10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Temporal Distribution of New Particle Formation Events in Brisbane, Australia
BUDDHI PUSHPAWELA, Rohan Jayaratne, Lidia Morawska, Queensland University of Technology, Brisbane, Australia
Abstract Number: 655 Working Group: Remote/Regional Atmospheric Aerosol
Abstract The formation of secondary particles in the atmosphere through homogeneous nucleation is known as new particle formation (NPF). This is one of the major sources of particles in the atmosphere and has been observed in different environments around the world including in urban, industrial, agricultural and coastal sites, as well as in boreal forests and the polar regions.
In this study, we collected data of charged and uncharged particle concentrations in the urban environment of Brisbane using a Neutral cluster and air ion spectrometer (NAIS) on nearly 500 days over three calendar years. The NAIS is able to measure particles down to their smallest size of 2 nm. The measurements were carried out at the Gardens Point Campus of the Queensland University of Technology in Brisbane, Australia. The main sources of atmospheric pollution at this site are motor vehicle exhaust emissions, emissions from the Brisbane port and oil refineries.
We identified NPF events using the rate of change of total particle concentration, dN/dt, where N is the number of particles in the size range 2.0 -10.0 nm. Events with N > 10,000 cm-3 for at least 1 hour and dN/dt >10,000 cm-3 h-1 were defined as “strong” NPF events. Events with 5000 < N < 10,000 cm-3 for at least 1 hour and 5000 < dN/dt < 10,000 cm-3 h-1 were classified as “weak” NPF events. These events generally exhibit a “banana” shape in the contour plot of particle number concentration (PNC) vs time.
Every NPF event was characterised by a sharp increase of the PNC in the intermediate size range from 2.0-7.0 nm. In this study, the starting time of a strong NPF event was determined by noting the time of first occurrence of dN/dt > 10,000 cm-3h-1, where N is the number of particles in the size range 2.0-10 nm. The starting time of a weak NPF event was determined by noting the time of first occurrence of dN/dt > 5000 cm-3h-1.
During the three-year period of monitoring, we acquired 485 complete days of data and observed 177 strong NPF events (an occurrence rate of 37%). We also observed 59 weak NPF events (an occurrence rate of 12%). These high occurrence rates suggest a possible link between NPF and high concentrations of gaseous precursors from motor vehicles and industry in the urban environment of Brisbane. Seasonal dependency on NPF showed that the probability of occurrence of NPF was highest during the summer (43.8% in November-February) and lowest during the winter (32.1% in May-August). However, the difference in mean temperature in Brisbane between summer and winter is not as great as in more temperate climates.
Further, we derived the first diurnal variation chart of NPF events anywhere in the world. We found that 74% of NPF events began during the morning, with a high likelihood occurrence between 8:00 and 8:30 am. This starting time coincides very well with the morning rush hour traffic when the production rate of gaseous precursors is generally at a maximum.
Another important observation from this study was that the NPF starting times were not dependent on season. There is a two-hour time difference in sunrise between winter and summer in Brisbane. Therefore, we conclude that NPF events in Brisbane are driven more by traffic density rather than by solar radiation intensity.