Earth Observation Data Group

Abstract:

AbstractThe high‐energy‐density synthesis of NxOy species is simulated in gas mixtures representing an O2‐free early‐Earth atmosphere by terawatt‐kilojoule‐class laser‐induced dielectric breakdown (LIDB). These experiments differ from previous LIDB experiments due to the 100 times greater energy delivered per pulse and sensitive analysis of products by high‐resolution infrared spectroscopy. The measured yields of NO, N2O, and NO2 are 0.08–8 × 1015, 5 × 1012, and 0.03–7 × 1014 molec J −1. The high N2O yield is above the upper‐limit constraint of previous tabletop LIDB experiments and the expected yield of a thermochemical freeze‐out at any temperature between 2000 and 5000 K, while the NO and NO2 yields are in broad agreement with freeze‐out models. Using a one dimensional chemical model of the Hadean atmosphere and a simple model of late bombardment, we compute the source flux of N2O assuming the same high production yield as measured experimentally and find the steady‐state partial pressure of N2O is insufficient to warm the climate.

Abstract:

Sulfuric acid is shown to form a core–shell particle on a micron-sized, optically-trapped spherical silica bead. The refractive indices of the silica and sulfuric acid, along with the shell thickness and bead radius were determined by reproducing Mie scattered optical white light as a function of wavelength in Mie spectroscopy. Micron-sized silica aerosols (silica beads were used as a proxy for atmospheric silica minerals) were levitated in a mist of sulfuric acid particles; continuous collection of Mie spectra throughout the collision of sulfuric acid aerosols with the optically trapped silica aerosol demonstrated that the resulting aerosol particle had a core–shell morphology. Contrastingly, the collision of aqueous sulfuric acid aerosols with optically trapped polystyrene aerosol resulted in a partially coated system. The light scattering from the optically levitated aerosols was successfully modelled to determine the diameter of the core aerosol (±0.003 μm), the shell thickness (±0.0003 μm) and the refractive index (±0.007). The experiment demonstrated that the presence of a thin film rapidly changed the light scattering of the original aerosol. When a 1.964 μm diameter silica aerosol was covered with a film of sulfuric acid 0.287 μm thick, the wavelength dependent Mie peak positions resembled sulfuric acid. Thus mineral aerosol advected into the stratosphere would likely be coated with sulfuric acid, with a core–shell morphology, and its light scattering properties would be effectively indistinguishable from a homogenous sulfuric acid aerosol if the film thickness was greater than a few 100 s of nm for UV-visible wavelengths.

Abstract:

Aerosol–cloud interactions (ACIs) are considered to be the most uncertain driver of present-day radiative forcing due to human activities. The nonlinearity of cloud-state changes to aerosol perturbations make it challenging to attribute causality in observed relationships of aerosol radiative forcing. Using correlations to infer causality can be challenging when meteorological variability also drives both aerosol and cloud changes independently. Natural and anthropogenic aerosol perturbations from well-defined sources provide “opportunistic experiments” (also known as natural experiments) to investigate ACI in cases where causality may be more confidently inferred. These perturbations cover a wide range of locations and spatiotemporal scales, including point sources such as volcanic eruptions or industrial sources, plumes from biomass burning or forest fires, and tracks from individual ships or shipping corridors. We review the different experimental conditions and conduct a synthesis of the available satellite datasets and field campaigns to place these opportunistic experiments on a common footing, facilitating new insights and a clearer understanding of key uncertainties in aerosol radiative forcing. Cloud albedo perturbations are strongly sensitive to background meteorological conditions. Strong liquid water path increases due to aerosol perturbations are largely ruled out by averaging across experiments. Opportunistic experiments have significantly improved process-level understanding of ACI, but it remains unclear how reliably the relationships found can be scaled to the global level, thus demonstrating a need for deeper investigation in order to improve assessments of aerosol radiative forcing and climate change.