Midlevel cloud layers (2–7 km) pertinent to transported Asian dust layers were observed on 26 occasions from the polarization lidar measurements during the period from December 2012 to December 2013 over Wuhan (30.5°N, 114.4°E), China. The events mainly occurred in winter. Cloud layers were mostly detected (19 out of 26) on top of the aloft Asian dust layers. The boundary between cloud layer and dust layer appeared at altitudes of 4–7 km. The observed events are classified into three categories: (1) category A (6 cases), supercooled liquid cloud layers persistently existed without an evident sign of freezing; (2) category B (9 cases), supercooled liquid cloud appeared initially and then ice crystals formed on the boundary between the cloud layer and dust layer; (3) category C (11 cases), only a cloud containing ice crystals was present throughout while the pure liquid stage did not present previously, indicating that deposition nucleation might take place. Cloud top temperatures (CTTs) for heterogeneous ice formation ranged from −38.5°C to −18.3°C; no ice formation occurred with CTT warmer than −15°C. After thousands of kilometers transport, ice nucleation efficiency of Asian dust over Wuhan substantially did not present a distinct change compared with that of pure dust.

Aerosol–cloud interactions (ACIs) are the largest contributor to the uncertainty in the global radiation budget. To improve the current consideration of ACIs in global circulation models, it is necessary to characterize the 3-D distribution of dust-related cloud condensation nuclei concentration (CCNC) and ice-nucleating particle concentration (INPC) globally. This can potentially be realized using the POlarization LIdar PHOtometer Networking (POLIPHON) method together with spaceborne lidar observations. However, dust-related conversion factors that convert bulk aerosol optical properties from lidar measurements to aerosol microphysical properties are still less constrained in many regions, which limits the applications of the POLIPHON method. Here we retrieve the essential dust-related conversion factors at remote oceanic and coastal sites using the historical AErosol RObotic NETwork (AERONET) database. Depolarization-ratio-based dust ratios R_d at 1020 nm are applied to identify the dust-occurring cases, thus enabling us to contain fine-mode dust-dominated cases (after the preferential removal of large-sized dust particles during transport), study the evolution of dust microphysical properties along the transoceanic pathway, and mitigate occasional interference of large-sized marine aerosols. The newly proposed scheme is proven to be valid and feasible by intercomparisons with previous studies at nine sites in/near deserts. The dust-related conversion factors are calculated at 20 oceanic and coastal sites using both pure dust (PD) and PD plus dust-dominated mixture (PD+DDM) datasets. At nearly half of the sites, the conversion factors are solely calculated using the PD datasets, while at the remaining sites, the participation of DDM datasets is required to ensure a sufficient number of data for the calculation. Evident variation trends in conversion factors are found for c_v,d (extinction-to-volume concentration, gradually decreasing), c_250,d (extinction-to-particle (with a radius > 250 nm) number concentration, gradually increasing), and c_s,d (extinction-to-surface-area concentration, gradually decreasing) along both the transpacific and transatlantic dust transport pathways. The retrieved dust-related conversion factors are anticipated to inverse 3-D dust-related CCNC and INPC distributions globally, thereby improving the understanding of ACIs in atmospheric circulation models.

Conventional lidar methods for deriving particle optical properties suffer from the fact that two unknowns (backscatter and extinction coefficients) need to be determined from only one lidar equation. Thus, additional assumptions (constant lidar ratio or Ångström relationship) have to be introduced to settle this problem. In contrast, a single-line-extracted pure-rotational-Raman (PRR) lidar method allows the strict retrieval of backscatter and extinction coefficients without additional assumptions. Based on the observations of our single-line-extracted PRR lidar from February 2016 to December 2017, the optical properties (backscatter coefficient, extinction coefficient and lidar ratio) of continental polluted aerosols, dust aerosols, and cirrus cloud particles over Wuhan (30.5°N, 114.4°E) are well characterized. The mean values of the measured lidar ratios are respectively 60 ± 7 sr for continental polluted aerosols, 47 ± 4 sr for dust aerosols and 22 ± 4 sr for cirrus cloud particles. The backscatter and extinction coefficients measured by the single-line-extracted PRR lidar deviate as a whole by 7-13% and 13-16%, respectively, from those retrieved by the traditional Fernald method. The optical properties measured by the single-line-extracted PRR lidar can serve as observational standards for particle optical properties (backscatter/extinction coefficient and lidar ratio) at 532 nm wavelength.

The POLIPHON (polarization lidar photometer networking) method is a powerful pathway to retrieve the height profiles of dust-related particle mass and ice-nucleating particle (INP) concentrations. The conversion factors fitted from the sun photometer observation data are the major part of the POLIPHON computations, which can convert the polarization-lidar-derived dust extinction coefficients into dust-related particle mass and INP concentrations. For the central Chinese megacity of Wuhan (30.5^∘ N, 114.4^∘ E), located at the downstream area several thousands of kilometers far away from the source regions of Asian dust, dust particles always mix with other aerosols from local emissions. Therefore, very few dust case data sets can be available when using the column-integrated Ångström exponent (for 440–870 nm) <0.3 and aerosol optical depth (at 532 nm) >0.1 recorded by a sun photometer as the filtering criteria. Instead, we present another dust case data set screening scheme that applies the simultaneous polarization lidar observation to verify the occurrence of dust. Based on the 33 dust-intrusion days identified during 2011–2013, the extinction-to-volume (c_v,d) and extinction-to-large particle (with radius >250 nm) number concentration (c_250,d) conversion factors are determined to be $(\mathrm{0.52}\pm \mathrm{0.12})\times {\mathrm{10}}^{-\mathrm{12}}\mathrm{Mm}{\mathrm{m}}^{\mathrm{3}}{\mathrm{m}}^{-\mathrm{3}}$ and 0.19±0.05 Mm cm⁻³, respectively. The c_250,d for Wuhan is 27 % larger than that observed at Lanzhou SACOL (36.0^∘ N, 104.1^∘ E), a site closer to the Gobi Desert, and tends to be closer to those observed in North Africa and the Middle East, indicating dust aerosols from these two sources are also possibly involved in the dust events observed over Wuhan. As a comparison, the conversion factor c_290,c of $\mathrm{0.11}\pm \mathrm{0.02}\mathrm{Mm}{\mathrm{cm}}^{-\mathrm{3}}$ for continental aerosol is much smaller than c_250,d, indicating that there is no significant influence of urban aerosols on the retrievals of dust-related conversion factor over Wuhan. The conversion factors are applied in a dust event in Wuhan to reveal the typical dust-related immersion-mode INP concentration over East Asian cities. The proposed dust case data set screening scheme may potentially be extended to the other polluted city sites that are more influenced by mixed dust.