New Particle Formation and Growth in CMAQ: Application of Comprehensive Modal Methods to Observations during CalNex 2010

Abstract:

Secondary formation and subsequent growth of ultrafine atmospheric particles is an important source of larger particles that can activate clouds and affect their microphysical properties. It is likewise important for models attempting to quantify cloud-aerosol feedbacks to realistically account for this pervasive pathway. Representing these phenomena accurately in models requires in-depth knowledge of the chemical interactions that lead to new particle formation as well as the availability of condensable species to sustain a growth event. Further, models must represent the numerical aspects of particle growth reasonably well in order to preserve the essential characteristics of the aerosol size distribution (e.g. unimodal vs. bimodal, peak diameter, etc). Such characteristics are critical for calculating the number of particles participating as nuclei for liquid and solid hydrometeors.

We implement into the Community Multi-scale Air Quality (CMAQ) model a new aerosol processing module designed for robust prediction of particle number concentrations, sources and sinks. The new module leverages the speed and flexibility of modal aerosol techniques with state-of-the-art, schemes for treating new particle formation, coagulation, and intermodal transference. Moreover, we incorporate an updated treatment of organic aerosol (OA) formation and explore the sensitivity of growth rates predicted by the CMAQ model to the uncertain OA formation parameters. We apply the new model to observations made during the CalNex 2010 campaign and evaluate model performance against observed number concentrations, time-dependent growth rates, and size distributions.