Abstract. Depolarization ratio (δ) of backscattered light is an
applicable parameter for distinguishing the sphericity of particles in real time, which
has been widely adopted by ground-based lidar observation systems. In this study, δ values of particles and chemical compositions in both PM2.5 (aerodynamic diameter
less than 2.5 µm) and PM10 (aerodynamic diameter less than
10 µm) were concurrently measured on the basis of a bench-top optical particle
counter with a polarization detection module (POPC) and a continuous dichotomous aerosol
chemical speciation analyzer (ACSA-14) from November 2016 to February 2017 at an urban
site in Beijing megacity. In general, measured δ values depended on both size and
sphericity of the particles. During the observation period, mass concentrations of
NO3- in PM2.5 (fNO3) were about an order of
magnitude higher than that in PM2.5−10 (cNO3) with a mean
fNO3∕cNO3 ratio of 14±10. A relatively low
fNO3∕cNO3 ratio (∼5) was also observed under higher
relative humidity conditions, mostly due to heterogeneous processes and particles in the
coarse mode. We found that δ values of ambient particles in both PM2.5 and
PM2.5−10 obviously decreased as mass concentration of water-soluble species
increased at unfavorable meteorological conditions. This indicated that the morphology of
particles was changed as a result of water-absorbing processes. The particles
with optical size (Dp) of Dp = 5 µm were used to represent mineral dust
particles, and its δ values (δDp=5) decreased by 50 % as
the mass fraction of cNO3 increased from 2 % to 8 % and
ambient relative humidity increased up to 80 %, suggesting that mineral dust
particles were likely to be spherical during humid pollution episodes. During the
observation, relative humidity inside the POPC measuring chamber was stable at 34±2 %, lower than the ambient condition. Its influence on the morphology was estimated
to be limited and did not change our major conclusion. This study highlights the evident
alteration of non-sphericity of mineral dust particles during their transport owing to a
synergistic effect of both pollutant coatings and hygroscopic processes, which plays an
important role in the evaluation of its environmental effect.
A water-soluble boronic acid sensor for caffeic acid based on double sites recognition
