| Atmospheric aerosols have great effects on ambient air quality, climate change and human health. With the rapid urbanization and development of industries, Guangzhou faces with high concentrations of atmospheric aerosols, which directly cause visibility impairment and deteriorate air quality. These effects depend on the physical and chemical properties of atmospheric aerosols, such as size distribution, chemical components, mixing state, hygroscopicity, and volatility. Volatility is an important physical property of aerosol particles. Some species may evaporate when aerosol particles are heated or diluted with less polluted air masses. Volatility can be used to apportion the original sources of particles in highly aged environments, and provide some information on the formation mechanism of secondary aerosols. In this study, a new method of measurement of aerosol particle volatility was developed, and then applied to Guangzhou aerosol during four seasons in 2013. Particle volatility in different seasons was studied. In addition, vehicle emission is the important source of urban PM2.5. The chemical characteristic of vehicle emission PM2.5 is very important for assessing the effectiveness of control policy issued by Guangdong Province.This paper focuses on four topics as follows:1) A new method of particle volatility measurement was developed using tandem thermodiluter-single particle aerosol mass spectrometry: The previous studies generally utilized thermodenuder(TD) to investigate the particles volatility. Using TD, semivolatile species are vaporized at different temperature, and the vaporized gas is adsorbed by activated charcoal. However, carbon might be emitted from activated charcoal under high temperature or activated charcoal ageing. In this study, a new method was developed for the measurement of particle volatility by coupling a thermodiluter system to an online single particle aerosol mass spectrometer(SPAMS). Laboratory tests show that the diluter can avoid the re-condensation of volatiles to the particles.2) The characteristics of aerosol particles volatility were quite different during four seasons in Guangzhou: Particles volatility in autumn was the highest among four seasons. The particle number remaining in autumn under the temperature of 75, 150, 300 °C were 0.780, 0.274, 0.041, lower than that in the other three seasons. Winter exhibited the lowest proportion of medium volatility(<150 °C volatile). Particle massspectrum characteristics under different temperatures shows that sulfate(97HSO4-), and nitrate(46NO2- and 62NO3-) were the main ions which changed greatly from unheated to 300 °C. Nitrate nearly volatilized under 150 °C, and sulfate volatilized under 300 °C. 26CN-, 42CNO- and carbon ion cluster peaks became more prominent under 300 °C. They are non-volatile, and could be regarded as the core of particles.3) In-depth analysis was done for the single particle volatility of Guangzhou spring aerosol. All the particles were general characterized into eight classes(EC, OCEC, Na K-EC, Metal-EC, OC, K-rich, Na K-rich, and Metal-rich) by Art-2a and manual classification. Particles types would make an infulence on the volatility of the main secondary species. The contribution of nitrate to the particle volatility in Metal-EC, Na K-EC, Metal-rich, Na K-rich particles are greater than that of sulfate; Instead, the contribution of sulfate to the particle volatility in EC, ECOC, OC, K-rich particles are greater than or comparable to that of nitrate. According to the volatility of these secondary species, compounds in different particle types would be partly infered. In the particles of Metal-EC, EC, K-rich and Na K-rich types, the volatilization loss rate under 300 °C exceeded 90%, indicating that the form of sulfate was high-volatile sulfate, including ammonium sulfate and ammonium hydrogen sulfate; while in the particles of ECOC, Na K-EC, and OC types, the volatilization loss rate under 300 °C was about 75% or even lower, indicating that the form of sulfate was low-volatile sulfate, including potassium sulfate and organic sulfate compounds.4) During the haze period, the volatility of particles would decaresed when compared to that in the sunny period. The content of most residual ions under 300 oC in haze period were 5-35% higher than that in sunny period. The reason could attribute to the lower relative humidity, which made the particle surface not benefit for absorbing the semi-volatile water-soluble compounds; higher NOx and O3 concentration, which counld influence the volatility of SOA.5) Characteristic of PM2.5 emited from on-road vehicle in the PRD region suggested that: emission factor of PM2.5 mass was 92.4 ± 8.9 mg vehicle-1 km-1; Emission factors of OC, EC, water-soluble inorganic ions(WSII), water-soluble organic carbon, and 27 metal elements were 16.7, 16.4, 9.06, 1.31, 14.0 mg vehicle-1 km-1;105 types of organic compound was also determined, includingn-alkanes, priority polycyclic aromatic hydrocarbons(PAHs), MW302 PAHs, Nitro-PAHs, Oxy-PAHs, hopanes and steranes, with the emission factors of 91.9, 5.02, 0.752, 0.095, 1.96, 32.0, 7.58 μg vehicle-1 km-1. Stable carbon isotopic composition δ13C value was-25.0‰ on average.An isotopic fractionation of 3.2‰ was found during fuel combustion.6) Compared with a previous study in Zhujiang Tunnel in year 2004, emission factors of PM2.5 mass, EC, OC, and major WSII decreased by 16.0~ 93.4%, which mostly attribute to control policy induced changes throughout the nine years from 2004 to 2013, that is, change of fleet composition, implementation of more stringent gasoline and diesel emission standards and improvement in fuel quality, clean fuel used for taxi and buses, and enforced replacement of high emission vehicles to environmental friendly ones that can attain to the more stringent standards. However, the total emissions of vehicle exhaust of PM2.5 mass increased by 25.2% during this 9-years period due to the growth of vehicle population, therefore more stringent emission and aggressive control policies are necessary. Additionally, the increase in emission of most metal elements should raise the awareness of the government, since metal elements, especially heavy metals could affect human health. |
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