Emission reduction from fossil fuel burning have reduced SOx in the atmosphere for the past 15 years. Sulfur is a lose lose for industry and public health. Sulfur poisons catalysts that are often required for processes and controlling the emissions of other harmful gases, and sulfates should increase aerosol pH. However, an interesting study by Weber et. al shows that aerosol acidity is still very high despite the reduction of sulfates. Results are shown below:
While this graph may initially seem like a lot, it shows exactly the problem that's being highlighted by Weber et. al. We see that that over the 15 years, pH hasn't changed much. We expect that with the declining sulfates that there should be fewer H+ ions in solution since sulfuric acid is a strong acid. But infact, there is pH buffering as ammonia and ammonium partition differently, and so reduce the effect caused by the decreasing sulfate concentration. We see this by the fact that ammonia gas has increased in concentration (purple inverted triangles). The rising NH3 in the gas phase means that ammonium released its H+ in solution and went into the gas phase, resulting in pH buffering. Essentially the problem lies that sulfates aren't volatile and ammonium is. Ammonium can partition as ammonia, while sulfates can only exist in solution. This pH very insensitive to changes in sulfate concentration since it is only ammonia and ammonium that can partition differently to maintain equilibrium. And when ammonium becomes ammonia, H+ is released into solution as mentioned above.
So what does this mean for the future? Well the study suggests that pH won't really change much in the future, and there is really very little hope that aerosol pH can be increased drastically soon. The study says that unless sulfate concentrations are reduced to pre-anthropogenic levels, there will really be no change. But, over time, if the trend of reducing SOx emissions continues, the pH reduction can occur for future generations.
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