Why has the threat from acid deposition been steadily increasing in the 21st century?

The threat from acid deposition has been steadily increasing in the 21st century because of the increase in the number of industries during this period. The acid particles get trapped in the atmosphere and falls as raid on earth. These acid particles are increasing with the booming of the industrial sector.

The investiture controversy began as a power struggle between pope gregory vii (1072–85) and henry iv, holy roman emperor (1056–1106).[2] a brief but significant struggle over investiture also occurred between henry i of england and pope paschal ii in the years 1103 to 1107, and the issue played a minor role in the struggles between church and state in france, as well.by undercutting the imperial power established by the salian emperors, the controversy led to nearly 50 years of civil war in germany, and the triumph of the great dukes and abbots. imperial power was finally re-established under the hohenstaufen dynasty. historian norman cantor:

The answer is 1-plate movement along fault lines