2013 Progress Report: Anthropogenic influence on biogenic VOC oxidation: the role of NOx pollution in secondary organic aerosol production in the Southeast U.S.

EPA Grant Number: R835399
Title: Anthropogenic influence on biogenic VOC oxidation: the role of NOx pollution in secondary organic aerosol production in the Southeast U.S.
Investigators: Fry, Juliane L
Institution: Reed College
EPA Project Officer: Hunt, Sherri
Project Period: April 1, 2013 through March 31, 2016
Project Period Covered by this Report: April 1, 2013 through March 31,2014
Project Amount: $299,995
RFA: Anthropogenic Influences on Organic Aerosol Formation and Regional Climate Implications (2012) RFA Text |  Recipients Lists
Research Category: Air Quality and Air Toxics , Global Climate Change , Climate Change , Air

Objective:

Determine the reactive fate of NO2 in the summertime southeastern U.S., with particular focus on the role of chemistry that results in secondary aerosol production, via organic (NO3 + biogenic VOCs) or inorganic (HNO3 uptake) mechanisms.

Progress Summary:

During the summer 2013 SOAS field campaign, all instruments described in the proposal were employed to successfully collected relevant NOy data: NO3 and N2O5 were measured atop the SOAS tower and from the SENEX aircraft by cavity ringdown spectroscopy (CRDS); HNO3(gas), NO3- (aero), NH3(gas), NH4+ (aero), HONO(gas), SO2(gas), SO42- (aero), and other inorganic ions were measured at the SOAS ground site by a Metrohm Monitor for AeRosols and GAses (MARGA); and Potential Aerosol Mass (PAM) measurements were carried out at the SOAS ground site with known NO3 exposure and measurements of aerosol enhancement by Aerosol Mass Spectrometer (AMS).
 
By comparison of NO3/N2O5 data with organonitrate measurements made by collaborators, we find evidence of rapid formation of both gaseous and aerosol-phase organonitrate from NO3 + BVOC, with aerosol organonitrate arising primarily from the oxidation of the monoterpenes alpha-pinene, beta-pinene and limonene, with much gaseous organonitrate formation attributable to NO3 + isoprene. This is a key finding because NO3+BVOC provides an efficient sink of atmospheric anthropogenic NOx to higher oxidized forms of nitrate, with the formation of aerosol assumed to lead to local deposition of NOx on ecosystems rather than transport away as gaseous NOx. This has implications for the regional nitrogen cycle and the fate of naturally emitted VOCs, as their oxidation is accelerated by the presence of anthropogenic NOx. Furthermore, this chemistry appears to be of more widespread importance than previously realized – although the NO3 radical is traditionally thought of as a nighttime oxidant because of its relatively rapid photolysis, a careful NO3 loss rate analysis for this site finds that because the local concentrations of BVOC are so high in the summertime, reactive losses (NO3+BVOC) are responsible for the loss of approximately half of the NO3/N2O5 reactive pool even during the day!
 
The production rate of organonitrate aerosol via this mechanism is estimated to be on average 0.1 μg m-3 hr-1, with episodic spikes up to 0.6 μg m-3 hr-1. This is a substantial production rate compared to typical background organonitrate concentrations of 5-15 μg m-3 . A comparable source term of inorganic nitrate aerosol (average production rate of 0.1 μg m-3 hr-1, with episodic spikes up to 1.0 μg m-3 hr-1) was discovered at SOAS, which we interpret as due to a very different chemistry, and thus peaking at different times.
 
Peaks in observed inorganic nitrate aerosol are hypothesized to be due to heterogeneous uptake of HNO3 onto salt/dust aerosol. We arrive at this interpretation because high inorganic nitrate episodes correspond to periods of high aerosol surface area and depletion events of HNO3(g). These high surface area events are also correlated with higher concentrations of sea salt (Na+ and Cl-) and/or mineral dust (Ca2+, Mg2+, K+) ions observed in the aerosol aqueous phase by the MARGA, suggesting that aerosol surface area is elevated by transported dust and/or sea salt. We can estimate the production rate of inorganic nitrate aerosol during these periods using the known rate of HNO3 uptake by aqueous aerosol surfaces, and thus arrive at the production rate reported above. Based on these two interpretations of sources of organic and inorganic nitrate aerosol, we predict varying periods of elevated concentrations of each, but with similar average concentrations. Both of these predictions are corroborated by the observations at SOAS: we observe varying periods of inorganic vs. organic nitrate dominance, as well as an overall similar average concentration of each, suggesting deposition rates are similar.
 
We are in the process of writing up these results into a first observational paper on nitrate aerosol production at SOAS. Following this work, we will move to analysis of the Potential Aerosol Mass data collected to help us better understand nitrate aerosol formation, and a comparison of vertical profiles of NO3 and N2O5 observed from aircraft with ground site observations.

Future Activities:

We will continue to work on the analysis of field data and preparation of the manuscript above. Thereafter, we plan a set of collaborative chamber experiments at the University of Colorado, with collaborators Brown and Jimenez, to help interpret field PAM data.

Journal Articles:

No journal articles submitted with this report: View all 19 publications for this project

Supplemental Keywords:

ambient air, ozone, acid deposition, global climate, chemical transport, environmental chemistry, analytical, Alabama (AL), EPA region 4.

Relevant Websites:

http://academic.reed.edu/chemistry/fry/research.html

Progress and Final Reports:

Original Abstract
2014 Progress Report
Final Report