2016 Progress Report: The Effect of Ammonia on Organic Aerosols in a Changing Climate

EPA Grant Number: R835882
Title: The Effect of Ammonia on Organic Aerosols in a Changing Climate
Investigators: Weber, Rodney J. , Huey, Greg , Ng, Nga Lee , Russell, Armistead G.
Institution: Georgia Institute of Technology
EPA Project Officer: Chung, Serena
Project Period: January 1, 2016 through December 31, 2018
Project Period Covered by this Report: January 1, 2016 through December 31,2016
Project Amount: $789,261
RFA: Particulate Matter and Related Pollutants in a Changing World (2014) RFA Text |  Recipients Lists
Research Category: Air , Climate Change

Objective:

The overall objective of this research is to investigate how changes in emissions of key species that affect aerosol acidity (pH) influence the formation and chemical and physical properties of PM2.5, impacting air quality, human health, and climate. A specific research focus is to assess secondary organic aerosol (SOA) formed under enhanced ammonia concentrations.

Progress Summary:

This project has three parts: (1) a field study, (2) an environmental chamber study, and (3) an air quality modeling study. Following the project plan for completing tasks, the first year of the project has focused on Part 1, the field study. Some initial work has begun on pH modeling for Part 3. Part 2, chamber studies, will be informed by the field study results and begin in Year 2.
 
Field Study
 
To investigate the effects of ammonia (NH3) on organic aerosols, the SEARCH Yorkville (YRK) network site was selected as the main field study location for this project. A 6-week intensive field study was undertaken from August 15 to September 30, 2016, a period where analysis of historical data showed highest NH3 concentrations, likely from air mass advection from nearby confined animal feeding operations. The period of late summer also was chosen because it included anticipated periods of high temperature-driven biogenic volatile organic compound (VOC) emissions. A further advantage of the site is that it had long been used as a regionally representative site and provided a large suite of ancillary air quality measurements (i.e., SEARCH data) that was available for this research. A suite of state-of-the-art instruments were deployed throughout the study period.
 
Preliminary analysis of the data shows highly variable NH3 levels that were on average roughly an order of magnitude higher than that of the rural Southeastern United States. Average NH3 concentrations were 5.5 μg/m3 compared to 0.4 μg/m3 recorded at a forested site near Centreville, Alabama (SOAS study), resulting in a roughly 1 unit higher pH from what we have reported for the clean southeast (this study's mean pH = 2.1 ± 0.6 versus 0.9 ± 0.6 for SOAS). The impact of higher pH was observed by higher levels of nitric acid partitioning to the particle phase. A similar observation was found for oxalate (i.e., higher levels of oxalate were observed in this study compared to SOAS). Similar pH effects are expected for other organic acids. Overall, we found that gas-phase organic acids had a distinctive diurnal profile similar in shape to that of ozone. In the particle phase, organic acids accounted for roughly 30 percent during the day and 15 percent at night of the overall PM2.5 organic aerosol mass. Although the field data analysis is in an early stage, the overall YRK data set appears to be very rich.
 
Air Quality Modeling Study
 
As a first step toward understanding the role of fine particle pH on air quality in the continental United States through model simulations, accurate model pH fields must be generated. Assessment of PM2.5 pH in models such as CMAQ are lacking. In Year 1, preliminary work has begun on examining pH on a nationwide scale using ambient concentration data from three monitoring networks—AMoN, CASTNET, and SEARCH—along with thermodynamic (ISORROPIA II) and chemical transport (CMAQ) models to predict aerosol acidity. How the models handle nonvolatile crustal material in the fine mode was found to greatly affect predicted PM2.5 pH. A consequence of this was that models tended to overpredict particle phase nitrate concentrations. The degree of bias depends on location and time of year (cold versus warm). Similar overprediction of organic acids, such as oxalate, can be expected. Assuming no crustal material for PM1, results in CMAQ predicted pH levels in agreement with what we have reported in a number of our other studies. Once accurate pH fields are generated, the modeling will move to assessing effects of pH on PM2.5 mass conconcentrations.

Future Activities:

In Year 2, field study data will undergo intensive analyses. We anticipate that a number of papers will be submitted by the end of Year 2. Two papers currently are in progress, one focusing on the novel Chemical Ionization Mass Spectrometer method used in the field study for measuring a large suite of gas phase organic acids, along with organic acid data. The second focuses on the pH-dependent gas-particle partitioning of oxalate. Chamber studies will begin in the next year. The types of experiments planned will be guided by the field study data. Modeling pH and assessment of the models will continue. Once some confidence in model predicted pH is achieved, the impacts of changing emissions on pH and the effects on fine aerosols through inorganic (nitrate) and organic (organic acids, acid catalyzed) reactions will be further assessed. Other areas of investigation, including brown carbon and reactive oxygen species, will be the focus of Years 2 and 3.

Supplemental Keywords:

fine particulates, measurement methods, environmental chemistry, source characterization, Georgia, Southeast, organics, biogenic, biosphere, health effects, human health, air quality modeling, air quality analysis, organic acids, pH, particle acidity, SOA, PM2.5, brown carbon, reactive oxygen species, oxidative potential.

Progress and Final Reports:

Original Abstract