Distribution and Fate of Selected Oxygenated Organic Species in the Troposphere and Lower Stratosphere Over the Atlantic
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. D3, PAGES 3795-3805, FEBRUARY 16, 2000 Distribution and fate of selected oxygenated organic species in the troposphere and lower stratosphere over the Atlantic H. Singh,• Y. Chen,• A. Tabazadeh,• Y. Fukui,• I. Bey,2 R. Yantosca,2D. Jacob,2 F. Arnold,3 K. Wohlfrom,3 E. Atlas,4 F. Flocke,4 D. Blake,• N. Blake,• B. Heikes,6 J. Snow,6R. Talbot,7G. Gregory,8G. Sachse,8S. Vay,8 and Y. Kondo9 Abstract. A large numberof oxygenatedorganic chemicals (peroxyacyl nitrates,alkyl nitrates, acetone,formaldehyde, methanol, methylhydroperoxide, acetic acid and formic acid) were measured during the 1997 SubsonicAssessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) airbornefield campaignover the Atlantic. In this paper,we presenta first pictureof the distribu- tion of theseoxygenated organic chemicals (Ox-organic) in the troposphereand the lower strato- sphere,and assesstheir sourceand sink relationships.In both the troposphereand the lower stratosphere,the total atmosphericabundance of theseoxygenated species (ZOx-organic) nearly equalsthat of total nonmethanehydrocarbons (ZNMHC), which have been traditionallymeasured. A sizablefraction of the reactivenitrogen (10-30%) is presentin its oxygenatedorganic form. The organicreactive nitrogen fraction is dominatedby peroxyacetylnitrate (PAN), with alkyl ni- tratesand peroxypropionyl nitrate (PPN) accounting for <5% of totalNOy. Comparisonof obser- vationswith the predictionsof the Harvard three-dimensionalglobal model suggeststhat in many key areas(e.g., formaldehydeand peroxides)substantial differences between measurements and the- ory are presentand must be resolved. In the caseof CH3OH, there appearsto be a large mismatch betweenatmospheric concentrations and estimatedsources, indicating the presenceof major un- known removalprocesses. Instrument intercomparisons as well as disagreementsbetween obser- vationsand model predictionsare usedto identify neededimprovements in key areas. The atmos- phericchemistry and sourcesof this groupof chemicalsis poorly understoodeven thoughtheir fate is intricatelylinked with uppertropospheric NOx and HOx cycles. 1. Introduction per troposphere(UT) and sequesterreactive nitrogen [Singh et al., 1995; Jaeglg et al., 1997; Wennberg et al., 1998; Craw- Oxygenated hydrocarbonspecies have the potential to play ford et al., 1999]. In general, measurementsof organic oxy- an important role in processes of atmospheric ozone forma- genated species in remote troposphere are extremely sparse tion [Loyd, 1979; Singh et al., 1995; Wennberg et al., 1998]. and often limited to a few species. In the stratosphere, these Unlike most hydrocarbons, many oxygenated species can be measurementsare virtually nonexistent [Singh et al., 1997]. photolyzed throughoutthe troposphere. Carbonyls and perox- The Subsonic Assessment (SASS)Ozone and Nitrogen Oxide ides are potential sourcesof HOx free radicals, and molecules Experiment (SONEX)/DC-8 airborne field campaign of Octo- such as acetonecan form peroxyacetylnitrate (PAN) in the up- ber/November1997, with its focus on the NOx-HOx-O3 chem- istry, provided a unique opportunity to characterize the distri- 1EarthScience Division, NASA Ames Research Center, Moffett bution of several important oxygenatedorganic species in the Field, Califomia. troposphereand the "lowermost" stratosphere (LS) over the 2Departmentof Earthand PlanetarySciences, Harvard University, Atlantic. An overview of the SONEX mission has been pro- Cambridge, Massachusetts. 3MaxPlanck Institute ftir Kemphysik,Heidelberg, Germany. vided by Singh et al. [1999]. 4AtmosphericChemistry Division, National Center for Atmospheric In this paper, we provide a description of the atmospheric dis- Research, Boulder, Colorado. tribution of selectoxygenated organic speciesas measureddur- 5Departmentof Chemistry,University of Califomia,Irvine. ing the 1997 SONEX mission. These species include organic 6Centerfor AtmosphericChemistry Studies, University of RhodeIs- nitrates (per.oxyacyl nitrates, alkyl nitrates), carbonyls land, Narragansett. 7Institutefor theStudy of Earth,Oceans, and Space, University of (acetone, formaldehyde), alcohols (methanol, ethanol), perox- New Hampshire,Durham. ides (methylhydroperoxide), and acids (acetic acid, formic 8EarthScience Division, NASA Langley Research Center, Hampton, acid). Instrumentation and measurement methods are not de- Virginia. scribedhere but have been previously published [Singh et al., 9SolarTerrestrial Environmental Laboratory, Nagoya University, 1999; Schumann et al, 1999; and references therein]. Toyokawa, Japan. SONEX/DC-8 and the Pollution From Aircraft Emissions in the North Atlantic Flight Corridor (POLINAT-2)/Falcon aircraft Copyright2000 bythe American Geophysical Union. platformsalso offered limited opportunitiesfor instrument in- Papernumber 1999JD900779. tercomparisonsand theseresults are discussed. In addition, the 0148-0227/00/1999JD900779509.00 Harvard three-dimensional(3-D) global model has been used to 3795 3796 SINGH ET AL.' DISTRIBUTION OF SELECTED OXYGENATED ORGANIC SPECIES compare these observationswith model predictions. We also pospheric observations have also been compared with the provide an assessmentof the sourcesand sinks of these chemi- Harvard3-D globalmodel (4 ølatitude x 5 ø longitude grid size), cals. an earlier versionof which has been describedby Wang et al. [1998a, b]. The presentversion has improved descriptionsof chemistry and emissions, and higher vertical resolution (20 2. Discussion of Results layers versus9 layers) making it more appropriate for upper It is pertinent to note that during SONEX attention was fo- troposphericsimulations. The model transports 24 tracers, cusedon the UT regionof the atmospherethat has been previ- including oddoxygen (Ox = O3 + NO2 + 2NO3), NOx, HNO3, ously minimally studied. SONEX objectives also were such N205, HNO4, three peroxyacylnitrates, a lumpedalkyl nitrate, that much of the data were collected between 40ø-65øN lati- CO, alkanes (ethane, propane, higher alkanes), alkenes, iso- tudes with only one flight going as far south as 20øN. The prene, carbonyls(acetone and higher alkyl ketones, formalde- stratospherewas frequentlysampled although O_•concentra- hyde, and higher alkyl aldehydes,methacrolein, methylvinyl tions never exceeded450 ppb, and N20 was never below 300 ketone), and peroxides(H202, CH3OOH). A present shortcom- ppb. This region of the stratospherehas been commonly de- ing is that the model contains 1993 meteorology. Compari- son of the 1993 simulation with SONEX observations should fined as the "lowermoststratosphere" (LS). With this perspec- still be useful in terms of mean concentrations and concentra- tive in mind, a greater emphasis is placed on the UT/LS re- gions. Only SONEX data collectedon the DC-8 are usedin this tion gradients. Fuelberg et al. [this issue] conclude that, study. POLlNAT Falcon data are occasionally consideredfor withinthe normalyear-to-year variability, 1997 wasa clima- purposes of comparisons. tologically averageyear. In the following section, we shall TheSONEX NOy data were examined against several tracers discussthe distribution,sources, and sinks of selectedorganic that may be usedto distinguish the stratospherefrom the tro- speciesin the troposphereand the LS. posphere(03, N20 , and CO). It was determinedthat a simple 2.1. Tropospheric Distributions filter (O3>100ppb and Z>6 km) was adequateto define periods of stratosphericinfluences. This filter has been applied to all Figure1 showsthe vertical distribution of 03 andCO (a use- tropospheric/stratosphericobservations discussedhere. Tro- ful tracer) as observedduring SONEX over the North Atlantic 12 12 10 10 E 8 E 8 50 100 150 200 50 100 150 200 CO, ppb 03, ppb 150 150 lOO lOO 50 50 10 20 30 40 50 60 70 10 20 30 40 50 60 70 LATITUDE,ON LATITUDE,ON Figure 1. Troposphericvertical and latitudinal distribution of 03 andCO over the Atlanticduring SONEX. Periodsof stratosphericinfluences (O3>100 ppb; Z>6 kin) are excluded. The vertical structureis for 45ø-65øN with averagescalculated over 2-km bins. Latitudinaldistribution is for the uppertroposphere based on data collectedbetween 8-12 km with averagescalculated over 5ø bins. Resultsfrom the Harvard3-D global model are averagedover 5ø-70øW longitude (SONEX regime) and are shownin similarlines (no datapoints). SINGH ET AL.' DISTRIBUTION OF SELECTED OXYGENATED ORGANIC SPECIES 3797 12 I I I 2.1.1. Organic nitrates. An onboard gas chroma- tograph (GC)operated by the NASA Ames group measured 10 peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), E methyl nitrate (MN), and ethyl/i-propyl nitrates in real time. The stable alkyl nitrates (methyl, ethyl, i-propyl, and 2-butyl) were also measuredfrom the whole air samplescollected by the _ %% NM H C University of California, Irvine group. Figure 3 shows a scat- Ox-org _• terplot of thesemeasurements obtained by the two groups dur- ing the SONEX mission. Recognizingthat the two techniques of sample collection are widely different, sampling times are not exactly identical, and mixing ratios are quite low, agree- ment between the onboard GC and canister sampled measure- I I I ments is consideredquite good. Statistical measuresof ensem- 0 1000 2000 3000 4000 ble averages indicated that these two data sets could be com- Concentration, ppt bined for all intents and purposes. PAN measurementswere 2OOO not duplicatedon the DC-8, and it was also not measuredon the 8-12 km AIt. POLINAT-2/Falcon, so no comparisonis possible. Figure 4