Systematic characterization of aerosol over the oceans is needed to understand the aerosol effect on climate and on transport of pollutants between continents. We report the results of a comprehensive optical and physical characterization of ambient aerosol in 5 key island locations of the Aerosol Robotic Network (AERONET) of sun and sky radiometers, spanning over 2-5 years. The results are compared with aerosol optical depths and size distributions reported in the literature over the last 30 years. Aerosol found over the tropical Pacific Ocean (at 3 sites between 20°S and 20°N), still resembles mostly clean background conditions dominated by maritime aerosol. The optical thickness is remarkably stable with mean value of τ a (500 nm)=0.07, mode value at τ am =0.06 and standard deviation of 0.02 to 0.05. Average Ångström exponent ranged from 0.3 to 0.7 characterizes the wavelength dependence of the optical thickness. Over the tropical to subtropical Atlantic (2 stations at 7°S and 32°N) the optical thickness is significantly higher: τ a (500 nm)=0.14 and τ am =0.10 due to the frequent presence of dust, smoke and urban/industrial aerosol. For both oceans the atmospheric column aerosol is characterized by a bimodal log-normal size distribution with a fine mode at effective radius R eff =0.11±0.01 µm and coarse mode at R eff =2.1±0.3 µm. A review of the published 150 historical ship measurements from the last three decades shows that τ am was around 0.07 to 0.12 in general agreement with the present finding. The information should be useful as a test-bed for aerosol global models and aerosol representation in global climate models. With global human population expansion and industrialization, these measurements can serve in the 21st century as a basis to assess decadal changes in the aerosol concentration, properties and radiative forcing of climate.

Abstract. Ground-based remote sensing observatories have a crucial role to play in providing data to improve our understanding of atmospheric processes, to test the performance of atmospheric models, and to develop new methods for future space-borne observations. Institut Pierre Simon Laplace, a French research institute in environmental sciences, created the Site Instrumental de Recherche par Télédétection Atmosphérique (SIRTA), an atmospheric observatory with these goals in mind. Today SIRTA, located 20 km south of Paris, operates a suite a state-of-the-art active and passive remote sensing instruments dedicated to routine monitoring of cloud and aerosol properties, and key atmospheric parameters. Detailed description of the state of the atmospheric column is progressively archived and made accessible to the scientific community. This paper describes the SIRTA infrastructure and database, and provides an overview of the scientific research associated with the observatory. Researchers using SIRTA data conduct research on atmospheric processes involving complex interactions between clouds, aerosols and radiative and dynamic processes in the atmospheric column. Atmospheric modellers working with SIRTA observations develop new methods to test their models and innovative analyses to improve parametric representations of sub-grid processes that must be accounted for in the model. SIRTA provides the means to develop data interpretation tools for future active remote sensing missions in space (e.g. CloudSatCorrespondence to: M. Haeffelin (martial.haeffelin@lmd.polytechnique.fr) and CALIPSO). SIRTA observation and research activities take place in networks of atmospheric observatories that allow scientists to access consistent data sets from diverse regions on the globe.

Abstract. Long-range-transported Canadian smoke layers in the stratosphere over northern France were detected by three lidar systems in August 2017. The peaked optical depth of the stratospheric smoke layer exceeds 0.20 at 532 nm, which is comparable with the simultaneous tropospheric aerosol optical depth. The measurements of satellite sensors revealed that the observed stratospheric smoke plumes were transported from Canadian wildfires after being lofted by strong pyro-cumulonimbus. Case studies at two observation sites, Lille (lat 50.612, long 3.142, 60 m a.s.l.) and Palaiseau (lat 48.712, long 2.215, 156 m a.s.l.), are presented in detail. Smoke particle depolarization ratios are measured at three wavelengths: over 0.20 at 355 nm, 0.18–0.19 at 532 nm, and 0.04–0.05 at 1064 nm. The high depolarization ratios and their spectral dependence are possibly caused by the irregular-shaped aged smoke particles and/or the mixing with dust particles. Similar results are found by several European lidar stations and an explanation that can fully resolve this question has not yet been found. Aerosol inversion based on lidar 2α+3β data derived a smoke effective radius of about 0.33 µm for both cases. The retrieved single-scattering albedo is in the range of 0.8 to 0.9, indicating that the smoke plumes are absorbing. The absorption can cause perturbations to the temperature vertical profile, as observed by ground-based radiosonde, and it is also related to the ascent of the smoke plumes when exposed in sunlight. A direct radiative forcing (DRF) calculation is performed using the obtained optical and microphysical properties. The calculation revealed that the smoke plumes in the stratosphere can significantly reduce the radiation arriving at the surface, and the heating rate of the plumes is about 3.5 K day−1. The study provides a valuable characterization for aged smoke in the stratosphere, but efforts are still needed in reducing and quantifying the errors in the retrieved microphysical properties as well as radiative forcing estimates.