AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Can Nucleation in the Residual Layer Explain “Class-B” New Particle Formation Events?
NICHOLAS MESKHIDZE, Juan Jaimes-Correa, Markus Petters, Taylor Royalty, Brittany Phillips, Alyssa Zimmerman, Robert Reed, NC State
Abstract Number: 203 Working Group: Urban Aerosols
Abstract The sources and sinks affecting ambient concentrations of ultrafine particles (Dp < 100 nm) are a topic of interest in air quality and climate sciences. New particle formation is a secondary source of ultrafine particles. Different types of new particle formation events have been observed. During Class-A events, nucleation mode particles are starting with molecular clusters ~1.5 nm in size, followed by continuous modal growth. During Class-B events, particles also grow, but the first detected particle mode exceeds 10 nm. The lack of observed sub-10 nm sized particles during Class-B events has been interpreted as that nucleation happened upwind of the measurement site. Here we propose a different explanation the for observed size distributions during Class-B events. Size distributions were measured in Raleigh, NC, during November–December 2017. Five well-defined Class-B events that lead to a broad regional scale increases in ultrafine particle concentration were observed. Data analysis involving the time series of particle number size distribution, meteorological variables, air parcel back trajectories, surface weather maps, gas-phase chemical composition (i.e., CO, SO2, NO, NO2, O3), and particle tracers (i.e., refractory black carbon, non-hygroscopic particles, condensation sink, PM2.5, and PM10) suggests that the nucleation events may have occurred inside the residual layer. The sudden appearance of nucleation mode particles could then be explained by turbulent mixing related to the growth of the daytime boundary layer, which results in downward mixing of particles that nucleated aloft and grew prior to vertical transport. This process leads to an increase in ultrafine particles during late morning/mid-day, a time when contributions from traffic to ultrafine particles are less.