Modeling ultrafine particle growth at a pine forest site influenced by anthropogenic pollution during BEACHON-RoMBAS 2011

Formation and growth of ultrafine particles is crudely represented in chemistry-climate models, contributing to uncertainties in aerosol composition, size distribution, and aerosol effects on cloud condensation nuclei (CCN) concentrations. Measurements of ultrafine particles, their precursor gases, and meteorological parameters were performed in a ponderosa pine forest in the Colorado Front Range in July-August 2011, and were analyzed to study processes leading to small particle burst events (PBEs) which were characterized by an increase in the number concentrations of ultrafine 4-30 nm diameter size particles. These measurements suggest that PBEs were associated with the arrival at the site of anthropogenic pollution plumes midday to early afternoon. During PBEs, number concentrations of 4-30 nm diameter particles typically exceeded 10⁴ cm⁻³, and these elevated concentrations coincided with increased SO₂ and monoterpene concentrations, and led to a factor-of-2 increase in CCN concentrations at 0.5% supersaturation. The PBEs were simulated using the regional WRF-Chem model, which was extended to account for ultrafine particle sizes starting at 1 nm in diameter, to include an empirical activation nucleation scheme in the planetary boundary layer, and to explicitly simulate the subsequent growth of Aitken particles (10-100 nm) by condensation of organic and inorganic vapors. The updated model reasonably captured measured aerosol number concentrations and size distribution during PBEs, as well as ground-level CCN concentrations. Model results suggest that sulfuric acid originating from anthropogenic SO₂ triggered PBEs, and that the condensation of monoterpene oxidation products onto freshly nucleated particles contributes to their growth. The simulated growth rate of ~ 3.4 nm h⁻¹ for 4-40 nm diameter particles was comparable to the measured average value of 2.3 nm h⁻¹. Results also suggest that the presence of PBEs tends to modify the composition of sub-20 nm diameter particles, leading to a higher mass fraction of sulfate aerosols. Sensitivity simulations suggest that the representation of nucleation processes in the model largely influences the predicted number concentrations and thus CCN concentrations. We estimate that nucleation contributes 67% of surface CCN at 0.5% supersaturation in this pine forest environment.