 |
| Titel |
Homogeneous nucleation of NAD and NAT in liquid stratospheric aerosols: insufficient to explain denitrification |
| VerfasserIn |
D. A. Knopf, T. Koop, B. P. Luo, U. G. Weers, T. Peter |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1680-7316
|
| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 2, no. 3 ; Nr. 2, no. 3 (2002-08-30), S.207-214 |
| Datensatznummer |
250000616
|
| Publikation (Nr.) |
 copernicus.org/acp-2-207-2002.pdf |
|
|
|
|
|
| Zusammenfassung |
| The nucleation of NAD and NAT from
HNO3/H2O and HNO3/H2SO4/H2O solution droplets is
investigated both theoretically and experimentally with respect to the formation of polar stratospheric clouds (PSCs). Our analysis
shows that homogeneous NAD and NAT nucleation from liquid aerosols is insufficient to explain the number densities of large nitric
acid containing particles recently observed in the Arctic stratosphere. This conclusion is based on new droplet freezing
experiments employing optical microscopy combined with Raman spectroscopy. The homogeneous nucleation rate coefficients of NAD
and NAT in liquid aerosols under polar stratospheric conditions derived from the experiments are
< 2 x 10-5 cm-3 s-1 and < 8 x 10-2
cm-3 s-1, respectively. These nucleation rate coefficients are smaller by orders of magnitude than the value of
~103 cm-3 s-1 used in a recent denitrification modelling study that is based on a linear
extrapolation of laboratory nucleation data to stratospheric conditions (Tabazadeh et al.,
Science, 291, 2591--2594, 2001). We show that this linear extrapolation is in
disagreement with thermodynamics and with experimental data and, therefore, must not be used in microphysical models of
PSCs. Our analysis of the experimental data yields maximum hourly production
rates of nitric acid hydrate particles per cm3 of air of about
3 x 10-10 cm-3 (air) h-1 under polar stratospheric conditions. Assuming PSC particle production to
proceed at this rate for two months we arrive at particle number densities of
< 5 x 10-7 cm-3, much smaller than the value of
~10-4 cm-3 reported in recent field observations. In addition, the nitric acid hydrate production rate
inferred from our data is much smaller than that required to reproduce the observed denitrification in the modelling study
mentioned above. This clearly shows that homogeneous nucleation of NAD and NAT from liquid supercooled ternary solution aerosols
cannot explain the observed polar denitrification. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|