Evolution of Altitude-Resolved Particle Properties within the 2020 Godzilla Dust Plume As Revealed By CALIPSO Lidar Profiling

To characterize how dust particle size and shape evolve during the long-range transport, it is ideal to follow the full life cycle of a dust plume on an event basis, particularly for intense dust events when observations tend to have high accuracy. This is only made possible by routine and large-scale satellite measurements because aircraft and ship cruises usually cannot follow the movement of same dust plume at the desired spatial and temporal scales. CALIPSO altitude-resolved observations are particularly useful because the data allow for following dust plumes in the free atmosphere (FA) where the interference of marine aerosol is minimized. This can help reduce the ambiguity as revealed by passive measurements of columnar aerosol. In the mid-June of 2020, a record-breaking Godzilla dust plume was formed by haboobs and a unique synoptic system over West Africa. The closed atmospheric circulation accumulated and recirculated dust plumes in the coastal region for about four days till a strong African Easterly Jet erupted to carry enormous amount of dust across the Atlantic Ocean to reach the southern U.S. (Yu et al., Atmos. Chem. Phys., 2021). In this follow-on study we use CALIPSO lidar observations to characterize vertical distributions of particulate depolarization ratio (PDR) at 532 nm and backscatter color ratio (CR, 1064nm/532nm) along the trans-Atlantic transit of the Godzilla dust plume. Both PDR and CR depend on particle size, albeit with different sensitivity. The analysis shows that PDR in the FA (above 2 km) is largely independent of altitude with a value of about 0.3, except near the very top of plume. Within the FA plume, sub-micron fine dust accounts for about 20% (16-24%) of total dust backscatter. However, CR in the Godzilla dust plume increases with increasing altitude, apparently suggesting an increase of particle size with altitude. This unique CR vertical profile is also different from that in another weaker dust plume just a week later. Based on a back trajectory analysis, this seemingly counter-intuitive CR profile in the Godzilla dust plume reflects that the dust particles at different altitudes have come from distinct air mass with different residence time and hence different size distribution. The dust at high altitudes came from the recirculated dust plumes containing smaller particles due to preferential settling of large particles, while the dust at lower altitudes came from relatively fresh dust plumes containing coarser particles. We also found that both PDR and CR profiles do not show discernible changes during the transport from African coast to the Caribbean. The CALIPSO observations of particle size change during the trans-Atlantic transport are consistent with particle size parameters (e.g., fine-mode fraction and extinction Angstrom exponent) retrieved from MODIS and AERONET observations.