to be significant and physically meaningful. Figure 6 shows the number of interstitial CCN, N, active at two supersaturations (S=0.92% and at S=0.35% ; the latter supersaturation picked essentially at random and to ensure a size dichotomy) normalized to the respective value of N given by the representative spectra (Fig. 2a-c) before and after this event. Such a ratio normalized relative to the representative spectra before the event that is greater than unity, or very much smaller

600

WNW

'W

'

"

W~W ' W

'

B

'

'

WNW '

~ a)

C

A' 4o0

•"~

CCN

z k~

.° V"

200

0 300

P

I 400

I

J

I

500

I

I

600

700

Time

"

,

N d

i

1

800

900

1000

lEST)

' 1200

,~.._..------

'

'

(b)

>200

CCN/ccth

SUiFATE

1000 "6

E

800

A M M O ~ I o ~ o

==

600 400

200

0

400

i

NITRA

i

I

500

L

I

600

I

700

800

29

VARIATIONOF CLOUD CONDENSATION NUCLEI

10 4

10 3

m t_

10 2

0

"¢~

101

10 °

3.5

4.5

$.5

6.5

7.5

8.5 9.$ 10.5 11.$ Modal Radius (~tm)

12.$

13.5

14.5

15.$

Fig. 7. Same as Fig. 4, except d u r i n g the J u n e 30, 1988 episode. T h e circled p denotes a 0.05 k P a rise in the 0.25 h average a t m o s p h e r i c pressure.

than unity, would suggest a change in air-mass, and perhaps particle generation in the greater than unity case. The additional consideration of the ratio normalized with respect to the representative spectra after the event would further the likelihood that the measured value during evaporation was the result of particle generation, especially if it were also greater than unity. Figure 6 shows the particles with critical supersaturations at 0.35% to be enhanced by a factor of 600_+2% (200_+2%) times those before (after) this event. Note that there are more S = 0.35% sized particles after the event compared to before it. Those particles with critical supersaturations at S = 0.92% are seen to be enhanced by a factor of 11 (9) times those before (after) this event. In summary, Figs. 2a, 3-6 all suggest that there was a change in air-mass to a more continental one. This new air-mass consisted of particles with a slightly different chemical composition than that prior to the event. The temporal changes in the physical and chemical nature of this event support the conceptual model shown in Fig. lb. They also suggest that a physically significant enhancement of the CCN concentration occurred during the evaporative stages of this event. II: June 30, 1988 Event Episode II began at 0403 EST and was accompanied by light to moderate precipitation until just prior to its end at 0745 EST. The event ended at 0815 EST. A weak cold front associated with an occluded Canadian low pushed slowly southeastward and passed through the site at approximately 0610 EST.

30

T.P. DEFELICE AND V.IC SAXENA

Meanwhile a low was also forming along the southern fringe of this cold front and was positioned over northeastern Georgia by 0700 EST. The 85 kPa chart, although saturated in the vicinity of our site and northward, showed no major synoptic feature in our vicinity 9 h prior to this event, except for a low north of Montreal. This low moved southward to the northern tip of New York State by 0700 EST. The 85 kPa wind field (including the 24-48 h back trajectory ending 0700 EST June 30) showed very little change during this period and was primarily westnorthwesterly. The 0403 EST event start was most likely due to the lifting of the saturated 85 kPa level air as the surface front neared the site. It is also likely that pressure perturbations moved across the site as this front approached. The temporal Nd and total CCN concentration (within and without clouds) measurements (S=0.92%) are shown in Fig. 7a. The Nd varied between 150 and 376 cm -3. The total droplet concentration has a set of minima (~200 drops c m- 3 ) beginning about 2 h after the start of the event. The initial and second (0615 EST) minimum is coincident with a 0.25 h average change in wind direction from west northwest (WNW) to west (W), a concurrent decrease in relative humidity from 100% to 82%, and a 0.25 h pressure drop of 0.05 kPa for the initial minimum and 0. I kPa for the second minimum (0615 EST). All indicative of the frontal approach and passage noted above. The interstitial CCN concentration ranged between 415 and 574 cm -3, with a maximum at 0613 EST. The value of the k parameter, 2. l, at 0613 EST suggests that dynamics is influencing the cloud droplet spectra (e.g. evaporation, precipitation), or that inhomogeneous mixing (i.e., that near a frontal boundary) is occurring. The open boxes (located between 0700 and 0730 EST) denote a time rate of change in the magnitude of the interstitial CCN concentration of 325 cm -3 h-1. The effect of evaporation at the end of this event (open oval) is not nearly as strong as the previous case, perhaps due to the low that was forming over northeastern Georgia at 0700 EST, and the resulting flow of moisture associated with it. The relative humidity during the evaporative stage of this event was ~ 95%. A second cloud event began at 0945 EST. In addition, note that the CCN concentration is about twice that of Nd which could possibly be the result of an advection of surface CCN with some enhancement due to evaporation, and/or the production of CCN. This event relieved a > 100 ppb 03 episode that began at 1800 EST on June 29. Figure 7b shows the temporal variations of the dominant ions, excluding H +, found in the hourly cloudwater collections. Each of the ionic species show a gradual decrease in concentration throughout the event. The final concentrations are approximately half of the respective values from the June 24 episode. Figure 7c shows the dN (dLog r)-1 versus r droplet spectra obtained during each of the periods denoted by A', A, B, C, D in Fig. 7a. The droplet spectra before and after the concurrent period of the pressure drop and the shift in wind direction, namely ~0530 and ~0630 EST, showed a higher

VARIATION

OF CLOUD

CONDENSATION

NUCLEI

31

(a) 0.6

!

.

!

g= o .m

LEGEND (1) (2)

0A o~ gl.

(3) (4)

(4)

0.2

(3)

0.0

Precipitation began Beginning of open square sector (Fig. 7a] Precipitation ended Non cloudy air

!

'

0200

0600 T i m e (EST)

0400

0800

10 3 '

'

'

(bi -,-'-'41

i

S B

°/°'°

./;~ ..." "~

10 2

.'

I

t" I

I= O

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/

/

/

I

..... '~ ....

0608 EST

..... • .....

01555EST

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0812 EST

t

101 0.20

6 i

I

i

0.40

I

w

0.60

Supersaturation

I

0.80 (%)

Fig. 8. S a m e as Fig. 5, e x c e p t d u r i n g t h e J u n e 30 e v e n t .

minimum detectable radius and more of the larger drops before (A) compared to that afterwards (B). The spectra at C, representative of the open boxed segment in Fig. 7a, was very similar to that at A. The last spectra, D, at 0735 EST, had no droplets above the modal radius of 9.5/zm in concurrence with the observation that the rain had ended at this time. As in the June 24 case, the temporal variation of Seff (Fig. 8a) was obtained. This figure suggests that the same aerosol type and composition was sampled throughout this event. There appears to be a relative minimum in association with the passage of the weak surface frontal boundary, and a wind shift from west to west northwest. The normalized comparison of the number of CCN mea-

32

T.P.

DEFELICE

AND

V.K.

SAXENA

sured at S = 0.92% and S = 0.35% to the respective before and after representative values is shown in Fig. 9. This figure shows little to no enhancement of N at S = 0.92% and N at S = 0.35% with respect to before or after this event. The maximum at S = 0.35% is unexplained, but it does coincide with the onset of the open box segment shown in Fig. 7a. The minima in this figure occurs for all curves at 15 minutes before the precipitation period ended. In summary, Figs. 2b and 7-9, and the general synoptic picture suggest no change in air-mass has occurred in association with this event. They (especially Fig. 9 ) fail to indicate a significant enhancement of the CCN during evaporation. This confirms the notion that the June 24 value is not likely due to a significant systematic error. It is also noted that ( 1 ) the relative humidity during evaporation associated with the June 30, 1988 event was ~. 95% and (2) the chemistry of the evaporating droplets was slightly different (approximately one half of the primary ion concentrations) than the June 24 episode. There was no reversal or rebound once the event trigger passed through. These are consistent with the results of Twomey and McMaster ( 1955 ), and others, who have not found any particle generation under these conditions. The resuits from this case also support that of the conceptual model proposed in Fig. lb.

III: July 22, 1988 Event This event, began at 0705 EST and ended at 0930 EST, and was not accompanied by any precipitation. The surface charts indicate a very weak surface 1.5

•

.

•

/

i

•

~

/

\

.

!

~-

After (S=0.92%)

--

Before(s=o35~)

e-

1.0 gg

z z

0.5 06

i

,

,

I

t

iw

-

O7 Time lhr, Est)

Fig. 9. Same as Fig. 6, except associated with the June 30 case.

'

08

VARIATIONOF CLOUDCONDENSATIONNUCLEI

33

500

WSW

WNW

SSE $SW

W

(a)

400 B d-

300

-

Nd

•

e~ 0 I--

200

= ¢,0

e. @

100

|

I

t

0

401

I

I

I

600

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800

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I

I

I000

I

1200

,

I

1400

1600

Time (EST)

101

10 o

1.s

z.s

3.s

4.s s.s 6.5 Modal Radius (~tm)

7.S

s.s

Fig. 10. (a) The temporal variations of cloud droplet concentration, Na, and the cloud condensation nuclei (CCN) concentration during the July 22, 1988 event. The circled p denotes a 0.05 kPa rise in the 0.25 h average atmospheric pressure. The open boxes indicate the period during which the magnitude of the time rate of change of the CCN concentration is > 200 cm -3 h-1. The oval represents the same as the open squares, except where evaporation is suspected. The event was too short to show the temporal cloudwater chemistry. (b) The dN (dLog r) -1 versus r droplet spectra obtained during each of the periods denoted by the A - D letters in (a) grouped with respect to cloud only, and precipitation and cloud (c).

34

T.P. DEFELICE AND V.IC SAXENA

"frontal" (i.e., a small dewpoint difference and no pressure gradient across it) passage around 0900 EST. The 0700 EST 85 kPa chart is generally nondescript; however, there are two noteworthy features: ( I ) the air below the site is saturated and (2) the 85 kPa 24-28 h back trajectory ending at 0700 EST shows a west southwesterly flow. Figure 10a shows the total droplet concentration to vary between 88 and 336 cm -3, and the total interstitial CCN concentration to range between 78 and 200 cm-3 during this episode. The event did not last long enough for a comparison of the temporal cloudwater ions. The greatest magnitude of the ta)

1.0 I tO om

~

"

0.8

(2) LEGEND

i

t-

Lgh "S

0.6

(1)

O p e n boxed s e g m e n t

(2)

EST) Non cloudy air

Fig. 10a (0845-0859 0.4

overhead 0.2

(I)

0.0 0700

'

-

'

0800

0900

Time

(EST)

(b) 103

E

l0 2

/ fK "/ W¢

.2

t-

....,

~

--"'"'Y:~':':....

t

10 1

10 0 0.50

~

I 0.60

~

I 0.70

Supersaturation

1 0.80 (%)

i

|

0.90

---i.--.

0635 EST

..... • - ....

0700 EST

. . . . O- - -

0903 EST

. . . . . • .....

0933 EST

---m--

1000 EST

35

VARIATIONOF CLOUDCONDENSATIONNUCLEI

(c)

2.0

...... ~ " ~----

B e f . (S~".92%) After (S=0.92%) Before(S=:0.35%)

"

'

//~ /

~

J ]

e °m 4-, ee', Z

1.0 I

7.

0.0

!

07

08

09

Time (hr, Est) Fig. 11. (a) Sameas Fig. 5a, (b) same as Fig. 5b, (c). Sameas Fig. 6, except duringthe July 22 case.

time rate of change of CCN concentration ( 304 c m - 3 h - ~) occurred between 0845 and 0859 EST implying that the boundary of the frontal inversion had reached the site. The local wind and pressure (see circled p in Fig. 10a) fields, and the CCN spectra measured during this period support the notion of sampiing near the boundary of two air-masses. By the end of the event the interstitial CCN were characteristic of the air that was below the observed preevent valley inversion. This is consistent with the observation of the inversion rising throughout the event, and that the base of the associated cloud was overhead by 0930 EST. Figure 10b shows little difference in the droplet spectra measured before and after the pressure increase near 0800 EST. The critical supersaturation near the beginning of the event was 0.90% and changed to 0.72% by 0903 EST, suggesting a change in the measured aerosol composition to that of a slightly more soluble aerosol, or to an air-mass with a greater number of larger CCN (e.g. Twomey, 1977). The temporal effective supersaturation is given in Fig. 11 a and the respective CCN activity spectra measured within and without cloud appears in Fig. 1 lb. The Seer decreases throughout the event from its maximum of ,~ 0.97% at its beginning. The overall variation of Sefr suggests a slight change in cloud dynamics, aerosol composition or in the number of particles with similar chemical composition whose critical supersaturation is Self. The number of CCN active at S = 0.92%

36

T.P. DEFELICEAND V.ICSAXENA

and at S = 0.35% were normalized as in the previous cases and are presented in Fig. 11c. This figure does illustrate a de-enhancement by a factor of 0.1 _+6% (0.03 __+6%) of those particles whose critical supersaturation equals S=0.35% compared to before (after) the event. In contrast, those with critical supersaturations of S = 0.92% show an enhancement by a factor of 2.4 (1.0) compared to before (after) this event. This supports a change to an air-mass with less relatively large particles and more relatively small particles. Similarly, it is interesting to note that, nuclei with critical supersaturations of 0.65% show an enhancement by a factor of 3.1 ( 1.0) compared to before (after) this event. The relative humidity during this period of evaporation was ~ 91%, and the evaporating water was chemically different (primarily less sulfate) than the other two events. CONCLUDING REMARKS AND RECOMMENDATION

The CCN, and other, measurements were made during and between three well documented cloud episodes (namely June 24-25, 1988, June 30, 1988 and July 22, 1988). The in-cloud CCN measurements did not include cloud droplets. The other measurements included standard meteorological, microphysical and aqueous-phase chemical quantities. The combined meteorological, microphysical and chemical data exemplified by the aforementioned events fit a conceptualized temporal pattern. Briefly, a sampled air-mass has a characteristic meteorology, microphysical and chemical (all phases) structure that persists until a physical or chemical change. The change (especially physical, e.g. a change in air-mass ) is detectable in each of the three aforementioned measured components. Subsequent measurements approach the characteristic meteorology, microphysical and chemical (all phases) structure of the "new" air. Measurements at the end of the aqueous phase sampling will not only indicate the obvious physical change, but they may also detect a change in the microphysical structure (i.e., primarily that of the CCN spectra) within this region depending on the meteorology and the chemistry of the cloud and "its" air-mass (Fig. l b ). Generally, the CCN concentration ( S = 0.92%) measurements (including those made during cloudy periods) ranged between < 100 and 3600 cm -3, and are consistent with other studies (e,g. Hobbs et al., 1985). The < 100 cm -3 CCN concentrations were also measured in the Olympic Mountains (e.g. Radke and Hobbs, 1969 ) and have been observed to follow widespread rain (Twomey, 1959). CCN concentrations <200 cm -3 (e.g. Jiusto, 1967; Twomey and Wojciechowski, 1969) are typical of maritime air-masses. Unpolluted continental air-mass concentrations are usually between 200 and 2000 c m - ~, while polluted continental air-masses have concentrations above 2000 cm -3 (e.g. Twomey, 1959, 1977; Twomey and Wojeiechowski, 1969).

VARIATION OF CLOUD CONDENSATION NUCLEI

37

The following highlights the meteorological, microphysical and chemical discussion of the three events. Figures 2a, 3-6 all suggest that there was a change in air-mass to a more continental one during the June 24-25, 1988 event. This new air-mass consisted of particles with a slightly different chemical composition than that prior to the event. The composition of the primary cloudwater ions (namely, H +, SO7, NO~- and NH~ respectively) reversed order with respect to their abundances after the precipitation period. The temporal changes in the physical and chemical nature of this event suggest that a physically significant enhancement of the CCN concentration (compared to the representative noncloud CCN spectra obtained both before and after this event) occurred during the evaporative stages of this event. The relative humidity at the end of this event is estimated to be 59% (based on NWS 12Z June 25, 1988 weather charts). Figures 2b and 7-9, and the general synoptic picture suggest no change in air-mass occurred in association with the June 30, 1988 event. The "boundary" of the event trigger passed through between 0700 and 0730 EST as denoted by a time rate of change in the magnitude of the interstitial CCN concentration of 325 cm -3 h - 1. Each of the primary ionic species show a gradual decrease in concentration throughout the event. Their final concentrations are approximately half of the respective values from the June 24 episode. There was no reversal or rebound in the chemistry of the cloudwater once the event trigger passed through. The effect of evaporation at the end of this event was not nearly as strong as in the June 24 case. There was little to no enhancement of CCN (with respect to the non-cloud representative spectra for before and after this event) during the evaporative stage of this cloud event. The relative humidity during the evaporative stage of this event was ~ 95%. The surface charts indicate a very weak surface frontal (i.e., a small dewpoint difference and no pressure gradient cross it) passage around 0900 EST associated with the July 22, 1988 event. The greatest magnitude of the time rate of change of CCN concentration ( 304 cm- 3 h - 1) occurred between 0845 and 0859 EST implying that the boundary of the frontal inversion had reached the site. The data does indicate a weak notion of "large" particle generation (compared to the representative non-cloud CCN spectra before and less so after the event) during evaporation. The relative humidity during the period of evaporation was ~ 91%, and the evaporation water was chemically different (primarily less sulfate) than the other two events. The analysis of the temporal interstitial CCN data of these three well documented events may be summarized into three noteworthy statements: ( 1 ) Concurrent physical and chemical characteristics of a cloud system follow the conceptual physical and chemical model shown in Fig. lb. (2) There is a threshold magnitude of the time rate of change in the total interstitial CCN concentration to be 200 cm-3 h - ~ (0.92% supersaturation),

38

T.P. DEFELICEANDV.K.SAXENA

associated with sampling near the boundary of different air-masses (i.e., that associated with the passage of a surface front or upper level disturbance) or air-parcels, and evaporation. Temporal changes in total interstitial CCN concentration less than the threshold are associated with synoptic scale or greater processes. This is a first-quantification (based on our dataset) of the observations of Radke and Hobbs (1969). (3) The number of CCN measured in association with areas of evaporation of cloud systems seems to be related to the rate of evaporation (i.e., relative humidity), and chemistry of the air-mass. Twomey and McMaster (1955 ), and others, observed particle generation when the relative humidity of the environment was below that required for the crystallization of the particular salt that emerged from the evaporating droplets. Further research using CCN measurements in conjunction with microphysical, chemical and meteorological processes is highly recommended. ACKNOWLEDGEMENTS

This study was funded through the Southeast Regional Center of the National Institute for Global Environmental Change by the U.S. Department of Energy under Cooperative Agreement No. DE-FC03-90ER61010 and by the University of Wisconsin-Milwaukee Graduate School and the College of Letters and Science (especially, Drs. W. Halloran and R. Hall ). The authors wish to thank Drs. N. Fukuta, D. Hegg and L. Radke for their helpful comments. The inciteful comments by the two anonymous reviewers helped us improve the manuscript considerably.

REFERENCES Charlson, R.J., Lovelock, J.E., Andreae, M.O. and Warren, G.S., 1987. Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature, 326:655-661. DeFelice, T.P. and Saxena, V.K., 1990. Mechanisms for the operation of three cloudwater collectors, comparison of mountain-top results. Atmos. Res., 25: 277-292. DeFelice, T.P. and Saxena, V.K., 1991. Characterization of extreme episodes of wet and dry deposition of air pollutants deposition on an above cloudbase forest during its growing season. J. Appl. Meteorol., 30: 1548-1561. Fukuta, N. and Saxena, V.K., 1979a. A horizontal thermal gradient cloud condensation nucleus spectrometer. J. Appl. Meteorol., 18:1352-1362. Fukuta, N. and Saxena, V.K., 1979b. The principle of a new horizontal thermal gradient cloud condensation nucleus spectrometer. J. Rech. Atmos. (Atmos. Res. ), 13:169-188. Ghan, S.J., Taylor, K.E., Penner, J.E. and Erickson III, D.J., 1990. Model test of CCN-cloud albedo climate forcing. Geophys. Res. Lett., 17: 607-610. Hagen, D.E., Schmidt, J., Trueblood, M., Carstens, J., White, D.R. and Alofs, D.J., 1989. Condensation coefficient for water in the UMR simulation chamber. J. Atmos Sci., 46:803-816. Hegg, D.A., Radke, L.F. and Hobbs, P.V., 1990. Particle production associated with marine clouds. J. Geophys. Res., 95D: 13,917-13,926.

VARIATIONOF CLOUDCONDENSATIONNUCLEI

39

Hobbs, P.V., Bowdle, D.A. and Radke, L.F., 1985. Particles in the lower troposphere over the High Plains of the United States. Part II: cloud condensation nuclei. J. Clim. Appl. Meteorol., 24: 1344-1356. Hudson, J.G., 1984. Cloud condensation nuclei measurements within clouds. J. Clim. Appl. Meteorol., 23:42-51. Hudson, J.G., 1989. An instantaneous CCN spectrometer. J. Atmos. Oceanic Technol., 6: 10551065. Jiusto, J.E., 1967. Aerosol and cloud physics measurements in Hawaii. Tellus, 19: 359-367. Katz, J.L. and Mirabel, P., 1975. Calculation of supersaturation profiles in thermal diffusion cloud chambers. J. Atmos. Sci., 32: 646-652. Lodge, J.P. and Baer, F., 1954. An experimental investigation of the shatter of salt particles on crystallization. J. Meteorol., 11: 420-421. Mitra, S.K., Brinkmann, J. and Pruppacher, H.R., 1992. A wind tunnel study on the drop-toparticle conversion. J. Aerosol Sci., 23: 245-256. Radke, L.F. and Hegg, D., 1972. The shattering of saline droplets upon crystallization. J. Rech. Atmos. (Atmos. Res.), 6: 447-455. Radke, L.F. and Hobbs, P.V., 1969. Measurement of cloud condensation nuclei, light scattering coefficient, sodium containing particles, and aitken nuclei in the Olympic Mountains of Washington. J. Atmos. Sci., 26: 281-288. Radke, L.F. and Hobbs, P.V., 1991. Humidity and particle fields around some small cumulus clouds. J. Atmos. Sci., 48:1190-1195. Radk¢, L.F., Domonkos, S.V. and Hobbs, P.V., 1981. A cloud condensation nucleus spectrometer designed for airborne measurements. J. Rech. Atmos. (Atmos. Res.), 15: 225-229. Saxena, V.K., 1980. Some wintertime cloud aerosol interactions over Lake Michigan. J. Rech. Atmos. (Atmos. Res.), 14: 255-265. Saxena, V.K., 1991. Climate forcing due to non-sea-salt sulfates. In: V.M. Kotlyakov, A. Ushakov and A. Glazovsky (Editors), Proc. International Symposium on Glaciers-Ocean-Atmosphere Interactions. IAHS Publ. 208, Int. Assoc. Hydrol. Sci. Press, Oxfordshire, U.K., pp. 243-255. Saxena, V.K. and Fowler, J.L., 1973. Experimental investigations of transient supersaturations in a thermal diffusion chamber. J. Appl. Meteorol., 12: 984-990. Saxena, V.K. and Lin, N-H., 1990. Cloud chemistry measurements and estimates of acidic deposition on an above cloudbase coniferous forest. Atmos. Environ., 24: 329-352. Saxena, V.K., Stogner, R.E., Hendler, A.H., DeFelice, T.P., Yeh, R.J.-Y. and Lin, N.-H., 1989. Monitoring the chemical climate of the Mt. Mitchell State Park for evaluating its impact on forest decline. Tellus, 41B: 92-109. Twomey, S., 1959. The nuclei of natural cloud formation. Part I: The chemical diffusion method and its application to atmospheric nuclei. Geofis. Pura Appl., 43: 227-242. Twomey, S., 1963. Measurements of natural cloud nuclei. J. Rechs. Atmos. (Atmos. Res.), 1: 101-105. Twomey, S., 1977. Atmospheric Aerosols. Elsevier, New York, 302 pp. Twomey, S. and McMaster, K.N., 1955. The production of condensation nuclei by crystallizing salt particles. Tellus, 7:458-461. Twomey, S. and Wojciechowski, T.A., 1969. Observations of the geographical variation of cloud nuclei. J. Atmos. Sci., 26: 684-688.