Characteristics of dust clouds

Our observation location was affected frequently by outside aerosols such as dust storms from Northern China which mostly appear during the spring seasons.13-15 The term "dust cloud" has been used instead of using "dust layer" because we have measured higher values of backscattering ratio of the dust as compared with other literature. We think that this phenomenon is related with our location as it is of low latitude, where atmosphere is more humid and the dust particles can easily interact with humidity and pollution.12 Figure 9 shows one of the dust-cloud case on 31 March 2002, which is observed over our lidar site during the period (spring season) of dust storm which break out in Northern China. The peaks of BR of this dust cloud are at 1.8 and 2.1km with RH around 100% and 80%, and depolarization ratios are 0.27 and 0.30, respectively. The upper layer, which is around 2.1km has lower BR (^ 12.5) with RH (^80%), but higher DR (0.30). The result shows that the dust clouds, which are measured by lidar during spring seasons, accompanied with BGA and water clouds have higher extinction coefficient and DR.

Depolarization Ratio (8 a) 0.00 0.05 0.10 0.15 0.20 0.25 0.30

Depolarization Ratio (8 a) 0.00 0.05 0.10 0.15 0.20 0.25 0.30

Backscattering Ratio

Fig. 9. The height distribution of backscattering ratio (—), depolarization ratio (—), and relative humidity (Q) for a dust case on 31 March 2002.

Backscattering Ratio

Fig. 9. The height distribution of backscattering ratio (—), depolarization ratio (—), and relative humidity (Q) for a dust case on 31 March 2002.

It is worthy to notice that mineral dust does not absorb water as they were primarily hydrophobic, but from the result (Fig. 9), the DR shows the inverse relation with RH at 1.8 km, and 2.1km. The reason is that these dust cloud layers become hydrophilic as dust aerosols are modified during the long-range transport through chemical reactions by mixing with sulfate, sea salt, and other pollutants' activities with ambient humidity to act as giant CCN (cloud condensation nuclei) to form clouds.16'17

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