2017 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 (Extended to December 31, 2020)
Project Period Covered by this Report: January 1, 2017 through December 31,2017
Project Amount: $789,261
RFA: Particulate Matter and Related Pollutants in a Changing World (2014) RFA Text |  Recipients Lists
Research Category: Air , Climate Change


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 aerosols formed under enhanced ammonia concentrations.

Progress Summary:

This project has three parts: a field study, an environmental chamber study and an air quality modeling study. Following the project plan for completing tasks, the first year of the project focused on the field study. Year 2 focused on analyzing the field study data and performing preliminary chamber studies. Modeling work focused on applying CMAQ to the observations and evaluating the simulated pollutant concentrations against observations taken as part of this project, as well as other observations available from routine networks, SEARCH and satellite retrievals.

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. Results from the study showed that for the study period, NH3 concentrations were substantially higher than typical for the site or the southeastern United States. Study average NH3 concentration was roughly 4 ppb based on the YRK measurement method, roughly double the more typical levels recorded at the site since 2011.

A highly sensitive technique for measuring gas-phase organic acids was developed and then deployed during the field study to generate a highly unique data set on a suite of organic acid concentrations. Ambient concentrations ranged from a few parts per trillion by volume to several parts per billion by volume. On average, measured acids comprised 30 percent of gas-phase water-soluble organic carbon. All organic acids displayed similar strong diurnal behaviors, reaching maximum concentrations between 5:00 and 7:00 p.m. local time. The organic acid concentrations were dependent on ambient temperature, and the sources for all acids appear to be biogenic in nature.

The gas phase organic acid data, along with measurements of NH3 and HNO3, were combined with particle phase composition data and a thermodynamic model to predict the aerosol pH and gas-particle partitioning of inorganic and organic species. Despite the high NH3 concentrations, PM1 aerosols were highly acidic, with pH values ranging from 0.9–3.8 and a study-average pH of 2.2 ± 0.6. PM1 pH varied by approximately 1.4 units throughout the day. Formic and acetic acids were the most abundant gas-phase organic acid, and oxalate was the most abundant particle phase organic acid, as has been found at many sites worldwide. Particle-phase water-soluble organic acids, however, only contributed 6 percent to the total organic aerosol mass, on average. Despite being ubiquitous, thermodynamic predations of oxalic acid partitioning have never been rigorously assessed. While previous studies have shown that such analytical predictions can accurately model the gas-particle partitioning of semivolatile inorganic species (e.g., HNO3-NO3-, HCl-Cl- and NH3-NH4+) with respect to particle pH, predicting gas-particle partitioning behavior of organic species has not been established. Our results showed that the measured oxalic acid gas-particle partitioning ratios (molar fraction in the particle phase ranged from 47–90% at pH 1.2–3.4) and generally agreed with the analytical predictions. Partitioning of formic and acetic acids were not well predicted. We have explored possible reasons for the discrepancy but do not have an explanation at this point. We found that for this study, changes in NH3 concentrations had minor effects on organic aerosol mass concentrations because (1) the particle phase organic acids are small mass fractions of overall organic aerosol mass, and (2) although particle pH is shown to affect oxalic acid partitioning to the particle phase, pH is not highly sensitive to NH3 concentrations.

Chamber Study

We conducted a series of preliminary laboratory experiments at the Georgia Tech Environmental Chamber facility to investigate the formation and evolution of gas-phase organic acids from α-pinene ozonolysis at atmospherically relevant concentrations. Data analysis is underway. More extensive chamber experiments are planned for spring/summer 2018.

Air Quality Modeling Study

This last year, the modeling focused on simulating the specific time period during the experimental campaign. Specifically, WRF and the base CMAQ were both applied and evaluated for the time period. CMAQ results were compared to both our observations and those from other groups, as well as to the routine network observations nationally and historic observations for reference. Additional data sets used included SEARCH, SLAMS, IMPROVE, and satellite data from MeTOP and CrIS. Generally, ammonia levels were lower than the observations taken during our campaign but similar to historic observations in the area. Deposition levels and patterns were similar. Simulated organic acid levels were low, as expected, given the limited mechanisms in CMAQ forming organic acids.


Future Activities:

In project year 3, field study data will continue. We anticipate that the submitted paper and paper nearing submission will undergo positive review and be published. An additional paper from the AMS data analysis will also be published in year 3. Chamber studies and results from modeling work are expected to increase in year 3. Both of these research aspects are expected to further address the impacts of changing emissions on pH, and the effects on fine aerosols through inorganic and organic species will be further assessed, building on the findings of the organic acid –NH3 paper.

Journal Articles on this Report : 1 Displayed | Download in RIS Format

Other project views: All 3 publications 1 publications in selected types All 1 journal articles
Type Citation Project Document Sources
Journal Article Nah T, Ji Y, Tanner DJ, Guo H, Sullivan AP, Ng NL, Weber RJ, Huey LG. Real-time measurements of gas-phase organic acids using SF6− chemical ionization mass spectrometry. Atmospheric Measurement Techniques Discussions 2018;46 (40 pp.). R835882 (2017)
  • Full-text: Atmospheric Measurement Techniques-Full Text PDF
  • Abstract: Atmospheric Measurement Techniques-Abstract
  • 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
  • 2016 Progress Report
  • 2018