Ozone destruction through heterogeneous chemistry following the eruption of El Chichón

TLDR

Heterogeneous reactions on aerosol surfaces are a key driver of Antarctic ozone depletion and are also expected to occur on sulfuric‑acid particles at lower latitudes, albeit more slowly. This study examines whether the stratospheric ozone loss observed at northern mid‑latitudes after the 1982–1983 El Chichón eruption can be attributed to heterogeneous chemistry on the volcanic aerosol. The authors measured the aerosol surface‑area density and sulfuric‑acid mass fraction of El Chichón particles and used these data to model the rates of relevant heterogeneous reactions. The volcanic aerosol reached a peak surface area of ~50 μm² cm⁻³ at 18–20 km, comparable to Antarctic cloud values, and model results indicate that heterogeneous reactions on these particles contributed to the anomalous ozone depletion and altered reactive‑nitrogen partitioning at mid‑latitudes.

Abstract

It is now well established that heterogeneous reactions provide an important mechanism for Antarctic ozone depletion. Recent laboratory studies suggest that the same reactions that occur on HNO 3 /H 2 O ice clouds in the cold Antarctic stratosphere can also take place on sulfuric acid particles (e.g., volcanic and background aerosols) typical of lower latitudes, albeit at slower rates. The reduction in stratospheric ozone observed at northern mid‐latitudes in late 1982 through 1983 following the volcanic eruption of El Chichón is investigated in terms of ozone loss through heterogeneous chemistry on the aerosol which formed in the stratosphere. The rates of the relevant heterogeneous reactions are believed to be critically dependent on (1) the aerosol surface area density and (2) the percent by weight sulfuric acid in the liquid particles. Direct measurements of both of these important quantities for El Chichón aerosol are described and used as a basis for model calculations of their possible effects on ozone and other trace species. The observed volcanic particle surface area reached a maximum at mid‐latitudes of about 50 μm 2 cm −3 (above a typical background value of about 0.75) at an altitude of 18–20 km in early 1983. This enhancement of surface area is about the same as that encountered in stratospheric clouds in the Antarctic, suggesting a possible basis for ozone depletion through heterogeneous chemistry. Observations of NO 2 and HNO 3 also suggest that heterogeneous reactions on both background and volcanic aerosol play a significant role in partitioning reactive nitrogen species in middle and high latitudes in winter. It is shown that heterogeneous reactions similar to those occurring in Antarctica may have been responsible for at least a portion of the anomalous ozone reduction observed at mid‐latitudes in early 1983.