On the formation, growth and composition of nucleation mode particles

TLDR

A new analytical tool using time‑resolved aerosol spectra was developed to derive nucleation mode formation and growth properties, and applied to three BIOFOR cases over Boreal forests to estimate nucleation rates and vapor source rates. The tool derives growth‑rate from size spectra, computes condensation and coagulation sinks, estimates condensable vapor concentration and source rate, and, using these sinks with measured 3 nm particle concentrations, estimates 1 nm nucleation rates and concentrations. In Boreal forest cases, nucleation mode growth‑rate was 2–3 nm h⁻¹ requiring 2.5–4×10⁷ cm⁻³ condensable vapor and a source rate of 7.5–11×10⁴ cm⁻³ s⁻¹; 3 nm particle formation rate was 1 cm⁻³ s⁻¹; 1 nm particle formation rate 10–100 cm⁻³ s⁻¹ with concentrations 10⁴–10⁵ cm⁻³; insoluble vapor mass flux matched soluble vapor, with soluble/insoluble ratio 0.7–1.4.

Abstract

Taking advantage of only the measured aerosol particles spectral evolution as a function of time, a new analytical tool is developed to derive formation and growth properties of nucleation mode aerosols. This method, when used with hygroscopic growth-factors, can also estimate basic composition properties of these recently-formed particles. From size spectra the diameter growth-rate can be obtained, and aerosol condensation and coagulation sinks can be calculated. Using this growth-rate and condensation sink, the concentration of condensable vapours and their source rate can be estimated. Then, combining the coagulation sink together with measured number concentrations and apparent source rates of 3 nm particles, 1 nm particle nucleation rates and concentration can be estimated. To estimate nucleation rates and vapour concentration source rates producing new particle bursts over the Boreal forest regions, three cases from the BIOFOR project were examined using this analytical tool. In this environment, the nucleation mode growth-rate was observed to be 2–3 nm hour−1, which required a condensable vapour concentration of 2.5–4×107 cm−3 and a source rate of approximately 7.5–11×104 cm−3 s−1 to be sustained. The formation rate of 3 nm particles was =1 particle cm−3 s−1 in all three cases. The estimated formation rate of 1 nm particles was 10–100 particles cm−3 s−1, while their concentration was estimated to be between 10,000 and 100,000 particles cm−3. Using hygroscopicity data and mass flux expressions, the mass flux of insoluble vapour is estimated to be of the same order of magnitude as that of soluble vapour, with a soluble to insoluble vapour flux ratio ranging from 0.7 to 1.4 during these nucleation events.