Global radiation attenuation by air pollution and its effects on the thermal climate in Mexico City

Solar radiation attenuation in Mexico City is estimated by comparing global radiation intensity observed at a station in the downtown area, relative to that observed in a cleaner rural air at a site 8 km northeast of the airport. On average, the city receives on clear days during the dry season 21.6% less solar energy than the rural surroundings. Similar values of attenuation are observed during the rainy season when aerosol concentration is abated by a wash-out effect. As expected, solar dimming shows an inverse relation to wind and temperature. Attenuation reaches its maximum (25-35%) with prevailing weak winds (1-2 m/s) and high noon relative humidity. In the city, variation of global radiation by the day of week, shows that solar attenuation is highest on Tuesdays, while on Saturdays solar dimming by smog was less. Since a similar weekly variation (with lower values) was observed at the rural site, the conclusion was that the smog layer affects the solar radiation climate in rural areas beyond the immediate city limits. The declining annual mean of maximum temperatures observed near the limits of the Mexico City basin, suggests that the smog layer is affecting the radiation and thermal climate of a vast region downwind of the capital city. An attempt is made to show that while in the industrial and downtown areas the day time heat island effect dominates over the cooling effect of aerosols, in other suburban and rural sites downwind and away from the warm air mass, the reflecting properties of the smog layer seem to prevail resulting in an observed decreasing trend in maximum temperatures. From the above it may be concluded that maximum temperatures continue to rise in downtown Mexico City in spite of the attenuation due to absorption and reflection of incoming solar energy by the smog layer. Copyright (C) 1999 Royal Meteorological Society.