Ship-borne aerosol profiling with lidar over the Atlantic Ocean: from pure marine conditions to complex dust–smoke mixtures

Abstract

Abstract. The multi-wavelength Raman lidar Polly XT has been regularly operated aboard the research vessel Polarstern on expeditions across the Atlantic Ocean from north to south and vice versa. The lidar measurements of the RV Polarstern cruises PS95 from Bremerhaven, Germany, to Cape Town, Republic of South Africa (November 2015), and PS98 from Punta Arenas, Chile, to Bremerhaven, Germany (April/May 2016), are presented and analysed in detail. The latest set-up of Polly XT allows improved coverage of the marine boundary layer (MBL) due to an additional near-range receiver.Three case studies provide an overview of the aerosol detected over the Atlantic Ocean. In the first case, marine conditions were observed near South Africa on the autumn cruise PS95. Values of optical properties (depolarisation ratios close to zero, lidar ratios of 23 sr at 355 and 532 nm) within the MBL indicate pure marine aerosol. A layer of dried marine aerosol, indicated by an increase of the particle depolarisation ratio to about 10 % at 355 nm (9 % at 532 nm) and thus confirming the non-sphericity of these particles, could be detected on top of the MBL. On the same cruise, an almost pure Saharan dust plume was observed near the Canary Islands, presented in the second case. The third case deals with several layers of Saharan dust partly mixed with biomass-burning smoke measured on PS98 near the Cabo Verde islands. While the MBL was partly mixed with dust in the pure Saharan dust case, an almost marine MBL was observed in the third case.A statistical analysis showed latitudinal differences in the optical properties within the MBL, caused by the downmixing of dust in the tropics and anthropogenic influences in the northern latitudes, whereas the optical properties of the MBL in the Southern Hemisphere correlate with typical marine values. The particle depolarisation ratio of dried marine layers ranged between 4 and 9 % at 532 nm.Night measurements from PS95 and PS98 were used to illustrate the potential of aerosol classification using lidar ratio, particle depolarisation ratio at 355 and 532 nm, and Ångström exponent. Lidar ratio and particle depolarisation ratio have been found to be the main indicator for particle type, whereas the Ångström exponent is rather variable.

Novel aerosol extinction coefficients and lidar ratios over the ocean from CALIPSO–CloudSat: evaluation and global statistics

“…Interestingly, vast areas over the ocean feature AOD fractions of less than 40 %, suggesting a significant contribution of free tropospheric aerosols to the total AOD. These results are qualitatively consistent with the results of Bourgeois et al (2018) using CALIPSO version 4.1.…”

“…HSRL airborne measurements collected in September 2016 (Burton et al, 2018) show 532 nm LRs in the range 58-76 sr in the free troposphere, with CALIOP-SODA yielding values in the lower bound of the HSRL measurements (55-60 sr). In addition, shipborne Raman lidar observations south of the region dominated by biomass burning aerosols (30 • S, near the South African coast) reveal a transition from a lower troposphere dominated by smoke to one mainly composed of maritime aerosols (lidar ratios less than 25 sr; Bohlmann et al, 2018). This southward reduction in LR is reproduced by CALIOP-SODA.…”

How the Inhomogeneity of Wet Sea Salt Aerosols Affects Direct Radiative Forcing

“…Compared to the other types of aerosols, sea salt is the largest contributor to radiative forcing over oceans (Haywood et al, 1999;Murphy et al, 1998). Although significant progress has been made to assess the radiative effects of sea salts from various perspectives (Chi et al, 2015;Gong et al, 2002;Haywood et al, 1999;Monahan et al, 1986;Randles et al, 2004;Textor et al, 2006), the microphysical and optical properties of marine aerosols from laboratory, field measurements, and remote sensing have been continuously studied due to their importance in radiative transfer progresses of global atmospheric models (Bohlmann et al, 2018;Haarig et al, 2017;Zeng et al, 2013;Zieger et al, 2017). For regional climates, sea salt aerosols also have important impacts especially, over the high sea salt burden regions (Zakey et al, 2008).…”

“…Such treatment is believed to be reasonable, because the nonsphericity of sea salt only appears at low RHs, which is rare above open oceans. Haarig et al (2017) and Bohlmann et al (2018) studied dry and wet marine particles based on multiwavelength lidar By using the X-ray imaging technique, Zeng et al (2013) clearly studied the relationship between the inhomogeneity structure and the RHs.…”

Aerosol measurements with shipborne sun-sky-lunar photometer and collocated multiwavelength Raman polarization lidar over the Atlantic Ocean

“…From 1.7 km to 2.0 15 km, the particle depolarization ratio increased with peak value at 355 nm (532 nm) to be 0.09 (0.08) and RH decreased to 10 % according to the GDAS1 data. These are good indicators for dried sea salt particles (Haarig et al, 2017;Bohlmann et al, 2018).…”