Thermal and radiational aspects of the christmas eve 1977 air pollution episode in Mexico City.

Abstract

During the night of Dec. 24, 1977, thousands of rubber tires were burned in the streets of Mexico City. The additional tropospheric load of particulate and gaseous matter together with meteorological conditions of stability produced a severe episode of air pollution perturbing the heat balance. On Dec. 2.5 between 0700 and 1400 hours True Solar Time (TST), horizontal visibility was reduced to less that I km, reaching its minimum < 0.1 km between 1200-1400 hours TST. Morning mixing height was 90 m with average wind velocity of 1 m sec-1. The thermal effects of the atmospheric aerosol layer produced warming at night of 2-3°C (greenhouse effect). During the day, on the contrary it was cooler by l.5°C. Duration of sunshine was reduced 1 hour. Atmospheric turbidity reached very high values. Total downward radiation was diminished by 26.8%. The ratio diffuse direct radiation " ·as inverted indicating that much of the incoming radiation was diffused in the aerosol layer. The combined action of the absorption and backscattering processes of the additional aerosol layer seems to be the responsible of the diurnal air cooling at the surface; Both effects are separately analyse on the thermal balance. Backscattering is similar to the one produced from a cloud layer. Using the Yamamoto and Tanaka model, one estimates that global albedo may be increased from 0.15 to 0.30. The above described radiative effects may be responsible for the observed diurnal cooling.