2013 Progress Report: Emission, Fate, and Contribution of Biogenic Volatile Organic Compounds to Organic Aerosol Formation in the Presence of Anthropogenic Pollution: Measurements and Modeling during SOAS
EPA Grant Number:
Emission, Fate, and Contribution of Biogenic Volatile Organic Compounds to Organic Aerosol Formation in the Presence of Anthropogenic Pollution: Measurements and Modeling during SOAS
Mak, John E
, Goldstein, Allen H.
, Guenther, Alex
The State University of New York at Stony Brook
National Center for Atmospheric Research
University of California - Berkeley
EPA Project Officer:
April 1, 2013 through
March 31, 2016
Project Period Covered by this Report:
April 1, 2013 through March 31,2014
Anthropogenic Influences on Organic Aerosol Formation and Regional Climate Implications (2012)
Air Quality and Air Toxics
Global Climate Change
The overall goal of this project is to quantify biogenic Volatile Organic Compound (VOC) emission and VOC deposition to terrestrial ecosystems and characterize VOC atmospheric oxidation and the impact of anthropogenic pollution on secondary organic aerosol (SOA) formation. Our specific objectives include:
Constrain and understand the processes controlling biogenic VOC emission, atmospheric oxidation and deposition;
Elucidate the oxidation pathways of primary organics to form secondary organics in clean and polluted atmospheres;
Evaluate the relative contributions of biogenic and anthropogenic emissions to the regional SOA burden in the southeastern United States;
Search for previously unidentified/unmeasured semi-volatile organic compounds (SVOCs) that would help explain why observations of SOA are often up to an order of magnitude higher than traditional models predict;
Investigate the impacts of urban development patterns on biogenic and anthropogenic emissions and determine the implications for regional climate.
These objectives will provide policy-makers with needed tools to improve model representations of anthropogenic influences on organic aerosol formation and regional climate implications. Our scientific objectives have not changed during this project.
To address these objectives, and as outlined in our proposal, during the SOAS summer 2013 campaign we successfully deployed a proton transfer time of flight mass spectrometer (PTR-TOFMS) at the ground based SEARCH tower site in Centreville Alabama, with concentrations measured at the top of the tower, along with a sonic anemometer allowing for eddy covariance flux calculations. These measurements were coordinated with observations using the same type of instrument at the AABC flux tower site and with aircraft observations from the Long-EZ aircraft, providing equivalent observations at multiple ground sites and in vertical profiles through the atmospheric boundary layer. We also deployed a leaf level enclosure measurement system to characterize the response of isoprene emission to temperature and light at various heights within a forest canopy in Alabama. A lift was used to access sun and shade leaves at a range of above ground heights. In January 2014, we deployed and operated a PTR-TOFMS as participants in The Focused Isoprene eXperiment at the California Institute of Technology (FIXCIT) collaborative atmospheric chamber campaign. FIXCIT is the laboratory component of SOAS and was aimed at addressing Objective 2 above. Specifically, FIXCIT is designed to provide: (1) a better understanding the chemical details behind ambient observations relevant to the Southeastern United States, (2) an advance in the knowledge of atmospheric oxidation mechanisms of important biogenic hydrocarbons, and (3) characterization of the behavior of SOAS field instrumentation using authentic standards. Approximately 20 principal scientists from 14 academic and government institutions performed parallel measurements at the SOAS site in Alabama and at the atmospheric chambers at Caltech.
During the SOAS summer 2013 field campaign, a PTR-TOF-MS was deployed at the AABC site to measure the VOCs mixing ratios of the air samples both within and above the canopy. VOCs fluxes at the top of the AABC flux tower were determined by applying eddy covariance (EC) technique to provide direct quantification of emission of the compounds. However also of interest is the fate of VOCs above the canopy. Obtaining vertical profiles of the concentration of VOCs as a function of altitude over time can provide a way to better understand chemical processes occurring in the atmosphere. The PTR-TOF-MS was used to quantify the vertical profiles of VOCs species mixing ratios above the ground based AABC sites by measuring air samples collected with the Whole Air Sample Profiler (WASP). The EC measurements were carried out between June 2 and July 14 and the vertical profiles of VOCs were derived from aircraft observations between June 4 and June 14. Figure 1 provides an example of three selected VOCs species measured at the AABC tower site for isoprene, methylvinylketone and methacrolein (MVK+MACR), and monoterpenes, from June 2-July 4.
Figure 1. Atmospheric mixing ratios observed at the AABC tower site for isoprene, some of its major oxidation products MVK+MACR and monoterpenes.
Figure 1 illustrates the diurnal trends in VOCs mixing ratios above the canopy at the AABC site. In general isoprene concentrations were low at the beginning of the day and peaked midday to concentrations above 10 ppb most days, and above 15 ppb on certain days (June 12, June 16, June 26, June 29). This agrees well with shortwave radiation records from the site indicating that the days where isoprene concentrations were highest were likely the sunniest and warmest days during the field campaign. Diurnal trends for MVK+MACR closely reflect those of isoprene, a result of their being first order products of isoprene oxidation, although concentrations were lower on average, closer to 3 ppb. Interestingly, those days where MVK+MACR reached the highest levels did not correspond to those days where isoprene concentrations were highest. Monoterpenes and isoprene are synthesized from isopentenyl-diphosphate. Both monoterpene and isoprene emissions are governed by temperature and light. However, trends for the monoterpenes show highest concentrations at the start and end of the day, with minimums occurring when isoprene concentrations are highest.
It is also important to better understand the above canopy chemistry that occurs within and above the boundary layer. The WASP (whole air sample profiler) was deployed from our research aircraft in order to determine vertical profiles of the VOCs in question. A total of 20 vertical profiles collected over the AABC and SEARCH site were processed and are shown in Figure 2. In general the concentrations of isoprene, MVK+MACR and monoterpenes are higher later in the day, likely a result of higher temperatures, the cessation of the growth of the planetary boundary layer (PBL) and decreased photochemical oxidation capacity over time. Additionally the increase in the mixing ratios of monoterpenes at night could perhaps be explained by another mechanism of monoterpene emissions, where emission rate depends on other factors such as vapor pressure in plant tissue. The figure also shows that concentrations of isoprene and monoterpenes decrease with altitude likely due to chemical loss and entrainment. Mixing ratios of MVK+MACR are stable and do not change much with height. There are a few instances where the observations deviate from these trends. For example on June 5 there was a sudden increase in the mixing ratio of MVK+MACR at around 1,000 m at 10.1h. On June 6 there was a sudden drop in the mixing ratios of both isoprene and MVK+MACR at around 580 m. On June 14, the mixing ratio of isoprene fluctuates between about 100-500 m while the concentration of MVK+MACR remained stable. Further analysis is required to determine exact mechanisms dictating these observations.
Figure 2. Vertical profiles of selected VOCs, temperature, and relative humidity during the SOAS campaign. The sampling time (CST) for each research flight is shown in the corresponding legend.
Measurements using the second PTR-TOFMS, located at the SEARCH tower, were also coordinated with other researchers on the tower to make sure that our observations were coincident and co-located with as many other relevant measurements as possible on the flux tower. Measurements were made continuously in H3O+ mode through the whole campaign period during June and July, except for the last few days of the campaign when we tried measurements in O2+ mode, alternating E/N and inlet switching. All data were record at 10 Hz frequency, and ~240 organic ions with significant signal above the instrument zero were detected.
Isoprene concentrations were extremely high, peaking at up to approximately 10 ppb during the hottest and sunniest days. There was excellent agreement between PTR-TOFMS measurements of isoprene mixing ratios and those made by in-situ GC-MS (Figure 3). The temperature dependence of the ambient air isoprene mixing ratios (Figure 4) matched well with the expected temperature dependence observed in leaf level emission measurements (discussed below).
Figure 3. Isoprene mixing ratios observed at the Centreville Alabama SEARCH tower site for by PTR-TOFMS and in-situ GC-MS were in excellent agreement.
Figure 4. Isoprene mixing ratios observed at the Centreville Alabama SEARCH tower site increased exponentially with temperature, consistent with expectations from leaf level flux measurements (shown below).
The SOAS summer 2013 campaign goals include looking holistically at physicochemical processes of BVOC emission, oxidation, and subsequent SOA formation and the role of anthropogenic emissions in biogenic VOC oxidation chemistry. An example of three of the more important organic ions measured, isoprene, methylvinylketone+methacrolein, and hydroxyacetone, all representing the emission of isoprene and its subsequent oxidation in the atmosphere, are shown in Figure 5.
Figure 5. Atmospheric mixing ratios observed at the Centreville Alabama SEARCH tower site for isoprene and some of the major isoprene oxidation products (methyl vinyl ketone (MVK) + methacrolein (MAC), and hydroxyacetone).
The rate of isoprene oxidation and the abundance of the first (MVK, MAC, etc.) and second (hydroxyacetone, etc.) order products were significantly different under cleaner conditions than under more polluted conditions. Isoprene oxidation likely is more dominated by the hydroperoxyl pathway under clean conditions while the NO pathway is more important under pollution conditions. Analysis of this issue, and observations of the full range of detected isoprene oxidation products and how they differ under cleaner and more polluted conditions are currently being analyzed. Further analysis of the concentration and flux measurements from the Centreville site, and their comparison with the FIXCIT study, the AABC tower observations, and the airborne observations from the Long-EZ aircraft are also ongoing.
In June and July 2013, we deployed a leaf level enclosure measurement system to characterize the response of isoprene emission to temperature and light at various heights within a forest canopy in Alabama. A lift was used to access sun and shade leaves at a range of above ground heights. Figure 6 shows that shade leaves have a considerably different response to light than do sun leaves. Figure 7 shows that isoprene emission increases exponentially at temperatures up to 35 degrees. These observations are being used to evaluate isoprene emission model algorithms.
Figure 6. Results of leaf level measurements of isoprene response to photosynthetic photon flux density (PPFD) from sun and shade leaves of Tupelo trees in an Alabama forest.
Figure 7. Results of leaf level measurements of isoprene response to leaf temperature from leaves of Tupelo, white oak and sweetgum trees in an Alabama forest.
The Chamber Study
During the four-week FIXCIT campaign period, we operated the PTR-TOFMS for a series of chamber experiments was conducted to investigate the dark- and photo-induced oxidation of isoprene, α-pinene, methacrolein, pinonaldehyde, acylperoxy nitrates, isoprene hydroxy nitrates (ISOPN), isoprene hydroxy hydroperoxides (ISOPOOH), and isoprene epoxydiols (IEPOX) in a highly controlled and atmospherically-relevant manner. Pinonaldehyde and isomer-specific standards of ISOPN, ISOPOOH, and IEPOX were synthesized and contributed by campaign participants, which enabled explicit exploration into the oxidation mechanisms and instrument responses for these important atmospheric compounds. The FIXCIT data have been analyzed and an initial overview has been prepared for submission to Atmos. Chemistry and Physics. Insights from FIXCIT are anticipated to significantly aid in interpretation of SOAS field data and the revision of mechanisms currently implemented in regional and global atmospheric models.
We will be finalizing data and hopefully submitting several manuscripts for peer review publication.
No journal articles submitted with this report: View all 14 publications for this project
BEIS, isoprene, biogenic VOC oxidation products, deposition
Progress and Final Reports:
2014 Progress Report