CMAQv5.1 Aerosol size dist
Aerosol updates – Binary nucleation and PM2.5 emissions size distribution
PM Emissions: Prior to CMAQv5.1, PM2.5 emissions were distributed into CMAQ’s Aitken and accumulation modes according to species dependent modal mass fractions. These fractions and associated emissions size distribution parameters were based on historical measurements known to underestimate ultrafine particles. In an effort to improve upon these outdated measurements and better simulate aerosol size distributions, Elleman and Covert (2010) developed updated particulate emissions distributions based on a review of modern measurements from urban, power-plant, and marine source dominated regions at 4-15 km spatial scales. Here we implement their “urban” PM emissions distribution and modal mass fractions. The updated emission parameters and their base case values are listed in Table 1. Anthropogenic emissions of coarse PM, as well as sea salt and windblown dust, were unchanged.
Table 1. Updated and original parameters for Aitken and accumulation mode particulate emissions.
|mode||Mass fraction||Dgv (m m)||sg||Mass fraction||Dgv (m m)||sg|
Nucleation: The previously implemented binary H2SO4-H2O nucleation scheme of Kulmala et al. (1998) contains errors in the formulation that were corrected in the expanded nucleation parameterization of Vehkamaki et al. (2002). In CMAQv5.1 we replace Kulmala et al. (1998) with Vehkamaki et al. (2002).
Significance and Impact
While regulatory modeling applications have traditionally focused on particulate mass, applications have expanded to include climate change and health impacts of ultrafine particles. As such, it is important to characterize not only aerosol mass but also the aerosol number size distribution.
While changes in average PM2.5 mass concentrations are small (usually within +/- 5%), number concentrations are significantly increased with the updates and better match the magnitude and size distribution of observed particles (Fahey et al., 2014). Additionally, it was found that the updated PM emissions distribution parameters improve modeled mass size distributions compared to MOUDI observations for multiple periods and measurement sites (Nolte et al., 2015).
No significant impact on model run time is expected.
Elleman, R. A., and Covert, D. S. (2010) Aerosol size distribution modeling with the Community Multiscale Air Quality Modeling system in the Pacific Northwest: 3. Size distribution of particles emitted into a mesoscale model, J. Geophys. Res., 115, D3(16), doi:10.1029/2009JD012401.
Fahey, K.M., Sarwar, G., Appel, K.W., and C.G. Nolte (2014) Evaluation of Updated CMAQ Aerosol Treatments with a Focus on Ultrafine Particles. 13th Annual CMAS Conference, Chapel Hill, NC, October 27-29, 2014.
Kulmala, M., Laaksonen, A., and L. Pirjola (1998) Parameterizations for sulfuric acid/water nucleation rates. J. Geophys. Res., v103(D7), 8301-8308.
Nolte, C.G., Appel, K.W., Kelly, J.T., Bhave, P.V., Fahey, K.M., Collett, J.L., Zhang, L., and J.O. Young (2015) Evaluation of the Community Multiscale Air Quality (CMAQ) model v5.0 against size-resolved measurements of inorganic particle composition across sites in North America. Geosci. Model Dev., 8, 2877-2892.
Vehkamaki, H., Kulmala, M., Napari, I., Lehtinen, K.E.J., Timmreck, C., Noppel, M., and A. Laaksonen (2002) An improved parameterization for sulfuric acid–water nucleation rates for tropospheric and stratospheric conditions. J. Geophys. Res., v107(22).