Vertical distributions of aerosol microphysical and optical properties based on aircraft measurements made over the Loess Plateau in China

Aerosol microphysical properties, scattering and absorption characteristics, and in particular, the vertical distributions of these parameters over the eastern Loess Plateau, were analyzed based on aircraft measurements made in 2020 during a summertime aircraft campaign in Shanxi, China. Data from six flights were analyzed. Statistical characteristics and vertical distributions of aerosol concentration, particle size, optical properties, including aerosol scattering coefficient (Sigma sp), backscattering ratio (beta sc), Angstro spacing diaeresis m exponent (alpha), single-scattering albedo (SSA), partially-integrated aerosol optical depth (PAOD), and black carbon concentration (BCc), were obtained and discussed. Mean values of aerosol particle number concentration (Na), particle volume concentration (Va), mass concentration (Ma), surface concentration (Sa), and particle effective diameter (EDa) were 854.92 cm-3, 13.37 mu m3 cm- 3, 20.06 mu g/m3, 170.08 mu m3 cm- 3, and 0.47 mu m, respectively. Mean values of BCc, Sigma sp (450, 525, 635 nm), beta sp (525 nm), alpha(635/450), and SSA were 1791.66 ng m- 3, 82.37 Mm- 1 at 450 nm, 102.57 Mm- 1 at 525 nm, 126.60 Mm-1 at 635 nm, 0.23, 1.47, and 0.92, respectively. Compared with values obtained in 2013, Na decreased by 60% and Ma decreased by 45%, but the scattering coefficients increased in different degrees. In the vertical direction, aerosol concentrations were higher at lower altitudes, decreasing with height. Vertical profiles of Sigma sp, beta sp, alpha(635/450), and BCc measured during the six flights were examined. Two peaks in Na were identified near the top of the boundary layer and between 2000 and 2200 m. Fine particles with EDa smaller than 0.8 mu m are dominant in the boundary layer and coarse aerosols existed aloft. Aerosol scattering properties and BCc in the lowest layer of the atmosphere contributed the most to the total aerosol radiative forcing. SSA values were greater than 0.9 below 2500 m, with lower values at higher levels of the atmosphere. On lightly foggy days, SSA values were greater than 0.9, and aerosols played a cooling role in the atmosphere. On hazy days, lowerlevel SSA values were generally greater than 0.85, with aerosols likely having a warming effect on the atmosphere. 48-hour backward trajectories of air masses during the observation days showed that the majority of aerosol particles in the lower atmosphere originated from local or regional pollution emissions, contributing the most to the total aerosol loading and leading to high values of aerosol concentration and radiative forcing.

Automated summary

Based on aircraft measurements in the eastern Loess Plateau during a summer aircraft campaign in 2020, the aerosol microphysical properties and their vertical distributions were analyzed. The results showed that aerosol concentrations decreased with height, and the increase in scattering coefficients and black carbon concentration contributed the most to the total aerosol radiative forcing.