AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Fine Particle pH
RODNEY J. WEBER, Hongyu Guo, Aikaterini Bougiatioti, Nikolaos Mihalopoulos, Athanasios Nenes, Georgia Institute of Technology
Abstract Number: 503 Working Group: Aerosol Chemistry
Abstract pH is a critical aerosol property that impacts many atmospheric processes, including isoprene secondary organic aerosol formation, mineral dust and redox metal mobilization and has been linked to direct adverse health effects. pH also affects acidic gas-particle phase partitioning, with nitric acid-nitrate a common example. Based on data from field studies in widely differing locations, through thermodynamic modeling we assess particle pH and nitrate partitioning in the southeastern US summer, northeastern US winter, western US summer and eastern Mediterranean. Thermodynamics predicts nitrate formation when pH ranges between 2 and 3 for moderate ambient temperatures. At all sites fine particle pH ranged between approximately 0 and 3 and various levels of nitrate were observed. In the southeast summer, pH has remained consistently below 2 despite a 70 percent decrease in sulfate in the past 15 years. Thus, little nitrate is expected and confirmed by historical data, counter to widespread views based on sulfate trends that particles will be neutralized and nitrate displace sulfate to become a significant component of PM2.5. Lower temperatures shift nitric acid partition to slightly lower pHs and account for the substantial nitrate levels often observed in winter in the northeast, despite similar sulfate and pH levels as those of the southeastern summer. In California pH tends to be more often in the 2 to 3 range, resulting in frequent high levels of nitrate aerosol, whereas in the eastern Mediterranean pH at the higher end of the range is seen in biomass burning plumes, leading to nitrate, in part due to the presence of potassium,. In all cases, the aerosol is never near neutral, as often assumed or inferred from ion balances or molar ratios. A better understanding of the implications of a consistent highly acidic aerosol on geochemical cycles, aerosol-health linkages and aerosol atmospheric processing is needed.