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. Atmospheric particles can act as cloud condensationnuclei (CCN) and modify cloud properties and precipitation and thusindirectly impact the hydrological cycle and climate. New particle formation(NPF or nucleation), frequently observed at locations around the globe, isan important source of ultrafine particles and CCN in the atmosphere. Inthis study, wintertime NPF over the Northeastern United States (NEUS) issimulated with WRF-Chem coupled with a size-resolved (sectional) advancedparticle microphysics (APM) model. Model-simulated variations in particlenumber concentrations during a 2-month period (November–December 2013)are in agreement with corresponding measurements taken at Pinnacle StatePark (PSP), New York, and Appalachian State University (APP), North Carolina.We show that, even during wintertime, regional nucleation occurs andcontributes significantly to ultrafine-particle and CCN numberconcentrations over the NEUS. The model shows that, due to low biogenicemissions during this period, wintertime regional nucleation is solelycontrolled by inorganic species and the newly developed ternary ion-mediatednucleation scheme is able to capture the variations in observed particlenumber concentrations (ranging from ∼200 to 20 000 cm −3 )at both PSP and APP. Total particle and CCN number concentrationsdramatically increase following NPF events and have the highest values over the Ohio Valley region, where elevated [ SO2 ] is sustained by power plants.Secondary particles dominate particle number abundance over the NEUS, andtheir fraction increases with altitude from ≳85  % near the surface to ≳95  % in the uppertroposphere. The secondary fraction of CCN also increases with altitude,from 20 %–50 % in the lower boundary layer to 50 %–60 % in the middletroposphere to 70 %–85 % in the upper troposphere.

Wintertime new particle formation and its contribution to cloud condensation nuclei in the Northeastern United States