Recent laboratory chamber studies indicate a significant role forhighly oxidized low-volatility organics in new particle formation (NPF), butthe actual role of these highly oxidized low-volatility organics inatmospheric NPF remains uncertain. Here, particle size distributions (PSDs)measured in nine forest areas in North America are used to characterize theoccurrence and intensity of NPF and to evaluate model simulations using anempirical formulation in which formation rate is a function of theconcentrations of sulfuric acid and low-volatility organics fromalpha-pinene oxidation (Nucl-Org), and using an ion-mediated nucleationmechanism (excluding organics) (Nucl-IMN). On average, NPF occurred on~ 70 % of days during March for the four forest sites withspringtime PSD measurements, while NPF occurred on only ~ 10 % of days in July for all nine forest sites. Both Nucl-Org and Nucl-IMNschemes capture the observed high frequency of NPF in spring, but theNucl-Org scheme significantly overpredicts while the Nucl-IMN schemeslightly underpredicts NPF and particle number concentrations in summer.Statistical analyses of observed and simulated ultrafine particle numberconcentrations and frequency of NPF events indicate that the scheme withoutorganics agrees better overall with observations. The two schemes predictquite different nucleation rates (including their spatial patterns),concentrations of cloud condensation nuclei, and aerosol first indirectradiative forcing in North America, highlighting the need to reduce NPFuncertainties in regional and global earth system models.
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