- Email: [email protected]
Aerosol nucleation and mixing in the upper troposphere
~
Aerosol Sci k'bl. 31, Suppl. I, pp. $570--$57 I, 2000
) Pergamon
www.elsevier.com/locate/jaerosci S e s s i o n 6D - Particle f o r m a t i o n a n d c o a g u l a t i o n AEROSOL NUCLEATION AND MIXING IN THE UPPER TROPOSPHERE
C F. CLEMENT 1, I. J. FORD 2 and C. H, TWOHY 115 Witan Way, Wantage, Oxen OX12 9EU, U.K. 2Department of Physics and Astronomy, University College, London WC 1E 6BT, UK. 3College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 9733 I, U.S.A. KEYWORDS Atmospheric aerosols; Climate; Modelling; Nucleation; Mixing On May 8, 1996, as part of the SUCCESS study (Toon and Miake-Lye 1998), a NASA DC8 flew through and near the cirrus outflow of a large midwest storm. Measurements were made at 1 s intervals of various gas concentrations, condensation nuclei, cloud surface area and meteorological variables. The aim of this work is to interpret the origin and nature of the nuclei observed which were in the size range R = 10 - 25 nm which had fluctuating concentrations up to a maximum of 1.25 l04 cm "3 at the ambient temperature, T ~ 215 K, and pressure, p = 200 nab (in standard units at NTP the concentration would be a factor of 4 larger). In the rising region of the flight, most gas concentrations and the aerosol concentration are strongly correlated, and the almost perfect correlation found between the concentrations of CO and CH4 in this region have been interpreted as arising from recent mixing of two air masses with internally uniform concentrations (Clement et a12000). The aerosol correlation indicates that one of the air masses, that from the outflow of the storm, contained an almost uniform aerosol concentration which had been formed before the mixing took place. An examination of the behaviour of the correlations at different times and over what are effectively different length scales reveals differences in gas and aerosol mixing properties. The high volatility of the observed aerosol indicates it could consist of sulphuric acid droplets nucleated in the relatively aerosol free outflow of the storm where the acid is being formed by photonuclear reactions from SO2 carried up from the surface. We have attempted a quantitative analysis to verify this hypothesis, based on a molecular production rate in the region of P = 1011 m "3 s-1 which is consistent with estimates of the likely SO2 concentration. The nucleation phase was examined using an analytic model for nucleation bursts (Clement and Ford 1999) and a parametrized rate model for sulphuric acid/water nucleation (Kulmala et al 1998). At the ambient very Jew temperatures, the model predicts a dependence on acid concentration, c, close to c 2 which is characteristic ofbarriedess nucleation. The nucleation pulse is predicted to last about 5 mins and to produce an initial concentration, N O = 7. 106 cm-3, of tiny droplets containing only 3 - 4 acid molecules. This concentration is much lower in size and total mass, and much higher in number, than the observed concentration. We then allow the initial concentration to grow with the same acid production rate. Not all the actual aerosol was likely to have been observed, and, allowing a bounded range for this missing mass, the growth time needed to reproduce it with the above assumed molecular production rate
Abstracts of the 2000 European Aerosol Conference
$571
would be between 48 mins and 7 hours. This agrees satisfactorily with the timescale for this major storm which had certainly lasted for several hours. To reproduce the observed number concentration, we need a reduction by a factor of over 500 from its initial value. Coagulation from the initial nm to finall0-50 nm sizes reached is in the molecular regime and increases as R 1/2 for equal sizes, but can be much larger between unequal sized droplets. Preliminary estimates have some difficulty in producing enough coagulation from the model nucleated aerosol, but it is clear that at least several hours are required. Altogether, the nucleation burst observed was very large in extent ( > 200 kin), and such upper troposphere bursts following major storms are significant producers of the type of atmospheric aerosol which acts as CCN and plays a role in affecting the climate. Our results indicate that the burst was probably due to sulphuric acid nucleation from SO2 of a largely anthropogenic origin, but that the quantitative features of the burst would repay further observations and study. REFERENCES Clement, C. F. and Ford, I. J. (1997b) Gas-to-particle conversion in the atmosphere: II. analytical models of nucleation bursts. Atmos. Environment 33, 489-499. Clement, C. F., Ford, I. J., and Twohy, C. H. (2000) Mixing of atmospheric gas concentrations, submitted for publication. Kulmala, M., Laaksonen, A. and Pirjola, L. (1998) Parametrization for sulfuric acid / water nucleation rates. J. Geophys. Res. 103, D7 88301-88307. Toon, O.B. and R.C. Miake-Lye, R.C. (1998) Subsonic aircraft: contrail and cloud effects special study (SUCCESS). Geophys. Res. Lett. 25, 1109-1112.
Aerosol Sci k'bl. 31, Suppl. I, pp. $570--$57 I, 2000
) Pergamon
www.elsevier.com/locate/jaerosci S e s s i o n 6D - Particle f o r m a t i o n a n d c o a g u l a t i o n AEROSOL NUCLEATION AND MIXING IN THE UPPER TROPOSPHERE
C F. CLEMENT 1, I. J. FORD 2 and C. H, TWOHY 115 Witan Way, Wantage, Oxen OX12 9EU, U.K. 2Department of Physics and Astronomy, University College, London WC 1E 6BT, UK. 3College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 9733 I, U.S.A. KEYWORDS Atmospheric aerosols; Climate; Modelling; Nucleation; Mixing On May 8, 1996, as part of the SUCCESS study (Toon and Miake-Lye 1998), a NASA DC8 flew through and near the cirrus outflow of a large midwest storm. Measurements were made at 1 s intervals of various gas concentrations, condensation nuclei, cloud surface area and meteorological variables. The aim of this work is to interpret the origin and nature of the nuclei observed which were in the size range R = 10 - 25 nm which had fluctuating concentrations up to a maximum of 1.25 l04 cm "3 at the ambient temperature, T ~ 215 K, and pressure, p = 200 nab (in standard units at NTP the concentration would be a factor of 4 larger). In the rising region of the flight, most gas concentrations and the aerosol concentration are strongly correlated, and the almost perfect correlation found between the concentrations of CO and CH4 in this region have been interpreted as arising from recent mixing of two air masses with internally uniform concentrations (Clement et a12000). The aerosol correlation indicates that one of the air masses, that from the outflow of the storm, contained an almost uniform aerosol concentration which had been formed before the mixing took place. An examination of the behaviour of the correlations at different times and over what are effectively different length scales reveals differences in gas and aerosol mixing properties. The high volatility of the observed aerosol indicates it could consist of sulphuric acid droplets nucleated in the relatively aerosol free outflow of the storm where the acid is being formed by photonuclear reactions from SO2 carried up from the surface. We have attempted a quantitative analysis to verify this hypothesis, based on a molecular production rate in the region of P = 1011 m "3 s-1 which is consistent with estimates of the likely SO2 concentration. The nucleation phase was examined using an analytic model for nucleation bursts (Clement and Ford 1999) and a parametrized rate model for sulphuric acid/water nucleation (Kulmala et al 1998). At the ambient very Jew temperatures, the model predicts a dependence on acid concentration, c, close to c 2 which is characteristic ofbarriedess nucleation. The nucleation pulse is predicted to last about 5 mins and to produce an initial concentration, N O = 7. 106 cm-3, of tiny droplets containing only 3 - 4 acid molecules. This concentration is much lower in size and total mass, and much higher in number, than the observed concentration. We then allow the initial concentration to grow with the same acid production rate. Not all the actual aerosol was likely to have been observed, and, allowing a bounded range for this missing mass, the growth time needed to reproduce it with the above assumed molecular production rate
Abstracts of the 2000 European Aerosol Conference
$571
would be between 48 mins and 7 hours. This agrees satisfactorily with the timescale for this major storm which had certainly lasted for several hours. To reproduce the observed number concentration, we need a reduction by a factor of over 500 from its initial value. Coagulation from the initial nm to finall0-50 nm sizes reached is in the molecular regime and increases as R 1/2 for equal sizes, but can be much larger between unequal sized droplets. Preliminary estimates have some difficulty in producing enough coagulation from the model nucleated aerosol, but it is clear that at least several hours are required. Altogether, the nucleation burst observed was very large in extent ( > 200 kin), and such upper troposphere bursts following major storms are significant producers of the type of atmospheric aerosol which acts as CCN and plays a role in affecting the climate. Our results indicate that the burst was probably due to sulphuric acid nucleation from SO2 of a largely anthropogenic origin, but that the quantitative features of the burst would repay further observations and study. REFERENCES Clement, C. F. and Ford, I. J. (1997b) Gas-to-particle conversion in the atmosphere: II. analytical models of nucleation bursts. Atmos. Environment 33, 489-499. Clement, C. F., Ford, I. J., and Twohy, C. H. (2000) Mixing of atmospheric gas concentrations, submitted for publication. Kulmala, M., Laaksonen, A. and Pirjola, L. (1998) Parametrization for sulfuric acid / water nucleation rates. J. Geophys. Res. 103, D7 88301-88307. Toon, O.B. and R.C. Miake-Lye, R.C. (1998) Subsonic aircraft: contrail and cloud effects special study (SUCCESS). Geophys. Res. Lett. 25, 1109-1112.