New particle formation in coastal New Zealand with a focus on open-ocean air masses

Abstract

Abstract. Even though oceans cover the majority of the Earth, most aerosol measurements are from continental sites. We measured aerosol particle number size distribution at Baring Head, in coastal New Zealand, over a total period of 10 months to study aerosol properties and new particle formation, with a special focus on aerosol formation in open-ocean air masses. Particle concentrations were higher in land-influenced air compared to clean marine air in all size classes, from sub-10 nm to cloud condensation nuclei sizes. When classifying the particle number size distributions with traditional methods designed for continental sites, new particle formation was observed at the station throughout the year with an average event frequency of 23 %. While most of these traditional event days had some land influence, we also observed particle growth starting from nucleation mode in 16 % of the data in clean marine air, and at least part of this growth was connected to nucleation in the marine boundary layer. Sub-10 nm particles accounted for 29 % of the total aerosol number concentration of particles larger than 1 nm in marine air during the spring. This shows that nucleation in marine air is frequent enough to influence the total particle concentration. Particle formation in land-influenced air was more intense and had on average higher growth rates than what was found for marine air. Particle formation and primary emissions increased particle number concentrations as a function of time spent over land during the first 1–2 d. After this, nucleation seems to start getting suppressed by the pre-existing particle population, but accumulation mode particle concentration keeps increasing, likely due to primary particle emissions. Further work showed that traditional NPF events were favoured by sunny conditions with low relative humidity and wind speeds. In marine air, formation of sub-10 nm particles was favoured by low temperatures, relative humidity, and wind speeds and could happen even during the night. Our future work will study the mechanisms responsible for particle formation at Baring Head with a focus on different chemical precursor species. This study sheds light on both new particle formation in open-ocean air masses coming from the Southern Ocean and local aerosol properties in New Zealand.

Chemical precursors of new particle formation in coastal New Zealand

“…The rest of the data were considered landinfluenced. While in our previous work 7.3 % of the aerosol data were classified as clean marine air (Peltola et al, 2022), out of the APi-TOF data only 4.3 % were classified as clean marine air because of lower data availability.…”

“…This article follows the work of Peltola et al (2022), and most of the measurements are described in detail by them. Briefly, the aerosol measurements included measurements of aerosol particle size distribution between 10-500 nm with a scanning mobility particle sizer (SMPS), ion and particle size distribution measurements with a Neutral cluster and Air Ion Spectrometer (NAIS; Mirme and Mirme, 2013), and aerosol particle number concentration measurements for particles with diameters above 10 nm with a condensation particle counter (CPC) and for particles larger than 1 and 3 nm with a particle size magnifier (PSM; Vanhanen et al, 2011).…”

“…The PSM was first used in fixed mode with a saturation flow rate of 1 L min −1 , and from 17 September it was used in switching mode, with the saturation flow switching from 0.1 to 1 L min −1 every 60 s. Saturation flow rate of 1 L min −1 was assumed to correspond approximately to a cutoff diameter of 1 nm and 0.1 L min −1 to 3 nm. Additionally, we used the long-term measurements of meteorological variables available from the NIWA climate database (https: //cliflo.niwa.co.nz/, last access: 11 May 2022) and radon, tide height, and air-mass back trajectory data as described by Peltola et al (2022).…”

“…In addition to the measurements described by Peltola et al (2022), we measured the chemical composition of ambient anions with an HTOF atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF; Junninen et al, 2010). We used the instrument in negative ion mode without chemical ionisation.…”

“…We report the chemical composition of ambient anions in coastal New Zealand over a 7month period. We combine these data with the results from our previous study that focused on new particle formation at the site and showed that NPF can occur in open-ocean air masses (Peltola et al, 2022) to understand which chemical species drive new particle formation in both marine and landinfluenced air. This is the first time that the chemical composition of ambient anions has been measured in the area, and our results provide valuable information for understanding the air chemistry and NPF precursors at this site and in marine air in general.…”