Dry versus wet marine particle optical properties: RH dependence of depolarization ratio, backscatter, and extinction from multiwavelength lidar measurements during SALTRACE

Abstract

Triple-wavelength lidar observations of the depolarization ratio and the backscatter coefficient of marine aerosol as a function of relative humidity (RH) are presented with a 5 min time resolution. The measurements were performed at Barbados (13 • N, 59 • W) during the Saharan Aerosol Long-range Transport and Aerosol-Cloud interaction Experiment (SALTRACE) winter campaign in February 2014. The phase transition from spherical sea salt particles to cubic-like sea salt crystals was observed with a polarization lidar. The radiosonde and water-vapor Raman lidar observations show a drop in RH below 50 % in the marine aerosol layer simultaneously with a strong increase in particle linear depolarization ratio, which reaches values up to 0.12 ± 0.08 (at 355 nm), 0.15 ± 0.03 (at 532 nm), and 0.10 ± 0.01 (at 1064 nm). The lidar ratio (extinction-to-backscatter ratio) increased from 19 and 23 sr for spherical sea salt particles to 27 and 25 sr (at 355 and 532 nm, respectively) for cubic-like particle ensembles. Furthermore the scattering enhancement due to hygroscopic growth of the marine aerosol particles under atmospheric conditions was measured. Extinction enhancement factors from 40 to 80 % RH of 1.94 ± 0.94 at 355 nm, 3.70 ± 1.14 at 532 nm, and 5.37 ± 1.66 at 1064 nm were found. The enhanced depolarization ratios and lidar ratios were compared to modeling studies of cubic sea salt particles.

Hygroscopic growth study in the framework of EARLINET during the SLOPE I campaign: synergy of remote sensing and in situ instrumentation

“…This spectral dependency has also been reported in Kotchenruther et al (1999) for in situ measurements at 450, 550, and 700 nm, obtaining increasing enhancement factors with wavelength, and in Zieger et al (2013), where the same behavior is observed for marine aerosols. As is reported in Haarig et al (2017), the enhancement factor dependency with wavelength suggests that larger wavelengths have an enhancement factor larger than short ones, which in fact was also evidenced in this work.…”

“…A decrease in AE 355−532 nm (∼ 0.4 km −1 ) means an increase in the predominance of larger particles, and a decrease in the PLDR 532 nm (∼ 0.13 km −1 ) is related to particles becoming more spherical. This correlation between AE 355−532 and PLDR has been observed in previous studies associated with hygroscopic growth (Granados-Muñoz et al, 2015;Haarig et al, 2017).…”

“…Moreover, a decreasing Ångström exponent (AE) and particle linear depolarization ratio (PLDR) are related to larger and more spherical particles, which also points to aerosol water uptake (GranadosMuñoz et al, 2015;Fernández et al, 2015;Haarig et al, 2017). However, additional constraints need to be fulfilled when studying the aerosol hygroscopic growth in the atmosphere by remote sensing techniques due to the lack of control over the environmental conditions, as opposed to in situ measurements.…”

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

“…During PS98 at 5 • S, 2500 km from the African coast, the lidar ratio between 1.5 and 2.4 km is around 35 sr, which is considerably lower than the characteristic values of pure dust (S 532 ≈ 55 sr; Tesche et al, 2011a). This may result from marine aerosol transported upward by turbulent mixing processes (Haarig et al, 2017).…”

“…Murayama et al (1999) measured high depolarisation ratios (≈ 10 %) at 532 nm in the lower atmosphere associated with sea breeze events in the coastal area of Tokyo Bay. During the Saharan Aerosol Long-range Transport and Aerosol-CloudInteraction Experiment (SALTRACE) winter campaign 2014 at Barbados, Haarig et al (2017) detected an increase of the particle depolarisation ratio up to 12 % at 355 nm, 15 % at 532 nm, and 10 % at 1064 nm when the RH drops below 50 %. Sakai et al (2000) observed low depolarisation ratios (< 5 %) at 532 nm over a wide range of relative humidities, whereas δ par > 10 % was measured at low RH (< 50 %) in air masses which had passed over the Pacific Ocean.…”