skip to main content


Title: Dried, closed-path eddy covariance method for measuring carbon dioxide flux over sea ice

Abstract. The Arctic marine environment plays an important role inthe global carbon cycle. However, there remain large uncertainties in howsea ice affects air–sea fluxes of carbon dioxide (CO2), partially dueto disagreement between the two main methods (enclosure and eddy covariance)for measuring CO2 flux (FCO2). The enclosure method has appearedto produce more credible FCO2 than eddy covariance (EC), but is notsuited for collecting long-term, ecosystem-scale flux datasets in suchremote regions. Here we describe the design and performance of an EC systemto measure FCO2 over landfast sea ice that addresses the shortcomingsof previous EC systems. The system was installed on a 10m tower onQikirtaarjuk Island – a small rock outcrop in Dease Strait located roughly35km west of Cambridge Bay, Nunavut, in the Canadian Arctic Archipelago. Thesystem incorporates recent developments in the field of air–sea gasexchange by measuring atmospheric CO2 using a closed-path infrared gasanalyzer (IRGA) with a dried sample airstream, thus avoiding the known watervapor issues associated with using open-path IRGAs in low-flux environments.A description of the methods and the results from 4 months of continuousflux measurements from May through August 2017 are presented, highlightingthe winter to summer transition from ice cover to open water. We show thatthe dried, closed-path EC system greatly reduces the magnitude of measuredFCO2 compared to simultaneous open-path EC measurements, and for thefirst time reconciles EC and enclosure flux measurements over sea ice. Thisnovel EC installation is capable of operating year-round on solar and windpower, and therefore promises to deliver new insights into the magnitude ofCO2 fluxes and their driving processes through the annual sea icecycle.

 
more » « less
Award ID(s):
1840868
NSF-PAR ID:
10097326
Author(s) / Creator(s):
;
Date Published:
Journal Name:
Atmospheric Measurement Techniques
Volume:
11
Issue:
11
ISSN:
1867-8548
Page Range / eLocation ID:
6075 to 6090
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract. Chemical ionization massspectrometry (CIMS) instruments routinely detect hundreds of oxidized organic compoundsin the atmosphere. A major limitation of these instruments is the uncertaintyin their sensitivity to many of the detected ions. We describe thedevelopment of a new high-resolution time-of-flight chemical ionization massspectrometer that operates in one of two ionization modes: using eitherammonium ion ligand-switching reactions such as for NH4+ CIMS orproton transfer reactions such as for proton-transfer-reaction massspectrometer (PTR-MS). Switching between the modes can be done within 2 min.The NH4+ CIMS mode of the new instrument has sensitivities of upto 67 000 dcps ppbv−1 (duty-cycle-corrected ion counts per second perpart per billion by volume) and detection limits between 1 and 60 pptv at2σ for a 1 s integration time for numerous oxygenated volatileorganic compounds. We present a mass spectrometric voltage scanning procedurebased on collision-induced dissociation that allows us to determine thestability of ammonium-organic ions detected by the NH4+ CIMS instrument.Using this procedure, we can effectively constrain the sensitivity of theammonia chemical ionization mass spectrometer to a wide range of detectedoxidized volatile organic compounds for which no calibration standards exist.We demonstrate the application of this procedure by quantifying thecomposition of secondary organic aerosols in a series of laboratoryexperiments.

     
    more » « less
  2. Abstract. Triplet excited states of organic matter are formed when colored organicmatter (i.e., brown carbon) absorbs light. While these “triplets” can beimportant photooxidants in atmospheric drops and particles (e.g., theyrapidly oxidize phenols), very little is known about their reactivity towardmany classes of organic compounds in the atmosphere. Here we measure thebimolecular rate constants of the triplet excited state of benzophenone(3BP), a model species, with 17 water-solubleC3C6 alkenes that have either been found in theatmosphere or are reasonable surrogates for identified species. Measured rateconstants (kALK+3BP) vary by a factor of 30 and are in therange of (0.24–7.5) ×109 M−1 s−1. Biogenic alkenesfound in the atmosphere – e.g., cis-3-hexen-1-ol, cis-3-hexenyl acetate, andmethyl jasmonate – react rapidly, with rate constants above 1×109 M−1 s−1. Rate constants depend on alkene characteristicssuch as the location of the double bond, stereochemistry, and alkylsubstitution on the double bond. There is a reasonable correlation betweenkALK+3BP and the calculated one-electron oxidation potential(OP) of the alkenes (R2=0.58); in contrast, rate constants are notcorrelated with bond dissociation enthalpies, bond dissociation freeenergies, or computed energy barriers for hydrogen abstraction. Using the OPrelationship, we estimate aqueous rate constants for a number of unsaturatedisoprene and limonene oxidation products with 3BP: values are inthe range of (0.080–1.7) ×109 M−1 s−1, withgenerally faster values for limonene products. Rate constants with lessreactive triplets, which are probably more environmentally relevant, arelikely roughly 25 times slower. Using our predicted rate constants, alongwith values for other reactions from the literature, we conclude thattriplets are probably minor oxidants for isoprene- and limonene-relatedcompounds in cloudy or foggy atmospheres, except in cases in which the tripletsare very reactive.

     
    more » « less
  3. Abstract. Photoacoustic spectroscopy (PAS) has become a popular technique for measuringabsorption of light by atmospheric aerosols in both the laboratory andfield campaigns. It has low detection limits, measures suspended aerosols,and is insensitive to scattering. But PAS requires rigorous calibration to beapplied quantitatively. Often, a PAS instrument is either filled with a gasof known concentration and absorption cross section, such that the absorptionin the cell can be calculated from the product of the two, or the absorptionis measured independently with a technique such as cavity ring-downspectroscopy. Then, the PAS signal can be regressed upon the known absorptionto determine a calibration slope that reflects the sensitivity constant ofthe cell and microphone. Ozone has been used for calibrating PAS instrumentsdue to its well-known UV–visible absorption spectrum and the ease with whichit can be generated. However, it is known to photodissociate up toapproximately 1120nm via the O3 + hν(>1.1eV)O2(3Σg-) + O(3P) pathway, which is likely tolead to inaccuracies in aerosol measurements. Two recent studies haveinvestigated the use of O3 for PAS calibration but have reachedseemingly contradictory conclusions with one finding that it results in asensitivity that is a factor of 2 low and the other concluding that it isaccurate. The present work is meant to add to this discussion by exploringthe extent to which O3 photodissociates in the PAS cell and the rolethat the identity of the bath gas plays in determining the PAS sensitivity.We find a 5% loss in PAS signal attributable to photodissociation at 532nmin N2 but no loss in a 5% mixture of O2 in N2.Furthermore, we discovered a dramatic increase of more than a factor of 2in the PAS sensitivity as we increased the O2 fraction in the bathgas, which reached an asymptote near 100% O2 that nearly matched thesensitivity measured with both NO2 and nigrosin particles. Weinterpret this dependence with a kinetic model that suggests the reason forthe observed results is a more efficient transfer of energy from excitedO3 to O2 than to N2 by a factor of 22–55 depending onexcitation wavelength. Notably, the two prior studies on this topic useddifferent bath gas compositions, and although the results presented here donot fully resolve the differences in their results, they may at leastpartially explain them.

     
    more » « less
  4. Abstract. Following the Budyko framework, the soil wetting ratio (the ratio betweensoil wetting and precipitation) as a function of the soil storage index (theratio between soil wetting capacity and precipitation) is derived from theSoil Conservation Service Curve Number (SCS-CN) method and the variableinfiltration capacity (VIC) type of model. For the SCS-CN method, the soilwetting ratio approaches 1 when the soil storage index approaches ,due to the limitation of the SCS-CN method in which the initial soil moisturecondition is not explicitly represented. However, for the VIC type of model,the soil wetting ratio equals the soil storage index when the soil storageindex is lower than a certain value, due to the finite upper bound of thegeneralized Pareto distribution function of storage capacity. In this paper,a new distribution function, supported on a semi-infinite interval x[0,), is proposed for describing the spatial distribution of storagecapacity. From this new distribution function, an equation is derived for therelationship between the soil wetting ratio and the storage index. In thederived equation, the soil wetting ratio approaches 0 as the storage indexapproaches 0; when the storage index tends to infinity, the soil wettingratio approaches a certain value (≤1) depending on the initial storage.Moreover, the derived equation leads to the exact SCS-CN method when initialwater storage is 0. Therefore, the new distribution function for soil waterstorage capacity explains the SCS-CN method as a saturation excess runoffmodel and unifies the surface runoff modeling of the SCS-CN method and theVIC type of model.

     
    more » « less
  5. Abstract. Oxidation flow reactors (OFRs) are a promising complement toenvironmental chambers for investigating atmospheric oxidation processes andsecondary aerosol formation. However, questions have been raised about howrepresentative the chemistry within OFRs is of that in the troposphere. Weinvestigate the fates of organic peroxy radicals (RO2), which playa central role in atmospheric organic chemistry, in OFRs and environmentalchambers by chemical kinetic modeling and compare to a variety of ambientconditions to help define a range of atmospherically relevant OFR operatingconditions. For most types of RO2, their bimolecular fates in OFRsare mainly RO2+HO2 and RO2+NO, similar to chambers andatmospheric studies. For substituted primary RO2 and acylRO2, RO2+RO2 can make a significant contribution tothe fate of RO2 in OFRs, chambers and the atmosphere, butRO2+RO2 in OFRs is in general somewhat less important than inthe atmosphere. At high NO, RO2+NO dominates RO2 fate inOFRs, as in the atmosphere. At a high UV lamp setting in OFRs,RO2+OH can be a major RO2 fate and RO2isomerization can be negligible for common multifunctional RO2,both of which deviate from common atmospheric conditions. In the OFR254operation mode (for which OH is generated only from the photolysis of addedO3), we cannot identify any conditions that can simultaneouslyavoid significant organic photolysis at 254 nm and lead to RO2lifetimes long enough (∼ 10 s) to allow atmospherically relevantRO2 isomerization. In the OFR185 mode (for which OH is generatedfrom reactions initiated by 185 nm photons), high relative humidity, low UVintensity and low precursor concentrations are recommended for theatmospherically relevant gas-phase chemistry of both stable species andRO2. These conditions ensure minor or negligible RO2+OHand a relative importance of RO2 isomerization in RO2fate in OFRs within ×2 of that in the atmosphere. Under theseconditions, the photochemical age within OFR185 systems can reach a fewequivalent days at most, encompassing the typical ages for maximum secondaryorganic aerosol (SOA) production. A small increase in OFR temperature mayallow the relative importance of RO2 isomerization to approach theambient values. To study the heterogeneous oxidation of SOA formed underatmospherically relevant OFR conditions, a different UV source with higherintensity is needed after the SOA formation stage, which can be done withanother reactor in series. Finally, we recommend evaluating the atmosphericrelevance of RO2 chemistry by always reporting measured and/orestimated OH, HO2, NO, NO2 and OH reactivity (or at leastprecursor composition and concentration) in all chamber and flow reactorexperiments. An easy-to-use RO2 fate estimator program is includedwith this paper to facilitate the investigation of this topic in futurestudies.

     
    more » « less