Relationship between relative humidity and the hygroscopic growth characteristics of mixed clusters comprising black carbon and ammonium sulfate

Abstract

A change in the relative humidity (RH) of the surrounding air significantly affects the target detection and sensor imaging capability of visible light and infrared sensors. In this study, Packmol, i.e. a program to facilitate molecular dynamics simulations, was used to construct mixed-cluster models of black carbon (BC) and ammonium sulphuric acid (AS). The extinction hygroscopic growth factors (f(RH, λ)) of the models were calculated in accordance with the Mie scattering and multi-sphere T-matrix theories. Consequently, the f(RH, λ) of BC was found to decrease with increasing RH in the near- and mid-infrared bands when the particle size was in the range of 20–100 nm. The f(RH, λ) of AS indicated an overall trend of increase; specifically, as RH increased, f(RH,0.55) increased at a faster rate. Additionally, the rate of change of f(RH, λ) decreased as the particle size increased. Regarding the mixed clusters, the internal particle distribution and composition significantly influenced f(RH, λ). However, increasing the wavelength gradually diminished the influence of the particle distribution. Lastly, there was a distinct f(RH, λ) peak in the far-infrared band when the proportion of BC was 10%; the maximum value of the peak was 14.3. This research can provide a theoretical basis for the development of methods to enhance target detection accuracy and imaging performance of visible light and infrared sensors.