Post Date: 15 April 2016

Size characteristics of fine particular matter (PM) during high pollution episodes based on high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) measurements at the HKUST Supersite

Abstract

Size characteristics of inorganic and organic components provide another dimension to reflect their origin, formation and atmospheric processing mechanisms. On the other hand, particle size also affects light scattering and absorption which influence visibility and the radiative properties of the atmosphere. While most traditional aerosol sizing instruments are not capable of differentiating particle components in size distributions, the high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) resolves broad chemical classes within size distributions and thus yields valuable additional information on the composition of differently sized particles in the accumulation and larger nucleation size range. Although there are many studies of aerosols using the AMS, the size analyses are usually on a monthly average basis which lack high enough resolution to reveal the dynamics of size changes within shorter period of time. In this study, the high resolution size distribution information available from the AMS are exploited to examine the size changes related to different aging mechanisms of aerosols under different meteorological conditions during high pollution episodes. 

The raw 10-min size distributions of organics and sulfate of the episodic dates were first grouped according to their belonging episode types then averaged over each type to attain the type-averaged size distributions. The grouped data was also averaged every 3 hour to attain the diurnal 3-hour size variations for each type of episode. The type-averaged and 3-hour averaged size distributions were then fitted by the peak fitting tool Multipeak Fit V2 as provided by Igor Pro (Wavemetrics) using two log normal peaks. The peaks were chosen such that the fit residuals were minimized. The two fitted peaks of the size distributions were referred to small particle size mode and large particle size mode. 

Fitted mode diameter for both small and large mode of organics remain constant across different episode types, while sulfate has constant small mode diameter in different types but slightly elevating large mode diameter in IR type episode. The decreasing small particle fraction from LWC type to IR type then to LRT type further support that fact that aerosols from long-range transported are more aged and dominated by large particle while episodes under more influences from local processes have higher portion of fresher small particles. Internal mixing of large particle were found in LWC and LRT types, while external mixing is more likely happened in freshly formed small particles in all episode types. Under gas-phase oxidation, aerosols undergo obvious size increase while under aqueous-phase oxidation, the size increase is much less drastic.