10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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New Particle Formation and Severe Haze Events in Beijing, China
ROHAN JAYARATNE, Buddhi Pushpawela, Lidia Morawska, Hui Li, Jian Gao, Queensland University of Technology, Brisbane, Australia
Abstract Number: 1026 Working Group: Remote/Regional Atmospheric Aerosol
Abstract An estimated 2.5 million people in China die each year from the health effects of indoor and outdoor air pollution. The concentration of airborne pollutants in China’s megacities can reach many hundreds of times that of European and American cities. Severe haze pollution events are routinely observed in Beijing over the winter months, with PM2.5 levels often exceeding 10 times the China National Ambient Air Quality Standard. The formation and evolution mechanisms of these haze events are not fully understood. In addition to the nature of the emission sources, it is known that there is a significant influence of meteorological conditions such as wind direction and speed, temperature and humidity. Some studies have shown that the haze particle concentrations are driven to a large extent by secondary aerosol formation.
We carried out a detailed study of the pollution in Beijing over three consecutive months from November 2015 to January 2016 using a suite of instrumentation to monitor particle number and mass concentration and a range of gaseous concentrations. Explosive haze events were observed on five occasions where the PM2.5 concentration increased ten-fold within a few hours and lasted at these high values for up to 2-3 days. The corresponding increase in particle number concentration was less than two-fold. Particle number size distributions were measured using a scanning mobility particle sizer (SMPS) and new particle formation (NPF) events were monitored with a neutral cluster and air ion spectrometer (NAIS). The SMPS and NAIS monitored particles in the size ranges 10-400 nm and 2-42 nm, respectively. The combination of these two instruments enabled a clear differentiation between NPF events and growth events. NPF events were observed only during the periods between the haze events when the winds arrived from the cleaner areas to the north of the city. The results suggest that weak southerly surface winds and high humidity were responsible for the severe haze pollution events, rather than an abrupt increase in emissions. Deeper temperature inversions and lower boundary layers contributed to stronger accumulation of pollution in the city.
There was some evidence that the secondary particles formed during NPF events contributed to the subsequent haze events, but the meteorological data suggests that the majority of the particles originated within the city, with a less significant contribution from the industrialized south and east. Investigation of the chemical composition of the particles revealed a significant increase in sulphates, nitrates, ammonium and potassium during the haze events, with only a marginal increase in chlorides, magnesium and calcium. Concentrations of oxides of nitrogen and carbon monoxide also showed a sharp increase at these times suggesting that the haze originated from combustion sources.
The NAIS also provided information on the charge carried by particles in the air. On a normal day, approximately 15% of particles were charged. During an NPF event, this fraction decreased to about 10%. In contrast, during haze episodes, it increased to between 20% and 30%. This is explicable in terms of the mean particle size; haze particles are much larger than secondary particles. It is known that the fraction of particles that are charged increases sharply with size. It should be noted that the reported fractions refer to particles that are smaller than 42 nm – the upper size cut-off of the NAIS. During haze events, as confirmed by the SMPS, a large proportion of the particles are larger than this size. The proportion of particles that are charged increases sharply with size and, therefore, it is to be expected that the actual fraction of all particles that are charged during a haze episode is much larger than 30%.