Abstract
Natural mineral dust and heavy anthropogenic pollution and its complex interactions cause significant environmental problems in East Asia. Due to restrictions of observing technique, real-time morphological change in Asian dust particles owing to coating process of anthropogenic pollutants is still statistically unclear. Here, we first used a newly developed, single-particle polarization detector and quantitatively investigate the evolution of the polarization property of backscattering light reflected from dust particle as they were mixing with anthropogenic pollutants in North China. The decrease in observed depolarization ratio is mainly attributed to the decrease of aspect ratio of the dust particles as a result of continuous coating processes. Hygroscopic growth of Calcium nitrate (Ca(NO3)2) on the surface of the dust particles played a vital role, particularly when they are stagnant in the polluted region with high RH conditions. Reliable statistics highlight the significant importance of internally mixed, ‘quasi-spherical’ Asian dust particles, which markedly act as cloud condensation nuclei and exert regional climate change.
Highlights
Mineral dust is considered one of the key contributors to global aerosol loadings[1, 2], and it disturbs the atmospheric radiative balance[3, 4], affects nutrient supplies in the marine ecosystem[5, 6], and superimposes detrimental health impacts[7]
Theoretical calculation on ellipsoid particles indicated that depolarization ratio of particle decreased as its aspect ratio decrease, which provided the possibility to investigate the time-resolved morphological variability of dust particles as it mixed with anthropogenic pollutants
The hourly averaged depolarization ratio of the dust particles decreased by approximately 46%, with a mean of 0.34 ± 0.05, as dust particles were continuously mixed with secondary pollutants
Summary
Mineral dust is considered one of the key contributors to global aerosol loadings[1, 2], and it disturbs the atmospheric radiative balance[3, 4], affects nutrient supplies in the marine ecosystem[5, 6], and superimposes detrimental health impacts[7]. Theoretical calculation on ellipsoid particles indicated that depolarization ratio of particle decreased as its aspect ratio (defined as ratio of the longest dimension to its orthogonal width) decrease, which provided the possibility to investigate the time-resolved morphological variability of dust particles as it mixed with anthropogenic pollutants. There was no real-time measurement of morphological variations of dust particles in Beijing mega city, and the effect of neither chemical composition nor meteorology has been clarified. The northern half of the dust plume returned back and was stagnant over the Beijing area for approximately 3 days This made the continuous observation of the same dust plume throughout the event possible. We performed comprehensive measurements of the size distribution, polarization properties and chemical composition of aerosol particles. The purpose of this study is to quantitatively investigate morphological evolution of dust particles as mixing with pollutants, and to quantify the impact of both water-soluble inorganic (i.e. nitrate) and relative humidity (RH) on the formation of spherical dust particle in polluted area
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