PNNL

Abstract

New particle formation (NPF) and growth is a key source of cloud condensation nuclei, yet the specific mechanisms driving NPF remain uncertain, particularly in environments with limited observations. Here we present an analysis using regional WRF-Chem simulations and observations from four springtime NPF days and five non-NPF days at the Atmospheric Radiation Measurement (ARM) program’s Bankhead National Forest (BNF) observatory in Alabama, USA, in March and June 2025. WRF-Chem accurately reproduces the presence and absence of NPF at the surface across all analyzed days and captures number concentration vertical profiles on days with tethererd balloon system measurements. Among eight nucleation mechanisms evaluated, the dimethylamine + sulfuric acid mechanism dominates all four NPF event days, with extremely low volatility organic compounds from anthropogenic, biogenic, and biomass burning sources contributing subsequent particle growth. During June, simulations indicate that the absense of NPF at BNF is driven by orders-of-magnitude lower sulfuric acid concentrations, higher ambient temperatures, and elevated condensation sinks relative to March. A sensitivity simulation reducing June isoprene emissions predicted increased sulfuric acid concentrations but still lacked NPF, pointing to higher ambient temperatures as the dominant factor inhibiting summer NPF. The lower summertime sulfuric acid concentrations are only partly explained by isoprene supression of OH, since wind directions from anthropogenically influenced regions impacted BNF more frequently during spring than summer. Together, our results provide insights into the coupled roles of biogenic and non-biogenic VOCs, sulfuric acid, DMA, SOA formation, and meteorology in governing NPF in the forests of the southeastern United States.