Patrick Rairoux

Mineral dust photochemistry induces nucleation events in the presence of SO2

Large quantities of mineral dust particles are frequently ejected into the atmosphere through the action of wind. The surface of dust particles acts as a sink for many gases, such as sulfur dioxide. It is well known that under most conditions, sulfur dioxide reacts on dust particle surfaces, leading to the production of sulfate ions. In this report, for specific atmospheric conditions, we provide evidence for an alternate pathway in which a series of reactions under solar UV light produces first gaseous sulfuric acid as an intermediate product before surface-bound sulfate. Metal oxides present in mineral dust act as atmospheric photocatalysts promoting the formation of gaseous OH radicals, which initiate the conversion of SO2 to H2SO4 in the vicinity of dust particles. Under low dust conditions, this process may lead to nucleation events in the atmosphere. The laboratory findings are supported by recent field observations near Beijing, China, and Lyon, France.

Atmospheric non‐spherical particles optical properties from UV‐polarization lidar and scattering matrix

[1]xa0In this contribution, the optical backscattering properties of atmospheric non-spherical particles are analyzed after long-range transport with a highly sensitive and accurate UV-polarization lidar. Far from the source region, the aerosol cloud is considered as a mixture of spherical (s) and non-spherical (ns) particles. Aerosols UV-depolarization serves as an independent means to discriminate ns from s-atmospheric particles. Vertical profiles of aerosols backscattering coefficient βa and UV-depolarization ratio δa are provided for two ns-particles case studies, on volcanic ash and desert dust, in the troposphere of Lyon (45.76°N, 4.83°E, France). Achieved polarization-sensitivity and accuracy allows tracing different atmospheric layers with a 75 m-altitude resolution. The depolarization ratio δa of the mixed (a) = {s, ns} aerosol cloud is then analyzed in the frame of the scattering matrix formalism. Observed δa-values, which range from a few to 38.5% (19.5%) for volcanic ash (desert dust) particles, only equal the intrinsic depolarization ratio of ns-particles when there is no detectable s-particle, and in the presence of s-particles, δa is always below δa,ns. By coupling our accurate lidar measurements with scattering matrix, we retrieved vertical profiles of backscattering coefficient, specific to ash (dust) particles, which is new. This ash (dust) specificity is then discussed within our error bars. We hence developed a methodology giving access to the number concentration vertical profile of specific particulate matter in the troposphere.

Sensitive and accurate dual-wavelength UV-VIS polarization detector for optical remote sensing of tropospheric aerosols

An UV-VIS polarization lidar has been designed and specified for monitoring aerosols in the troposphere, showing the ability to precisely address low particle depolarization ratios, in the range of a few percent. Non-spherical particle backscattering coefficients as low as 5×10−8xa0m−1⋅sr−1 have been measured and the particle depolarization ratio detection limit is 0.6xa0%. This achievement is based on a well-designed detector with laser-specified optical components (polarizers, dichroic beamsplitters) summarized in a synthetic detector transfer matrix. Hence, systematic biases are drastically minimized. The detector matrix being diagonal, robust polarization calibration has been achieved under real atmospheric conditions. This UV-VIS polarization detector measures particle depolarization ratios over two orders of magnitude, from 0.6 up to 40xa0%, which is new, especially in the UV where molecular scattering is strong. Hence, a calibrated UV-VIS polarization-resolved time-altitude map is proposed for urban and free tropospheric aerosols up to altitude of 4 kilometers, which is also new. These sensitive and accurate UV-VIS polarization-resolved measurements enhance the spatial and time evolution of non-spherical tropospheric particles, even in urban polluted areas. This study shows the capability of polarization-resolved laser UV-VIS spectroscopy to specifically address the light backscattering by spherical and non-spherical tropospheric aerosols.

LIDAR mapping of ozone-episode dynamics in Paris and intercomparison with spot analyzers: Supplementary material available at http ://link.springer.de/journals/apb

Abstract.Continuous mapping of an ozone episode in Paris in June 1999 has been performed using a differential absorption lidar system. The 2D ozone concentration vertical maps recorded over 33xa0h at the Champ de Mars are compiled in a video clip that gives access to local photochemical dynamics with unprecedented precision. The lidar data are compared over the whole period with point monitors located at 0-, 50-, and 300-m altitudes on the Eiffel Tower. Very good agreement is found when spatial resolution, acquisition time, and required concentration accuracy are optimized. Sensitivity to these parameters for successful intercomparison in urban areas is discussed.

UV polarization lidar for remote sensing new particles formation in the atmosphere.

Understanding new particles formation in the free troposphere is key for air quality and climate change, but requires accurate observation tools. Here, we discuss on the optical requirements ensuring a backscattering device, such as a lidar, to remotely observe nucleation events promoted by nonspherical desert dust or volcanic ash particles. By applying the Mie theory and the T-matrix code, we numerically simulated the backscattering coefficient of spherical freshly nucleated particles and nonspherical particles. We hence showed that, to remotely observe such nucleation events with an elastic lidar device, it should operate in the UV spectral range and be polarization-resolved. Two atmospheric case studies are proposed, on nucleation events promoted by desert dust, or volcanic ash particles. This optical pathway might be useful for climate, geophysical and fundamental purposes, by providing a range-resolved remote observation of nucleation events.

Lidar remote sensing of laser-induced incandescence on light absorbing particles in the atmosphere

Carbon aerosol is now recognized as a major uncertainty on climate change and public health, and specific instruments are required to address the time and space evolution of this aerosol, which efficiently absorbs light. In this paper, we report an experiment, based on coupling lidar remote sensing with Laser-Induced-Incandescence (LII), which allows, in agreement with Plancks law, to retrieve the vertical profile of very low thermal radiation emitted by light-absorbing particles in an urban atmosphere over several hundred meters altitude. Accordingly, we set the LII-lidar formalism and equation and addressed the main features of LII-lidar in the atmosphere by numerically simulating the LII-lidar signal. We believe atmospheric LII-lidar to be a promising tool for radiative transfer, especially when combined with elastic backscattering lidar, as it may then allow a remote partitioning between strong/less light absorbing carbon aerosols.

Polarization-resolved exact light backscattering by an ensemble of particles in air

We present the first experimental observation of exact backscattering of light by an ensemble of particles in ambient air. Our experimental set-up operates in the far-field single scattering approximation, covers the exact backscattering direction with accuracy (θ = π ± ε with ε = 3.5 × 10(-3) rad) and efficiently collects the particles backscattering radiation, while minimizing any stray light. Moreover, by using scattering matrix formalism, the observation of the particles UV-backscattering signal allowed to measure the particles depolarization of water droplets and salt particles in air, for the first time, in the exact backscattering direction. We believe this result may be useful for comparison with the existing numerical models and for remote sensing field applications in radiative transfer and climatology.

Gas concentration measurement by optical similitude absorption spectroscopy: methodology and experimental demonstration

We propose a new methodology to measure gas concentration by light-absorption spectroscopy when the light source spectrum is larger than the spectral width of one or several molecular gas absorption lines. We named it optical similitude absorption spectroscopy (OSAS), as the gas concentration is derived from a similitude between the light source and the target gas spectra. The main OSAS-novelty lies in the development of a robust inversion methodology, based on the Newton-Raphson algorithm, which allows retrieving the target gas concentration from spectrally-integrated differential light-absorption measurements. As a proof, OSAS is applied in laboratory to the 2ν3 methane absorption band at 1.66 µm with uncertainties revealed by the Allan variance. OSAS has also been applied to non-dispersive infra-red and the optical correlation spectroscopy arrangements. This all-optics gas concentration retrieval does not require the use of a gas calibration cell and opens new tracks to atmospheric gas pollution and greenhouse gases sources monitoring.

Error budget of the MEthane Remote LIdar missioN (MERLIN) and its impact on the uncertainties of the global methane budget.

MEthane Remote LIdar missioN (MERLIN) is a German-French space mission, scheduled for nlaunch in 2024 and built around an innovative light detecting and ranging instrument that will retrieve nmethane atmospheric weighted columns. MERLIN products will be assimilated into chemistry transport nmodels to infer methane emissions and sinks. Here the expected performance of MERLIN to reduce nuncertainties on methane emissions is estimated. A first complete error budget of the mission is proposed nbased on an analysis of the plausible causes of random and systematic errors. Systematic errors are spatially nand temporally distributed on geophysical variables and then aggregated into an ensemble of 32 scenarios. nObserving System Simulation Experiments are conducted, originally carrying both random and systematic nerrors. Although relatively small (±2.9 ppb), systematic errors are found to have a larger influence on MERLIN nperformances than random errors. The expected global mean uncertainty reduction on methane emissions ncompared to the prior knowledge is found to be 32%, limited by the impact of systematic errors. The nuncertainty reduction over land reaches 60% when the largest desert regions are removed. At the latitudinal nscale, the largest uncertainty reductions are achieved for temperate regions (84%) and then tropics (56%) and nhigh latitudes (53%). Similar Observing System Simulation Experiments based on error scenarios for nGreenhouse Gases Observing SATellite reveal that MERLIN should perform better than Greenhouse Gases nObserving SATellite for most continental regions. The integration of error scenarios for MERLIN in another ninversion system suggests similar results, albeit more optimistic in terms of uncertainty reduction.