Coastal new particle formation: Environmental conditions and aerosol physicochemical characteristics during nucleation bursts

Nucleation mode aerosol was characterized during coastal nucleation events at Mace Head during intensive New Particle Formation and Fate in the Coastal Environment (PARFORCE) field campaigns in September 1998 and June 1999. Nucleation events were observed almost on a daily basis during the occurrence of low tide and solar irradiation. In September 1998, average nucleation mode particle concentrations were 8600 cm^-3 during clean air events and 2200 cm^-3 during polluted events. By comparison, during June 1999, mean nucleation mode concentrations were 27,000 cm^-3 during clean events and 3350 cm^-3 during polluted conditions. Peak concentrations often reached 500,000-1,000,000 cm^-3 during the most intense events and the duration of the events ranged from 2 to 8 hours with a mean of 4.5 hours. Source rates for detectable particle sizes (d > 3 nm) were estimated to be between 104 and 106 cm^-3 s^-1 and initial growth rates of new particles were as high as 0.1-0.35 nm s^-1 at the tidal source region. Recently formed 8 nm particles were subjected to hygroscopic growth and were found to have a growth factor of 1.0-1.1 for humidification at 90% relative humidity. The low growth factors implicate a condensable gas with very low solubility leading to detectable particle formation. It is not clear if this condensable gas also leads to homogeneous nucleation; however, measured sulphuric acid and ammonia concentration suggest that ternary nucleation of thermodynamically stable sulphate clusters is still likely to occur. In clear air, significant particle production (>10⁵ cm^-3) was observed with sulphuric acid gas-phase concentration as low as 2 × 10 ⁶ molecules cm^-3 and under polluted conditions as high as 1.2 × 10⁸ molecules cm^-3.