10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Assessment of the Influence of Cut-Off Shift Due to Particle Hygroscopic Growth on the Analysis of Its Chemical Composition
YING CHEN, Oliver Wild, Yu Wang, Liang Ran, Monique Teich, Johannes Größ, Lina Wang, Gerald Spindler, Hartmut Herrmann, Dominik van Pinxteren, Gordon McFiggans, Alfred Wiedensohler, Lancaster Uni. and TROPOS
Abstract Number: 1003 Working Group: Aerosol Chemistry
Abstract Particulate matter is of major concern in the world nowadays, due to its effects on public health and climate. The chemical composition of particles is a crucial factor in these effects. Filter sampling is one of the major ways to measure aerosol compositions. However, the cut-off size for particle sampling shifts in different ambient conditions due to hygroscopic growth. This leads to difficulties in consistent comparisons between measurements under different conditions (spatial and temporal). This problem cannot be easily solved by drying airflow, since there is no efficient way to dry it for a high-volume aerosol sampler; moreover, the drying or heating processes change the gas-particle equilibrium and lead to a loss of semi-volatile compounds (e.g. nitrate and secondary organic aerosol). Here, we propose a method to assess this influence based on κ-Köhler theory (Petters and Kreidenweis, 2007). A global perspective on this influence is reported. This study is important for a consistent analysis of the long-term characteristics and spatial distributions of particle chemical compositions, and brings more confidence to model validation.
To explore the cut-off shift in filter-based particle sampling due to hygroscopic growth, we calculate the aerodynamic diameter of ambient particles based on the hygroscopic growth factor (GF) of size. The GFs are derived from κ-Köhler theory, with consideration of particle composition and size as well as meteorological conditions. The κ value, representing the dependence of particle hygroscopicity on chemical composition, can be estimated with composition measurements following the Zdanovskii-Stokes-Robinson mixing rule and Liu et al. (2014). Here, we adopt the previous reported global distributions of κ value (Pringle et al., 2010), relative humidity (RH, Dai, 2006) and temperature (Willett et al., 2014) to estimate the GF of particles in different sizes. The particle volume size distributions (PVSD) of eight different aerosol types (Whitby, 1978) are used to assess the influence of cut-off shift on chemical analysis.
Generally, European background sites seem to be more greatly influenced by the cut-off shift than those in other continents, with 10-20% in median and may reach 50-60% in some cases when hygroscopic particles present in humid air mass. However, the influence is generally much smaller (less than 7%) at all urban sites, and is negligible for hydrophobic dust particles. Sea salt aerosol (SSA) experiences the largest influence (about 50% in median), resulting from their large size, high hygroscopicity and high RH in marine air masses. A difference of more than 30% in the influence on SSA sampling can be observed between relatively dry and humid conditions. This work highlights importance of considering cut-off shift in chemical analyses when compare between different spatial/temporal conditions.
References: [1] Dai, A., (2006) Journal of Climate, 19, 3589-3606. [2] Liu, H. J., et al. (2014) Atmos. Chem. Phys., 14, 2525-2539. [3] Petters, M. D., and Kreidenweis, S. M. (2007) Atmos. Chem. Phys., 7, 1961-1971. Pringle, K. J., et al. (2010) Atmos. Chem. Phys., 10, 5241-5255, 2010. [5] Whitby, K. T. (1978) Atmos. Env., 12, 135-159. [6] Willett, K. M., et al. (2014) Clim. Past, 10, 1983-2006.