Spectral dependence of light absorption and direct radiative forcing of the TSP, PM₁₀, PM_2.5 and PM_0.1 in a rural region of northwestern China

Black carbon (BC) and brown carbon (BrC) are major light absorbing components of aerosol, affecting visibility, radiative forcing balance and human health. In this study, we investigated the light absorption and radiative forcing of carbonaceous aerosol in the total suspended particle (TSP), coarse particle (PM₁₀: particulate matter with an aerodynamic diameter less than 10 μm, Dp ≤ 10 μm), fine particle (PM_2.5: Dp ≤ 2.5 μm), and nanoparticle (PM_0.1: Dp ≤ 0.1 μm) in a rural area of Guanzhong Plain, China. Similar variations of the absorption Ångstrom exponent (AAE) of TSP, PM₁₀, and PM_2.5 were observed. Lower light absorption coefficients and higher AAEs were obtained for PM_0.1 compared with other particle sizes. The direct radiative forcing (DRE) efficiency of BC decreased with size bins of TSP, PM₁₀, PM_2.5, and PM_0.1, respectively. The DRE of BCs for all particle sizes at top atmosphere (TOA), surface atmosphere (SUF) and the whole atmosphere (ATM) were estimated. The optical properties of primary and secondary BrC (PBrC and SBrC) in PM_0.1 were further analyzed. The AAE values indicated that the light absorbing of SBrC was more wavelength dependent than PBrC in PM_0.1. The DRE of BC, PBrC, and SBrC in PM_0.1 were estimated firstly with the values of 19.9 Wm^-2, 2.1 Wm^-2, and 1.1 Wm^-2 in the ATM, respectively.