Regularities of new particle formation and evolution of existing atmospheric aerosol particles in a large (3200 m³) isolated volume

Abstract. The paper reports on an investigation of nanometre-sized new particles formation (NPF) in aerosol-free outdoor air. This phenomenon was observed after filling of Large Aerosol Chamber (LAC) RPA Typhoon with the volume of 3200 m³ with outdoor air, passed through HEPA 13 class filter (H13). During the summer-autumn period of 2018, even in the full darkness and in presence ionizing radiation only in the shape of secondary galactic cosmic rays, new particle formation with the particle size greater than 15 nm starts 0.5–1 hour after the end of LAC filling. During the 2018–2019 winter periods the NPF event was not observed once only.

Approximately one day after NPF narrow bell-shaped spectra with number concentration up to 10⁴ cm⁻³ and mass concentration up to 0.6 µg per m³ are formed. During the next five or more days, these size distributions evolve due to coagulation, while their asymptotic shape remains constant with relative breadth σ_c ≈ 0.28, and relative asymmetry ras ≈ 2 (ras = skewness/σ_c). The value ras ≈ 2 defines the analytical description of the size distribution as the gamma-distribution.

During additional purification of newly formed particles with the inner H13 filter, aerosol particles concentration in LAC decreases down to a few particles per cm³. This concentration remained constant for more than a week. This demonstrates that new aerosol particles are formed by homogeneous gas-to-particle conversion of gaseous precursors, which passed through the external H13 filter. The mass concentration of newly formed particles depends on the concentration of precursors.

It was found that after filling LAC with outdoor unfiltered air, approximately after 10 hours the left-hand side of aerosol particle size distribution below 15 nm disappears, and after several days there forms an asymptotic bell-shaped size spectrum with σ_c ≈ 0.4–0.5 and ras = 2–3. The modal diameter becomes about 150 nm after five days, while the size distribution greater than 200 nm remains unchanged. This allows concluding that aerosol particles greater than 200 nm have a life-time of more than five days, while particles smaller than 15 nm, not more than five hours.

The observed regularities of NPF and pre-existing aerosol spectra evolution may contribute significantly to understanding the processes of the formation of atmospheric aerosols, which are responsible for cloud and precipitation formation. They also should be considered during the design of purification methods for facilities and living spaces.

During the investigation of size distribution evolution of aerosol particles generated by the spraying of tap water, it was found that this aerosol particles size distribution transforms from a power law to a bell-shaped distribution in five days with σ_c ≈ 0.4 and ras ≈ 2. These results may be used for the development of aerosol evolution models.