Articles | Volume 18, issue 15
Research article
10 Aug 2018
Research article |  | 10 Aug 2018

Differentiating between particle formation and growth events in an urban environment

Buddhi Pushpawela, Rohan Jayaratne, and Lidia Morawska

Abstract. Small aerosols at a given location in the atmosphere often originate in situ from new particle formation (NPF). However, they can also be produced and then transported from a distant location to the point of observation where they may continue to grow to larger sizes. This study was carried out in the subtropical urban environment of Brisbane, Australia, in order to assess the relative occurrence frequencies of NPF events and particle growth events with no NPF. We used a neutral cluster and air ion spectrometer (NAIS) to monitor particles and ions in the size range 2–42 nm on 485 days, and identified 236 NPF events on 213 days. The majority of these events (37 %) occurred during the daylight hours with just 10 % at night. However, the NAIS also showed particle growth with no NPF on many nights (28 %). Using a scanning mobility particle sizer (SMPS), we showed that particle growth continued at larger sizes and occurred on 70 % of nights, typically under high relative humidities. Most particles in the air, especially near coastal locations, contain hygroscopic salts such as sodium chloride that may exhibit deliquescence when the relative humidity exceeds about 75 %. The growth rates of particles at night often exceeded the rates observed during NPF events. Although most of these night time growth events were preceded by day time NPF events, the latter was not a prerequisite for growth. We conclude that particle growth in the atmosphere can be easily misidentified as NPF, especially when they are monitored by an instrument that cannot detect them at the very small sizes.

Short summary
New particle formation (NPF) is a common occurrence in urban environments. Using a 500-day dataset obtained in Brisbane, Australia, we observed that NPF events occurred on 37 % of days and 10 % of nights. However, particle growth occurred on 70 % of nights. We show that when particles are measured by an instrument with a detection limit of 3 nm or above, particle growth events may be easily misidentified as NPF events. This can lead to an overestimation of the occurrence of NPF events.
Final-revised paper