10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Aerosol Acidity in the Southeastern United States and Source Impacts on Fine Particle pH
YU QIAN, Armistead G. Russell, Georgia Institute of Technology
Abstract Number: 947 Working Group: Aerosol Chemistry
Abstract Aerosol acidity (pH) plays an important role by affecting aerosol formation, particle composition, human health, nutrient bioavailability, etc. This study used species concentration and meteorological data for three monitoring sites from the Southeastern Aerosol Research and Characterization Study (SEARCH) from 2001 to 2013, and thermodynamic model (ISORROPIA-II) to estimate the daily pH trends over that time period. By using the estimated pH time series data, with the help of multiple methods including both simulation and regression, the control factors of pH and the sensitivities of pH to those factors were investigated. Also based on the source apportionment results for the same domain, multiple-linear-regression models were developed to show the impacts of different source categories on fine particle pH. Aerosol pH in SE US kept low (average 2.61) and did not change much over the past decade with the SO2 emissions reduction, but significant seasonal patterns were observed: high pH during the winter and low pH during the summer, which mainly caused by higher sulfate concentration during the summer. And spatial variations were also observed: higher pH in the urban area and lower pH in the rural area, which were related to higher NH3 concentration emitted by mobile and biogenic sources (human activities) in the urban area. Model performance was evaluated by comparing estimated and observed NH3 gas phase partitioning ratio, which confirmed the model performance by showing a good agreement between those two (averaged slope = 0.87, intercept = 0.1 μg/m3, R2 = 0.65). Relative humidity and temperature are proved as two strong driven factors of pH, with high RH tend to raise pH and high temperature tend to decrease pH. pH showed largest sensitivity to two species in this study domain: sulfate (-0.11 unit of pH change per 1 μg/m3 change of sulfate concentration) and ammonia (+0.10 unit of pH change per 1 μg/m3 change of total ammonia concentration). Crustal ions in SE US have little effect on aerosol pH. With strong correlations (average R2 = 0.37), in most cases, models indicated vehicles emissions had the largest positive impact on pH and ammonia bisulfate contributed most negative impact on pH, even though specific results are different for different seasons and locations. Data withholding methods used for evaluating the model performance indicate that the model is trustable (R2 = 0.67).