On the potential high acid deposition in Northeastern China

This is inconsistent with the usual situation where high-acidity precursor pollutants result in low pH (high acidity) values and acid rain conditions. In other regions, such as southern China and the United States, high SO₂ concentrations are typically well correlated with high acidities. Using measured soluble particle measurements (including both positively and negatively charged ions), it is seen that there are high values of alkaline ions in northeastern China that play an important role in neutralizing acidity in this region.

This result strongly suggests that the high alkaline concentrations, especially Ca²⁺, increase warm season pH values by about 0.5 in northern China, partially explaining the inconsistency between sulfate concentrations and acidity. This has a very important implication for acid rain mitigation—especially in northeastern China. However, there are additional issues pertaining to the precursor-acidity relationship that need further investigation.

Why is it that the reduction in acidity due to the alkaline ions is only significant in summer? During winter, the measured alkaline ions play a much smaller role in explaining the discrepancy. The measured alkaline ions in this study were mostly obtained from particles in the PM₂.₠range.

However, the size of calcium particles is typically much larger -- extending well beyond 2.5 µm -- and so a significant amount of calcium may be underestimated by PM₂.₠measurements alone. The under-sampling of calcium particles is further exacerbated in that the sampling protocol excluded particle (and soluble ion and pH) measurements during dust storms. This all leads to the need for an improved understanding of pollutant-ion-particulate interactions in China, and their role in explaining the counter-intuitive conclusion that dust mitigation strategies in China could have the unintended consequence of exacerbating acid rain conditions.