Particle size characteristics based on HR-ToF-AMS measurements at two suburban sites in the Pearl River Delta, China

Abstract

Aerosol mass spectrometer (AMS) has been used in various ambient measurements. Most of the studies have focused on source apportionment and the formation of secondary organic aerosols based on chemical compositions, with relatively little analysis of the size distribution data. In this work, particle size characteristics based on AMS sampling at two suburban sites, HKUST and Panyu district near Guangzhou, China in 2011 and 2014 respectively were systematically analyzed. The PToF data from AMS was fitted using two log-normal peaks, and were thus separated into Aitken mode (~200nm) and accumulation mode (~500nm). In the HKUST campaign, particle size characteristics during three different episode types were identified. The accumulation mode dominated (>98%) during the long-range transport episode type, while the contribution of the Aitken mode increased up to 21% during the two episode types with more local influence. Mixing state information was also reflected in the correlations of mode diameters between different species. High Pearson’s R values together with close-to-unity diameter ratios may indicate internal mixture in long-range transport episodes while poor correlations were likely caused by external mixtures in local episodes. In Guangzhou, local photochemical processing of aerosols was obvious in November. The shapes of size distributions in November were clearly bi-modal, with an obvious Aitken mode representing smaller particles which are likely formed locally. Morning peaks at around 8:30-11:30 a.m. of Aitken mode can be observed, while the peaks of accumulation mode appeared later and lasted longer from 10:00 a.m. to 3:30 p.m. By contrast, regional transport was the dominant sources of aerosols in December, while local production can still play a role during the episodic event. The shapes of size distributions in December were uni-modal and symmetric with little contribution of Aitken mode particles. Much better correlation between diameters of different species as well as close-to-unity diameter ratios suggested that different species were more likely internally mixed. During the December episode, however, high nighttime primary emission during the Christmas Eve and the subsequent daytime photochemical processing could be observed.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology