Observation and Analysis of Particle Nucleation at a Forest Site in Southeastern US

This study examines the characteristics of new particle formation at a forest site in southeastern US. Particle size distributions above a Loblolly pine plantation were measured between November 2005 and September 2007 and analyzed by event type and frequency, as well as in relation to meteorological and atmospheric chemical conditions. Nucleation events occurred on 69% of classifiable observation days. Nucleation frequency was highest in spring. The highest daily nucleation (class A and B events) frequency (81%) was observed in April. The average total particle number concentration on nucleation days was 8,684 cm⁻³ (10 < D_p < 250 nm) and 3,991 cm⁻³ (10 < D_p < 25 nm) with a mode diameter of 28 nm with corresponding values on non-nucleation days of 2,143 cm⁻³, 655 cm⁻³, and 44.5 nm, respectively. The annual average growth rate during nucleation events was 2.7 ± 0.3 nm·h⁻¹. Higher growth rates were observed during summer months with highest rates observed in May (5.0 ± 3.6 nm·h⁻¹). Winter months were associated with lower growth rates, the lowest occurring in February (1.2 ± 2.2 nm·h⁻¹). Consistent with other studies, nucleation events were more likely to occur on days with higher radiative flux and lower relative humidity compared to non-nucleation days. The daily minimum in the condensation sink, which typically occurred 2 to 3 h after sunrise, was a good indicator of the timing of nucleation onset. The intensity of the event, indicated by the total particle number concentration, was well correlated with photo-synthetically active radiation, used here as a surrogate for total global radiation, and relative humidity. Even though the role of biogenic VOC in the initial nuclei formation is not understood from this study, the relationships with chemical precursors and secondary aerosol products associated with nucleation, coupled with diurnal boundary layer dynamics and seasonal meteorological patterns, suggest that H₂SO₄ and biogenic VOC play a role in nucleated particle growth at this site.