Hygroscopicity measurements of atmospheric aerosol using a humidified tandem differential mobility analyzer (HTDMA) at a suburban site in Hong Kong

Abstract

The importance of aerosols study arises from their influences on the global climate, visibility as well as human health. Atmospheric aerosol impacts the global climate via direct interaction with solar radiation and cloud modification by acting as cloud condensation nuclei (CCN). Hygroscopicity of aerosols governs their optical properties and CCN activity by altering their water content, size and chemical reactivity with changing relative humidity (RH) conditions. This study aims at characterizing the hygroscopicity of aerosols in Hong Kong and correlating the hygroscopic properties with varying chemical compositions, which are crucial for better prediction of aerosol climate forcing and visibility. A new commercial Humidified Tandem Differential Mobility Analyzer (BMI HTDMA) was deployed for hygroscopicity measurements at the HKUST Air Quality Research Supersite. The performance of the system was first evaluated with respect to DMA sizing accuracy and resolution, detection of bi-modal distribution and RH control for growth factor (GF) measurements. Real-time hygroscopic growth measurements were conducted starting May 2011. The GF at 90% RH were measured for four dry particle diameters between 75 and 200 nm in three individual campaigns (May up to 9^th, Sep and Nov 2011), covering springtime to early wintertime in 2011. Cluster analyses were performed to characterize the aerosol hygroscopicity based on the air parcel trajectories computed by the HYSPLIT model and the major chemical composition (sulfate, nitrate, ammonium, and organics) measured by the High-Resolution Time-of-Flight Aerosol Mass Spectrometer. Back trajectory analysis revealed that the air masses were mainly originated from the South China Sea (maritime) in the springtime and late summertime (early May and early Sep 2011). Maritime aerosol GF was higher (~1.6) than that of the continental aerosols of higher organic fractions (between 1.3 and 1.6) observed during the early autumn and early winter periods (late Sep and Nov 2011). GF reduction is attributed to the high organic fraction with the enhanced fraction of less oxygenated and less hygroscopic species. In the closure study, the measured GF values were compared against predictions based on chemical composition. Predictions on the water-uptake by inorganic species using the simplified ion pairing scheme and the E-AIM model were also compared. The E-AIM model was able to predict the presence of small water content in the sulfuric acid-containing aerosols under dry condition and gave a closer GF with the measured values than the ion pairing scheme did. For the organic fractions, three different approximations based on the degree of oxygenation were examined to estimate the GF of organic fraction (GF_org). However, all approximations could not yield better closure than using a constant GF_org of 1.18. The best fit GF_org ranged from 1.1 to 1.5 over the three separate measurements campaign, reflecting the large variability of the detected organic species in term of their hygroscopic properties.

Date of Award	2013
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology