Acid Rain can react with highway deposited lead wheel weights to form 
highly toxic and mobile lead nitrate. Acid rain is rain or any other 
form of precipitation that is unusually acidic containing, but not 
limited to, dilute forms of nitric, hydrochloric, sulphuric and 
carbonic acid. It has harmful effects on plants, aquatic animals, and 
infrastructure. Acid rain is mostly caused by human emissions of 
sulfur and nitrogen compounds which react in the atmosphere to produce
acids. In recent years, many governments have introduced laws to 
reduce these emissions.

Acid deposition
Processes involved in acid deposition 
(note that only SO2 and NOx play a significant role in acid rain).
Wet deposition
Wet deposition of acids occurs when any form of precipitation 
(rain, snow, etc.) removes acids from the atmosphere and delivers 
it to the Earth's surface. This can result from the deposition of 
acids produced in the raindrops (see aqueous phase chemistry above) 
or by the precipitation removing the acids either in clouds or below 
clouds. Wet removal of both gases and aerosols are both of importance 
for wet deposition.

Dry deposition
Acid deposition also occurs via dry deposition in the absence of 
precipitation. This can be responsible for as much as 20 to 60% of 
total acid deposition.[16] This occurs when particles and gases stick 
to the ground, plants or other surfaces.
PLEASE CLICK HERE FOR MORE DETAILED INFORMATION.