Chapter 2 Rainwater Analysis (Phân tích nước mưa) - Practical Environmental Analysis, 2nd Edition - M. Radojevic, V. Bashkin

50 Chapter 2
Figure 2.3 Mole fraction of S(IV) species as a function of pH
In addition, weak organic acids such as formic and acetic acids, originating
from natural sources, can make a significant contribution to the rainwater
acidity, especially at unpolluted rural and remote sites. Currently, about half
of the global emissions of sulfur and NO x compounds originate from anthropogenic
sources. Some 68% of anthropogenic emissions of SO 2 and NO x
originate in Europe and North America, but the contribution of developing
countries, especially in Asia is rapidly rising. Plumes of pollutants can travel
hundreds or even thousands of kilometers from their source before being
converted to acid rain. This so-called long-range transport of air pollution
(LRTAP) has become a major political issue because of the trans-boundary
movement of pollution between countries.
Sulfur compounds are removed from the atmosphere by deposition
processes. Gases dissolve in cloud droplets whereas dust particles are incorporated
into cloud droplets either as cloud condensation nuclei (CCN)
around which the droplets grow, or by scavenging mechanisms. These
processes are referred to as rainout. The incorporation of dust particles and
gaseous molecules into falling raindrops is called washout. Both rainout and
washout are termed wet deposition. Dry deposition involves sedimentation of
dust particles and diffusion of gaseous molecules onto surfaces. Dry deposition
processes are generally more important because they are closer to the
source of pollution.
2.1.2 Effects of Acid Rain
Since the initial link between acid rain and dying fish was made some 30
years ago, a number of other harmful effects of acid rain have been uncovered.
Some of these effects, given in Table 2.1, have been conclusively