Investigating Ultrafine Aerosol Turbulent Fluxes during Atmospheric New Particle Formation Events

Quantitatively predicting new particle formation (NPF) remains challenging, largely because the true origin of events observed on the surface is often ambiguous. This potential for misattributing particles formed aloft as ground-level events fundamentally compromises predictive models built on local measurements. Here, we propose and validate an analytical framework using airborne eddy covariance and continuous wavelet transform to directly quantify the vertical turbulent flux of newly formed particles and determine their dominant vertical transport direction. Analyzing data from a dedicated airborne campaign over the Southern Great Plains, we observed a consistent and strong downward turbulent flux of ultrafine aerosols during NPF events, with a mean value of -133.8 cm^-3 m s^-1 in the entrainment zone, whereas fluxes on non-NPF days were negligible. Our spectral analysis further confirms that these directional fluxes can be reliably captured by using standard 1 Hz aerosol instrumentation. These findings suggest that NPF events associated with entrainment from the overlying residual/stable layer during planetary boundary layer growth represent a significant and potentially underestimated source of boundary layer aerosols. The framework presented here provides a new methodology to correctly attribute NPF events to specific altitudes, thereby improving our process-level understanding of particle formation in the atmosphere.