Alan J. Hills

Eddy covariance measurement of isoprene fluxes

A system has been developed to directly measure isoprene flux above a forest canopy by eddy covariance using the combination of a fast response, real-time isoprene sensor and sonic anemometer. This system is suitable for making nearly unattended, long-term and continuous measurements of isoprene fluxes. Isoprene detection is based on chemiluminescence between isoprene and reactant ozone, which produces green light at 500 nm. The sensor has a noise level (1σ) of 450 pptv for a 1-s integration which is dominated by random high-frequency noise that does not significantly degrade eddy covariance flux measurements. Interference from the flux of other compounds is primarily due to the emission of monoterpenes, propene, ethene and methyl butenol and the deposition of methacrolein and methyl vinyl ketone. The average total interference for North American landscapes in midday summer is estimated to be about 5% for emissions and -3% for deposition fluxes. In only a few North American landscapes, where isoprene emissions are very low and methyl butenol emissions are high, are interferences predicted to be significant. The system was field tested on a tower above a mixed deciduous forest canopy (Duke Forest, North Carolina, USA), dominated by oak trees, which are strong isoprene emitters. Isoprene fluxes were estimated for 307 half-hour sampling periods over 10 days. Daytime fluxes ranging from 1 to 14 mg C m-2 h-1 were strongly correlated with light and temperature. The daytime mean flux of 6 mg C m-2 h-1 were strongly correlated with light and temperature. The daytime mean flux of 6 mg C m-2 h-1 is similar to previous estimates determined by relaxed eddy accumulation by Geron et al. [1997] at this site. Nighttime fluxes were near zero (0.01 ± 0.03 mg C m-2 h-1).

Measurement of isoprene fluxes at the PROPHET site

As part of the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET), isoprene fluxes were measured in the surface layer immediately above the forest canopy by relaxed eddy accumulation (REA) and eddy covariance (EC). Isoprene profiles obtained from aircraft flights at higher altitudes allowed larger-scale isoprene flux estimates based on a mixed layer gradient modeling technique. Emission results derived from the three methods have been compared and used to generate standard emission factors for use in biogenic emission modeling. A standard emission flux of 11.4 mg m -2 h -1 was determined for the canopy footprint region. The aspen and oak isoprene emitting biomass density in the footprint amounted to approximately 150 g m -2 which, when combined with the standard flux, gives a standard emission rate of 76 μg g -1 h -1 for this northern Michigan forest. We found good agreement between isoprene fluxes determined by REA and EC methods. The general emission pattern is the same for both methods, and in many cases, the REA and EC fluxes were nearly identical. The mixed layer modeling approach gave isoprene fluxes that were consistent with those made at the same time at the canopy scale. The continuous coverage of isoprene fluxes by eddy covariance has provided more detailed insight into emission variability during the daytime period. During the midday period, canopy fluxes often changed significantly from one 30 min period to the next.

Upper tropospheric ozone production from lightning NOx-impacted convection: Smoke ingestion case study from the DC3 campaign

As part of the Deep Convective Cloud and Chemistry (DC3) experiment, the National Science Foundation/National Center for Atmospheric Research (NCAR) Gulfstream-V (GV) and NASA DC-8 research aircraft probed the chemical composition of the inflow and outflow of two convective storms (north storm, NS, south storm, SS) originating in the Colorado region on 22 June 2012, a time when the High Park wildfire was active in the area. A wide range of trace species were measured on board both aircraft including biomass burning (BB) tracers hydrogen cyanide (HCN) and acetonitrile (ACN). Acrolein, a much shorter lived tracer for BB, was also quantified on the GV. The data demonstrated that the NS had ingested fresh smoke from the High Park fire and as a consequence had a higher VOC OH reactivity than the SS. The SS lofted aged fire tracers along with other boundary layer ozone precursors and was more impacted by lightning NO_x (LNO_x) than the NS. The NCAR master mechanism box model was initialized with measurements made in the outflow of the two storms. The NS and SS were predicted to produce 11 and 14 ppbv of O_3, respectively, downwind of the storm over 2 days. Sensitivity tests revealed that the ozone production potential of the SS was highly dependent on LNO_x. Normalized excess mixing ratios, ΔX/ΔCO, for HCN and ACN were determined in both the fire plume and the storm outflow and found to be 7.0 ± 0.5 and 2.3 ± 0.5 pptv ppbv^(−1), respectively, and 1.4 ± 0.3 pptv ppbv^(−1) for acrolein in the outflow only.

Airborne flux measurements of methane and volatile organic compounds over the Haynesville and Marcellus shale gas production regions

© 2015. American Geophysical Union. All Rights Reserved. Emissions of methane (CH 4 ) and volatile organic compounds (VOCs) from oil and gas production may have large impacts on air quality and climate change. Methane and VOCs were measured over the Haynesville and Marcellus shale gas plays on board the National Center for Atmospheric Research C-130 and NOAA WP-3D research aircraft in June-July of 2013. We used an eddy covariance technique to measure in situ fluxes of CH 4 and benzene from both C-130 flights with high-resolution data (10Hz) and WP-3D flights with low-resolution data (1Hz). Correlation (R=0.65) between CH 4 and benzene fluxes was observed when flying over shale gas operations, and the enhancement ratio of fluxes was consistent with the corresponding concentration observations. Fluxes calculated by the eddy covariance method show agreement with a mass balance approach within their combined uncertainties. In general, CH 4 fluxes in the shale gas regions follow a lognormal distribution, with some deviations for relatively large fluxes (>10μgm-2s-1). Statistical analysis of the fluxes shows that a small number of facilities (i.e., ~10%) are responsible for up to ~40% of the total CH 4 emissions in the two regions. We show that the airborne eddy covariance method can also be applied in some circumstances when meteorological conditions do not favor application of the mass balance method. We suggest that the airborne eddy covariance method is a reliable alternative and complementary analysis method to estimate emissions from oil and gas extraction.

Intercomparison of automated methodologies for determination of ambient isoprene during the PROPHET 1998 summer campaign

The Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) 1998 summer campaign, conducted at the University of Michigan Biological Station, provided a unique opportunity to compare isoprene measurement techniques that were automated, sampled and analyzed on-line, and provided relatively fast time resolution. Assessment of the data quality for fast isoprene measurements is important because isoprene dominates the surface chemistry at many rural sites and even some urban environments. An informal intercomparison was conducted by evaluating ambient isoprene mixing ratio data generated by five different instruments: quadrupole ion trap (QIT) MS, the chemiluminescent-based fast isoprene sensor (FIS), and three gas chromatograph/mass spectrometry (GC/MS) techniques. The GC/MS methods were deployed and maintained by Purdue University (GC/MS-P), the National Center for Atmospheric Research (GC/MS-NCAR), and the Rosenstiel School of Marine and Atmospheric Science (GC/MS-RSMAS). The FIS was deployed and maintained by NCAR, Hills-Scientific.com and Washington State University, while the QIT was implemented by Purdue University. The GC/MS-P was chosen as the reference method to evaluate the agreement of the data set. The data were evaluated for time-matched samples through regression analysis, ratio analysis, and percent difference analysis relative to GC/MS-P. For measurement data in the central 90th percentile relative to the median, the mean percent difference was 21% for GC/MS-NCAR, 41% for QIT, 42% for GC/MS-RSMAS, and 88% for the FIS. Potential sources of disagreement, especially for low-concentration data, such as variations in sampling time, interferences, method precision and accuracy, and limited cross-calibration, are discussed.

Kinetics of the reactions of diatomic sulfur with atomic oxygen, molecular oxygen, ozone, nitrous oxide, nitric oxide, and nitrogen dioxide

Rates of the reactions S2 + O → SO + S (10), S2 + O2 → products (1), S2 + O3 → products (18), S2 + N2O → products (20), S2 + NO → products (21), S2 + NO2 → products (19) were investigated at 409 K and low pressure (0.89-3.0 Torr) in a discharge-flow system with mass spectrometric detection. Reaction 10 was found to be fast with k10 = (1.12 ± 0.20) × 10-11 cm3 molecule-1 s-1. Under the experimental conditions, S2 did not react with O2, O3, N2O, NO, or NO2. Upper limits for the rate constants of these reactions are k1 < 2.3 × 10-19, k18 < 4.0 × 10-15, k20 < 3.2 × 10-17, k21 < 8.5 × 10-17, and k19 < 1.1 × 10-15 cm3 molecule-1 s-1. Reaction 10 produced a large ion current at m/e 48 leading to the conclusion that S2 + O produces SO + S as primary products.

Atmospheric benzene observations from oil and gas production in the Denver‐Julesburg Basin in July and August 2014

High time resolution measurements of volatile organic compounds (VOCs) were collected using a proton-transfer-reaction quadrupole mass spectrometry (PTR-QMS) instrument at the Platteville Atmospheric Observatory (PAO) in Colorado to investigate how oil and natural gas (O&NG) development impacts air quality within the Wattenburg Gas Field (WGF) in the Denver-Julesburg Basin. The measurements were carried out in July and August 2014 as part of NASAs “Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality” (DISCOVER-AQ) field campaign. The PTR-QMS data were supported by pressurized whole air canister samples and airborne vertical and horizontal surveys of VOCs. Unexpectedly high benzene mixing ratios were observed at PAO at ground level (mean benzene = 0.53 ppbv, maximum benzene = 29.3 ppbv), primarily at night (mean nighttime benzene = 0.73 ppbv). These high benzene levels were associated with southwesterly winds. The airborne measurements indicate that benzene originated from within the WGF, and typical source signatures detected in the canister samples implicate emissions from O&NG activities rather than urban vehicular emissions as primary benzene source. This conclusion is backed by a regional toluene-to-benzene ratio analysis which associated southerly flow with vehicular emissions from the Denver area. Weak benzene-to-CO correlations confirmed that traffic emissions were not responsible for the observed high benzene levels. Previous measurements at the Boulder Atmospheric Observatory (BAO) and our data obtained at PAO allow us to locate the source of benzene enhancements between the two atmospheric observatories. Fugitive emissions of benzene from O&NG operations in the Platteville area are discussed as the most likely causes of enhanced benzene levels at PAO.

Convective transport of formaldehyde to the upper troposphere and lower stratosphere and associated scavenging in thunderstorms over the central United States during the 2012 DC3 study

We have developed semi-independent methods for determining CH2O scavenging efficiencies (SEs) during strong midlatitude convection over the western, south-central Great Plains, and southeastern regions of the United States during the 2012 Deep Convective Clouds and Chemistry (DC3) Study. The Weather Research and Forecasting model coupled with chemistry (WRF-Chem) was employed to simulate one DC3 case to provide an independent approach of estimating SEs and the opportunity to study CH2O retention in ice when liquid drops freeze. Measurements of CH2O in storm inflow and outflow were acquired on board the NASA DC-8 and the NSF/National Center for Atmospheric Research Gulfstream V (GV) aircraft employing cross-calibrated infrared absorption spectrometers. This study also relied heavily on the nonreactive tracers i-/n-butane and i-/n-pentane measured on both aircraft in determining lateral entrainment rates during convection as well as their ratios to ensure that inflow and outflow air masses did not have different origins. Of the five storm cases studied, the various tracer measurements showed that the inflow and outflow from four storms were coherently related. The combined average of the various approaches from these storms yield remarkably consistent CH2O scavenging efficiency percentages of: 54% ± 3% for 29 May; 54% ± 6% for 6 June; 58% ± 13% for 11 June; and 41 ± 4% for 22 June. The WRF-Chem SE result of 53% for 29 May was achieved only when assuming complete CH2O degassing from ice. Further analysis indicated that proper selection of corresponding inflow and outflow time segments is more important than the particular mixing model employed.

Dimethyl sulfide measurement by fluorine-induced chemiluminescence.

We have developed a high-speed sensor for dimethyl sulfide (DMS) based on its fast chemiluminescent reaction with molecular fluorine. Emission in the wavelength range 450-650 nm is monitored via photon counting. The instrument can continuously measure DMS with a response time of 0.1 s and is highly linear and sensitive. Limits of detection (S/N = 1) are 39, 12, and 4 pptv DMS for 0.1-, 1-, and 10-s integration times, respectively. Sensitivity and response time allow the direct measurement of DMS fluxes in the marine atmospheric boundary layer by the eddy correlation technique. Selectivity has previously been measured and is sufficient for monitoring DMS in the marine boundary layer without significant interferences.

Airborne measurements of BrO and the sum of HOBr and Br2 over the Tropical West Pacific from 1 to 15 km during the CONvective TRansport of Active Species in the Tropics (CONTRAST) experiment

A chemical ionization mass spectrometer was used to measure BrO and HOBr + Br2 over the Tropical West Pacific Ocean within the altitude range of 1 to 15 km, during the CONvective TRansport of Active Species in the Tropics (CONTRAST) campaign in 2014. Isolated episodes of elevated BrO (up to 6.6 pptv) and/or HOBr + Br2 (up to 7.3 pptv) were observed in the tropical free troposphere (TFT) and were associated with biomass burning. However, most of the time we did not observe significant BrO or HOBr + Br2 in the TFT and the tropical tropopause layer (TTL) above our limits of detection (LOD). The 1 min average LOD for BrO ranged from 0.6 to 1.6 pptv and for HOBr + Br2 ranged from 1.3 to 3.5 pptv. During one flight, BrO observations from the TTL to the extratropical lowermost stratosphere were used to infer a profile of inorganic bromine (Bry). Based on this profile, we estimated the product gas injection of bromine species into the stratosphere to be 2 pptv. Analysis of Bry partitioning further indicates that BrO levels are likely very low in the TFT environment and that future studies should target the measurement of HBr or atomic Br.