Effects of wind-blown dust emissions on size-resolved aerosol acidity over the US

Abstract

Wind-blown dust can affect the acidity of all particles, impacting a series of related chemical processes. In this work, we use a wind-blown dust emissions model along with the hybrid version of aerosol dynamics in the PMCAMx chemical transport model to quantify the effects of dust on size-resolved aerosol pH over the U.S during February and July 2017 as a function of altitude. The version of the model used can capture the most important range of pH values (less than 5), while particles that are more alkaline than this threshold are assumed to have pH equal to 5. Our simulations indicate that wind-blown dust can increase ground level PM₁ pH up to 1 unit during wintertime and up to 3.5 units during summertime in the western U.S. For PM_1−2.5, the corresponding increases are higher during wintertime (up to 1.5 units) and a little lower during summertime (up to 3 units) compared to PM₁. For coarse particles (PM_2.5−5 and PM₅₋₁₀), the impact of wind-blown dust is predicted to be lower since in most areas the corresponding pH is already quite high due to the presence of dust from anthropogenic activities (e.g., agriculture, resuspension due to traffic). The impact of wind-blown dust on aerosol acidity decreases with altitude for PM₁ mainly because of the reduction of aerosol water. On the other hand, for PM_1−2.5, the predicted effect increases with altitude due to the lower pH at higher altitudes. PM₁ and PM_1−2.5 acidity can be affected significantly by wind-blown dust for both simulated periods impacting at the same time secondary aerosol formation, emissions control strategies, solubility of metals, and nitrogen deposition.