Final Report: Assessing The Synergistic Impact Of Anthropogenic And Biogenic Emissions On Air Pollution Using Novel High-Sensitivity, Real-Time Monitors For Fundamental CarbonylsEPA Grant Number: R835138
Title: Assessing The Synergistic Impact Of Anthropogenic And Biogenic Emissions On Air Pollution Using Novel High-Sensitivity, Real-Time Monitors For Fundamental Carbonyls
Investigators: Keutsch, Frank N
Institution: University of Wisconsin - Madison
EPA Project Officer: Hunt, Sherri
Project Period: February 1, 2012 through January 31, 2015 (Extended to January 31, 2016)
Project Amount: $250,000
RFA: Developing the Next Generation of Air Quality Measurement Technology (2011) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
The goal of this project is to demonstrate instrumentation that will allow the use of novel high time resolution monitoring data sets as new metrics to determine the contribution of anthropogenic and biogenic emissions to ozone and organic aerosol (PM). We will demonstrate that the proposed novel instrumentation can obtain long-term, low-maintenance and accurate measurements of key carbonyl-containing compounds such as formaldehyde and glyoxal, consistent with the requirements for instrumentation employed in monitoring networks. The project will validate this approach by collecting a yearlong dataset at the Horicon National Core Monitoring Station, a rural site in Wisconsin that is part of the EPA Region 5 Ambient Monitoring Network. An objective of this work is to develop the ratio and absolute concentration of monitored glyoxal/formaldehyde as a metric of the contribution of biogenic and anthropogenic volatile organic compounds (VOCs) to atmospheric oxidation. This metric will allow distinction between the direct contribution of anthropogenic VOCs and the anthropogenic impact on biogenic VOC oxidation (resulting O3 and PM via NOx), an emerging issue in air quality control, in particular for PM. In addition, these data will be compared with WRF-CMAQ model output to evaluate and improve the representation of atmospheric oxidative chemistry in models, thereby helping to provide strategies to control air quality related to O3 and PM.
Oxidation of volatile organic compounds (VOCs) is directly coupled to the formation of secondary pollutants such as ozone and secondary organic aerosol (SOA), which affect human health and climate. In order to have reliable forecasts of ozone and SOA it is important to have an accurate understanding of VOC oxidation and how it is coupled to the formation of pollutants. There are numerous forms of anthropogenic influence on VOC oxidation and two major ones are influence from anthropogenic VOCs and anthropogenic nitrogen oxide (NOx = NO + NO2) emissions. It is of particular interest to not just measure the anthropogenic emissions themselves but rather the impact they have on VOC oxidation. Formaldehyde and glyoxal are two VOC oxidation products that represent an especially useful set of tracers of VOC oxidation, and more importantly, of anthropogenic influence on VOC oxidation. This arises from the fact that both are formed in the oxidation of numerous VOCs, but in varying amounts. However, monitors suitable for low-maintenance, long-term, high sensitivity and high time resolution measurements of formaldehyde and glyoxal able to realize the potential with respect to air quality monitoring described above have not been available.
The objectives of this grant were to develop such monitors, deploy them at the Horicon National Core Monitoring Station, a rural site in Wisconsin that is part of the EPA Region 5 Ambient Monitoring Network, and evaluate the utility of the ratio of glyoxal and formaldehyde (RGF) in combination with the absolute glyoxal and formaldehyde concentrations as a metric that allows distinction between anthropogenic influence on VOC oxidation via anthropogenic VOC emissions and anthropogenic NOx emissions.
We developed and deployed a new monitor for measuring formaldehyde based on a novel UV fiber laser used for laser-induced fluorescence (LIF) detection of formaldehyde. LIF detection of formaldehyde had previously been proven to be ideally suited for the desired formaldehyde monitor but was hindered by availability of suitable laser systems. The novel UV fiber laser successfully addressed this need. The instrument was deployed at the Horicon site and successfully collected an unprecedented near year-long set of high time resolution, high sensitivity, formaldehyde data. The monitor clearly showed the advantage of a continuous high time resolution system compared to sporadic or low time resolution systems as rapid changes in formaldehyde concentrations were observed. As chemical processing can be highly non-linear, averaged concentrations can fail to capture VOC oxidation faithfully. With this work, a novel formaldehyde monitor was successfully demonstrated.
Development of the analogous glyoxal monitor, based in this case on laser-induced phosphorescence, revealed that current compact laser systems, required for long-term monitors, are not yet capable of providing reliable monitors. Further developments in compacted laser technology are required for the full capability of the glyoxal monitor to become available. To address the objectives of the grant with respect to evaluating the utility of RGF in combination with glyoxal and formaldehyde concentrations as metrics of anthropogenic influence via NOx and anthropogenic VOCs, we used measurements of glyoxal using instrumentation not suitable for long-term monitoring due to cost, size or power and maintenance requirements.
One analysis was completed using airborne data collected under a different EPA grant as part of Southeast Nexus (SENEX) component of the Southeast Atmosphere Study (SAS). The analysis clearly shows the utility of glyoxal and formaldehyde as tracers for the composition VOC mixture. One important aspect of that work was that it clearly showed that RGF depends on VOC composition, directly addressing one of the main objectives of this grant: RGF was higher for air with high terpene to isoprene ratios and lower for high isoprene to terpene ratios. This clearly showed that RGF is sensitive to which VOC represents the more important reactive emission, which is interesting as for both species (formaldehyde and glyoxal) satellite retrievals exist. In contrast, RGF showed no observable dependence on the amount of anthropogenic nitrogen oxide (NOx) emissions. However, the absolute concentrations of glyoxal and formaldehyde increased significantly with NOx. The combination shows that the approach of the work proposed here is fundamentally sound: the ratio of glyoxal to formaldehyde reflects the dominant reactive VOC mixture and the absolute amounts reflect the degree of anthropogenic influence via NOx.
To further study this central, objective of the grant, we conducted a more detailed study, from measurements at a ground site in Alabama as part of the Southern Oxidant and Aerosol study (SOAS) component of SAS. At the SOAS site the reactive VOC mixture is nearly always dominated by isoprene and this allowed analysis of the dependence of RGF on a large number of parameters without the convolution of a changing VOC mixture. In agreement with our hypothesis and the SENEX results, RGF had no dependence on NOx. This is the result of small but opposing dependencies of RGF on NO and NO2, which when combined result in no dependence on NOx (NO+NO2). Our work also revealed that to what degree VOC oxidative processing occurs via reaction of organic peroxy radicals with NO versus reaction with the hydroperoxide radicals is reflected in the glyoxal and formaldehyde concentrations and in RGF. This trend is the result of the dependence of HO2, glyoxal and formaldehyde on isoprene emissions. This work thus shows an additional way in which RGF may be a tracer of anthropogenic influence, in this case on the fate of organic peroxy radicals. Fundamentally, the work leveraging other measurements with the formaldehyde monitor and traditional glyoxal instruments achieved one of the main objectives of the grant: We confirmed the hypothesis that RGF is a good indicator for the VOC species dominating the reactive VOC emissions, that RGF is independent of NOx emissions, and that the (absolute) concentrations of glyoxal and formaldehyde have a strong dependence on NOx emissions for a given VOC mixture. Thus RGF in combination with glyoxal and formaldehyde concentrations allows distinction between the direct contribution of anthropogenic VOCs, if they contribute significantly to VOC reactivity, and the anthropogenic impact of NOx on VOC oxidation, one of the main objectives of the grant.