Urban impacts on climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 241

Table 7.3 Major interactions between air pollutants (mainly fine particulates and ozone) and urban
climate components

Interaction Comment or impact

Scatter shortwave energy Reduction of available SWR for surface absorption;

increased diffuse SWR, reduced UHI?

Absorb shortwave energy Reduction of available SWR for surface absorption;

reduced UHI?

Absorb longwave radiation Increased UHI

Increased nuclei for cloud formation
and precipitation

Increased urban storms and rainfall; increased fog

Inclusion in UHIC Reduced visibility; increased pollution episodes

Inclusion in urban plume downwind Transfer of pollution impacts to rural areas

Photochemical reactions Increased atmospheric temperature; reduction of

available SWR

greenhouse gases such as carbon dioxide and methane. Gases can also be created
through secondary reactions in the atmosphere, between primary gases and
atmospheric components such as ultraviolet radiation and moisture.
Tropospheric ozone, the major component of photochemical smog is the most
prevalent example.
A second category of pollutant is particulate matter. Particles in the atmo-
sphere exist across a wide range of sizes. Sources range from windblown dust
from bare soil surfaces, to sea spray, to volcanic emissions, to biomass and fossil
fuel burning. Of most concern to the respiratory health and welfare of the urban
population is particulate matter below 10 mm in diameter (PM10) and that below
2.5 mm in diameter (PM2.5). The latter represents the size range called fine
particulates.
It is PM10, PM2.5, and photochemical smog that cause the greatest problems in
urban atmospheres today, and also have the strongest influence on urban climate.
Table 7.3 lists the major interactions of air pollutants with components that create
the urban climate. In Mexico City, the impact of air pollution (mainly ozone and
fine particulate matter) on incoming all-wavelength radiation is significant, with
an average reduction of 21.6% in the city compared to a rural location (Jauregui
and Luyando 1999 ). There is little difference in reduction between the dry andwet
seasons. Although rainfall will clean the atmosphere in the wet season, the
increased humidity and hygroscopic action create haze which enhances short-
wave scattering. Attenuation tends to be highest during weekdays (35%) and least
on weekends (25%), reflecting a major change in the traffic regime.
Shortwave radiation depletion in Mexico City is related positively to relative
humidity and air pollution concentrations, but inversely to wind speed and
temperature (Jauregui and Luyando 1999 ). Periods of maximum reduction