6.1.3 Acid rain cycle
Since the acid at the study site is a mixture of sulfuric and nitric acid (Zhang, 2015), the acid rain cycle was set up in such a way that the sandstone samples were submerged in solutions with pH=2.6, pH=4.1 and pH=5.6, respectively. They were configured with dilute hydrochloric solution and distilled water; 0.009 g/L of sodium nitrate solution, 0.0285 g/L of potassium sulfate solution and 0.0160 g/L of ammonium nitrate solution were added into the solutions with different pH values. After that, the sandstone samples were submerged in these solutions overnight (21:00- to 21:00 on the next day). Then, they were air-dried for the subsequent 24 hours. This procedure was considered a cycle, and three cycles were set as one period. The pH value of solution was adjusted to original value with dilute hydrochloric solution before each soaking, as shown in Fig. 7.