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Optical detection of metastable ambient aerosols
J. Aerosol Sci., Vol. 17, No. 3. p 304, 1986.
0021 8502/86$3.00+0.00 Pergamon Journals Ltd.
Printed in Great Britain.
OPTICAL DETECTION OF METASTABLE AMBIENT AEROSOLS* MARKJ.
R O O D "t,
T. V.
LARSON,
D. S. COVERTand A. P. WAGGONER
Department of Civil Engineering, University of Washington, Seattle, WA 98195, U.S.A.
The ubiquitous presence of liquid H20 in atmospheric aerosol particles at relative humidities (RH) less than 100~ has many important consequences, including effects on radiative transfer properties and atmospheric chemistry. We have focused our attention on metastable droplets, those that exist as droplets even though the surrounding RH is below the bulk saturation humidity of the solute. Our measurement technique involves the use of an integrating nephelometer operating as close to ambient conditions as possible. Upstream of this sensor, the aerosol is heated to a maximum temperature of 30°C above ambient temperature and then rapidly cooled to ambient temperature prior to entering the nephelometer. A rapid decrease in the sub-micron aerosol particle light scattering extinction coefficient (asp) over a small temperature difference of approximately 5°C in the upstream heater is attributed to the presence of metastable droplets. Measurements were performed during the months of August and September 1983 in Riverside, CA and approximately one year later at remote sites in Grand Canyon, AZ and the Mojave Desert, CA. Approximately 200 tests were performed at each site. In the Riverside experiment, metastable droplets were present 60 ~o of the time, and the RH ranged from 26 to 86 ~o during the entire test period. In the Grand Canyon and Mojave Desert, metastable droplets were present 36 and 12 ~o of the time, respectively, but the RH values were systematically lower than in Riverside (ranged from 12 to 70 ~o and 10 to 78 ~o, respectively). The percent asp attributed to metastable liquid H20 averaged 8, 14 and 6 ~o respectively for Riverside, Grand Canyon and the Mojave Desert. Overall, metastable droplets were detected between 30 and 78 ~o RH but existed most frequently between 50 and 76 ~o RH.
* This paper was also presented during the 1986 American Association for Aerosol Research Conference in New lpexico, U.S.A. resent address: International Meteorological Institute, Stockholm, Sweden. 304
0021 8502/86$3.00+0.00 Pergamon Journals Ltd.
Printed in Great Britain.
OPTICAL DETECTION OF METASTABLE AMBIENT AEROSOLS* MARKJ.
R O O D "t,
T. V.
LARSON,
D. S. COVERTand A. P. WAGGONER
Department of Civil Engineering, University of Washington, Seattle, WA 98195, U.S.A.
The ubiquitous presence of liquid H20 in atmospheric aerosol particles at relative humidities (RH) less than 100~ has many important consequences, including effects on radiative transfer properties and atmospheric chemistry. We have focused our attention on metastable droplets, those that exist as droplets even though the surrounding RH is below the bulk saturation humidity of the solute. Our measurement technique involves the use of an integrating nephelometer operating as close to ambient conditions as possible. Upstream of this sensor, the aerosol is heated to a maximum temperature of 30°C above ambient temperature and then rapidly cooled to ambient temperature prior to entering the nephelometer. A rapid decrease in the sub-micron aerosol particle light scattering extinction coefficient (asp) over a small temperature difference of approximately 5°C in the upstream heater is attributed to the presence of metastable droplets. Measurements were performed during the months of August and September 1983 in Riverside, CA and approximately one year later at remote sites in Grand Canyon, AZ and the Mojave Desert, CA. Approximately 200 tests were performed at each site. In the Riverside experiment, metastable droplets were present 60 ~o of the time, and the RH ranged from 26 to 86 ~o during the entire test period. In the Grand Canyon and Mojave Desert, metastable droplets were present 36 and 12 ~o of the time, respectively, but the RH values were systematically lower than in Riverside (ranged from 12 to 70 ~o and 10 to 78 ~o, respectively). The percent asp attributed to metastable liquid H20 averaged 8, 14 and 6 ~o respectively for Riverside, Grand Canyon and the Mojave Desert. Overall, metastable droplets were detected between 30 and 78 ~o RH but existed most frequently between 50 and 76 ~o RH.
* This paper was also presented during the 1986 American Association for Aerosol Research Conference in New lpexico, U.S.A. resent address: International Meteorological Institute, Stockholm, Sweden. 304