Heterogeneous reactions with NaCl in the El Chichon volcanic aerosols

Previous investigations of the effects of the 1982 eruption of the El Chichon volcano could not explain all the observations of changes in O_3, HCl, NO and NO_2 simultaneously without proposing unproven chemical reactions. Since reactions between solid NaCl and gaseous ClNO_3 and N_2O_5 rapidly produce photochemically active chlorine species and solid NaNO_3 in laboratory experiments, we suggest that these reactions could have occurred with the NaCl observed to be present in the El Chichon sulfuric acid aerosols. As a consequence, we predict that HCl should increase substantially, while NO_x should decrease, in agreement with the measurements after the eruption. Ozone should only be slightly affected by these reactions. Reactions between solid NaCl and the acids H_2SO_4 and HNO_3 might prove to be important, but we lack sufficient evidence regarding their efficiency and the presence of HNO_3 in the aerosols to be more conclusive

El Chichon Volcanic Aerosols: Impact of Radiative, Thermal, and Chemical Perturbations

We examine the consequences of the eruption of the El Chichon volcano on the Earth's stratospheric chemistry. Formed after the eruption, the volcanic aerosol cloud, with a peak particle density at 27 km, was very efficient at altering the radiation field. The results of a one-dimensional radiative transfer model show that the total radiation increased by 8% within the aerosol layer longward of 3000 Å. At certain altitudes and wavelengths below 3000 Å, the total radiation decreased by 15%. Consequently, there are changes in the photolysis rates obtained with a one-dimensional photochemical model: for example, O_2 photodissociation rate constants decrease by 10%, while O_3 photodissociation rate constants increase by a comparable amount. A combination of this radiation change and the effect of a temperature variation of a few degrees causes the abundance of O_3 to decrease by 7% at 24 km, in good agreement with the Solar Backscattered Ultraviolet experiment (SBUV) measurements of a 5–10% decrease. The combined radiative and thermal perturbations on the concentrations of O, O(1D), OH, HO_2, H_2O_2, NO, NO_2, NO_3, N_2O_5, HNO_3, HO_2NO_2, Cl, ClO, ClO_2, HOCl, ClNO_3, and HCl are computed and presented in detail. However, these changes as calculated are insufficient to explain the observations of significant decreases in NO and NO_2 and increases in HCl. A heterogeneous reaction catalyzed by aerosol surfaces which transforms ClNO_3 into HCl provides a pathway for sequestering NO_x, and at the same time reduces ClNO_3 in favor of HCl. The inclusion of this reaction in the model leads to a satisfactory single-step explanation of the otherwise puzzling observations of NO, NO_2, and HCl. The observed lack of change in HNO_3 cannot be explained by this hypothesis. The effects of a number of heterogenous reactions, some believed to be important for the Antarctic stratosphere, have been assessed with our model. We also examine the hypothesis of direct injection of gases from the volcano into the stratosphere. Only an unrealistically large injection (60% column increase above 12 km) results in an HCl increase in agreement with observations. An equally large water injection decreases HCl, and decreases the NO and NO_2 by as much as 20%, but still does not simulate the observed NO_x decrease

Enhancement of Atmospheric Radiation by an Aerosol Layer

The presence of a stratospheric haze layer may produce increases in both the actinic flux and the irradiance below this layer. Such haze layers result from the injection of aerosol-forming material into the stratosphere by volcanic eruptions. Simple heuristic arguments show that the increase in flux below the haze layer, relative to a clear sky case, is a consequence of “photon trapping.” We explore the magnitude of these flux perturbations, as a function of aerosol properties and illumination conditions, with a new radiative transfer model that can accurately compute fluxes in an inhomogeneous atmosphere with nonconservative scatterers having arbitrary phase function. One calculated consequence of the El Chichon volcanic eruption is an increase in the midday surface actinic flux at 20°N latitude, summer, by as much as 45% at 2900 Å. This increase in flux in the UV-B wavelength range was caused entirely by aerosol scattering, without any reduction in the overhead ozone column

SME observations of O_2(^1Δ_g) nightglow: An assessment of the chemical production mechanisms

Solar Mesosphere Explorer (SME) observations of the 3 a.m. 1.27 μm nightglow at 45°N latitude, averaged over the period 10–31 July 1984, are reported. From the deduced volume emission rates, we derive the O_2(a^1Δ_g) night-time production rates for the 80–100 km altitude range. Utilizing the mean SME-acquired 3 p.m. ozone profile for the same latitude and time period and an updated photochemical model, we determine night-time O, O_3, H, OH, HO_2, and H_2O_2 profiles. These are used in calculating the rates of reactions which are sufficiently exothermic to produce O_2(^1Δ) or excited states of OH or HO_2, which could transfer their energy to O_2 to form O_2(^1Δ). Of these reactions, most have rates that are quite small compared with the observed night-time O_2(^1Δ) production rate. For several others, laboratory experiments have found O_2(^1Δ) yields which are insufficient for simulating the observed O_2(^1Δ). Using yields of O_2(^1Δ) based on published laboratory and observational studies, we find that the sum of two reaction sequences can approximate the SME measurements: (1) O + O + M and (2) H + O_3 followed by OH^∗ + O_2