Abstract
Knowledge of hygroscopic properties is essential to prediction of the role of aerosol in cloud formation and lung deposition. Our objective was to introduce a new approach to classify and predict the hygroscopic growth factors (Gfs) of specific atmospheric sub-micrometre particle types in a mixed aerosol based on measurements of the ensemble hygroscopic growth factors and particle number size distribution (PNSD). Based on a non-linear regression model between aerosol source contributions from PMF applied to the PNSD data set and the measured Gf values (at 90% relative humidity) of ambient aerosols, the estimated mean Gf values for secondary inorganic, mixed secondary, nucleation, urban background, fresh, and aged traffic-generated particle classes at a diameter of 110 nm were found to be 1.51, 1.34, 1.12, 1.33, 1.09 and 1.10, respectively. It is found possible to impute (fill) missing HTDMA data sets using a Random Forest regression on PNSD and meteorological conditions.
Highlights
Processes affecting atmospheric aerosol and its effects on climate change are strongly dependent upon hygroscopic properties[1,2].Once a particle is emitted or formed in the atmosphere, it can grow or shrink in size by water vapour uptake due to its hygroscopicity, altering the scattering and absorption of solar radiation and changing the Earth’s radiation balance[3,4]
There is no significant difference between mean growth factors between summer and winter, but the peak mode of the nearlyhydrophobic and more-hygroscopic groups seems shifted to smaller Gf values in summer which was probably attributable to more atmospheric evaporation of hygroscopic components of aerosols or possibly more less-hygroscopic secondary organic aerosol (SOA) in summer than winter
Both an non-linear regression (NLR) model and a positive matrix factorisation (PMF) model applied to particle number size distribution (PNSD) and hygroscopicity data sets can predict well the hygroscopic growth factors of particles emitted from different sources without knowledge of chemical composition from each source
Summary
Processes affecting atmospheric aerosol and its effects on climate change are strongly dependent upon hygroscopic properties[1,2]. Once a particle is emitted or formed in the atmosphere, it can grow or shrink in size by water vapour uptake due to its hygroscopicity, altering the scattering and absorption of solar radiation and changing the Earth’s radiation balance[3,4]. Particle growth factor is considered as a vital parameter in determining the deposition of aerosols in the human respiratory system[2,7,8] as atmospheric aerosols at 200 nm can grow their size to more than double when exposed to a high relative humidity (>99.5%) in the human lung, enhancing their lung deposition efficiencies. The Gf is defined as the ratio between the particle diameter measured at a high relative humidity (RH) condition (dw, RH~ 90%) and a dry particle diameter measured at a low RH condition (dp, RH < 10%): GfðRHÞ
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