The connection of atmospheric new particle formation to fair-weather Earth-atmosphere electric field

Atmospheric new particle formation (NPF) generates secondary aerosol particles into the lower atmosphere via gas-to-particle phase transition. Secondary aerosol particles dominate the total particle number concentration and are an important source for cloud condensation nuclei ^[1]. NPF typically begins with clustering among gaseous molecules. Once the newly formed clusters attain a size larger than the critical cluster size (~1.5 nm), their growth to larger sizes is energetically favoured and eventually they become nanoparticles ^[2]. NPF is often observed with the participation of air ions ^[3] and sometimes is induced by ions ^[4]. Air ions are a constituent of atmospheric electricity. The presence of the Earth-atmosphere electric field poses an electrical force on air ions. The earth-atmosphere electric field exhibits variability at different time scales under fair-weather conditions ^[5]. It is therefore interesting to understand whether the Earth-atmosphere electric field influences atmospheric new particle formation.

We analysed the Earth-atmosphere electric field together with the number size distribution data of air ions and aerosol particles under fair-weather conditions measured at Hyytiälä SMEAR II station in Southern Finland ^[6]. The electric field were measured by two Campbell CS 110 field mills in parallel. Air ion data were obtained with a Balance Scanning Mobility Analyser (BSMA) and a Neutral and Air Ion Spectrometer (NAIS), and aerosol particle data with a Differential Mobility Particle Sizer (DMPS). We used condensation Sinks (CS) derived from the DMPS measurement, air temperature, relative humidity, wind speed, global radiation as well as brightness derived from the global radiation measurement to assist the analysis. The measured earth-atmosphere electric field on NPF days was higher than on non-NPF days. We found that under low CS conditions, the electric field can enhance the formation of 1.7-3 nm air ions, but the concentration of 1.7-3 nm ions decreased with an increasing electric field under high CS conditions.

References:

[1]       Kerminen V.-M. et al., Environ. Res. Lett. 2018, 13, 103003.

[2]       Kulmala M. et al., Science 2013, 339, 943-946.

[3]       Manninen H. E. et al., Atmos. Chem. Phys. 2010, 10, 7907-7927.

[4]       Jokinen T. et al., Science Advances 2018, 4, eaat9744.

[5]       Bennett A. J., Harrison R. G., Journal of Physics: Conference Series 2008, 142, 012046.

[6]       Hari P., Kulmala M., Boreal Environ. Res. 2005, 10, 315-322.