Abstract
Abstract. Using a three-dimensional general circulation model with sulfur chemistry and sectional aerosol microphysics (WACCM/CARMA), we studied aerosol formation and microphysics in the upper troposphere and lower stratosphere (UTLS) as well as the middle and upper stratosphere based on three nucleation schemes (two binary homogeneous schemes and an ion-mediated scheme related to one of the binary schemes). Simulations suggest that ion-mediated nucleation rates in the UTLS are 25 % higher than its related binary scheme, but that the rates predicted by the two binary schemes vary by two orders of magnitude. None of the nucleation schemes is superior at matching the limited observations available at the smallest sizes. However, it is found that coagulation, not nucleation, controls number concentration at sizes greater than approximately 10 nm. Therefore, based on this study, processes relevant to atmospheric chemistry and radiative forcing in the UTLS are not sensitive to the choice of nucleation schemes. The dominance of coagulation over other microphysical processes in the UTLS is consistent with other recent work using microphysical models. Simulations using all three nucleation schemes compare reasonably well to observations of size distributions, number concentration across latitude, and vertical profiles of particle mixing ratio in the UTLS. Interestingly, we find that we need to include Van der Waals forces in our coagulation scheme to match the UTLS aerosol concentrations. We conclude that this model can reasonably represent sulfate microphysical processes in the UTLS, and that the properties of particles at atmospherically relevant sizes appear to be insensitive to the details of the nucleation scheme. We also suggest that micrometeorites, which are not included in this model, dominate the aerosol properties in the upper stratosphere above about 30 km.
Highlights
The tropical upper troposphere is known to be a net source region of new particles (e.g. Brock et al, 1995; Clarke and Kapustin, 2002)
Three nucleation schemes are available in this model: two binary homogeneous nucleation (BHN) schemes – one based on classical nucleation theory (Zhao BHN) and one based on kinetic nucleation theory lookup tables (Yu BHN) – as well as an ion-mediated nucleation (IMN) scheme look-up Table (Yu IMN)
The two Yu schemes often are found to be out of the Table limits in the middle stratosphere and above due to limits on boundary conditions of Table inputs; this problem was mostly resolved by setting nucleation to zero if relatively humidity is less than the Table minimum value, but the tables should be used with caution in these regions
Summary
The tropical upper troposphere is known to be a net source region of new particles (e.g. Brock et al, 1995; Clarke and Kapustin, 2002). Kanawade and Tripathi (2006) calculated IMN with a sectional aerosol model and found agreement with UTLS observations, but did not compare with BHN simulations. Yu and Luo (2009) calculated IMN with a sectional microphysical aerosol model and found reasonable agreement in the troposphere, but did not compare to BHN, and did not compare to observations in the UTLS. Yu et al (2010) compared nucleation rates and number concentration from IMN and two different BHN schemes in the troposphere to aircraft observations, but did not study the aerosol evolution (size, mass, effective radius) and did not study stratospheric properties. Kazil et al (2010) found that simulations agree best with observations in the lower and mid-troposphere when IMN and BHN are included across the entire model domain and organic cluster formation is included but limited to the continental boundary layer, but did not compare to observations in the stratosphere. We compare our simulations with UTLS observations of size distribution (Lee et al, 2003; Deshler et al, 2003), number concentration (Borrmann et al, 2010; Brock et al, 1995; Heintzenberg et al, 2003), and Stratospheric Aerosol and Gas Experiment (SAGE) II aerosol extinctions and effective radii (Chu et al, 1989)
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