Spectral dependence of light absorption and direct radiative forcing of rural carbonaceous aerosol in TSP, PM10, PM2.5, and PM0.1 in 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 (PM2.5: Dp≤2.5 μm), and nanoparticle (PM0.1: Dp≤0.1 μm) in a rural area of Guanzhong Plain, China. Similar variations of light absorption coefficients and the absorption Ångstrom exponent (AAE) of TSP, PM10, and PM2.5 were observed. Lower light absorption coefficients and higher AAEs were obtained for PM0.1 compared with other particle sizes. The direct radiative forcing (DRE) efficiency of BC decreased with size bins of TSP, PM10, PM2.5, and PM0.1, respectively. The DRE of BCs for all particle sizes at top atmosphere (TOA), surface atmosphere (SUF) and the whole atmosphere (ATM) were estimated, and the levels in TSP were ~3.7 times higher than those in PM0.1. The optical properties of primary and secondary BrC (PBrC and SBrC) in PM0.1 were further analyzed. The levels of AAEs indicated that the light absorbing of SBrC was more wavelength dependent than PBrC in PM0.1. The DRE of BC, PBrC, and SBrC in PM0.1 were estimated firstly with the values of 19.9, 2.1, and 1.1 Wm-2 in the ATM, respectively.