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VOC reactivity in central California

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Atmos. Chem. Phys., 8, 351–368, 2008www.atmos-chem-phys.net/8/351/2008/© Author(s) 2008. This work is licensedunder a Creative Commons License.AtmosphericChemistryand PhysicsVOC reactivity in central California: comparing an air qualitymodel to ground-based measurementsA. L. Steiner1, R. C. Cohen2, R. A. Harley3, S. Tonse4, D. B. Millet5, G. W. Schade6, and A. H. Goldstein71Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI, USA2Department of Chemistry, University of California, Berkeley, CA, USA3Department of Civil and Environmental Engineering, University of California, Berkeley, CA, USA4Lawrence Berkeley National Laboratory, Berkeley, CA, USA5Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA6Department of Atmospheric Sciences, Texas A&M University, College Station, TX, USA7Department of Environmental Science, Policy and Management, University of California, Berkeley, CA, USAReceived: 16 August 2007 – Published in Atmos. Chem. Phys. Discuss.: 7 September 2007Revised: 6 December 2007 – Accepted: 20 December 2007 – Published: 29 January 2008Abstract. Volatile organic compound (VOC) reactivityin central California is examined using a photochemicalair quality model (the Community Multiscale Air Qualitymodel; CMAQ) and ground-based measurements to evalu-ate the contribution of VOC to photochemical activity. Weclassify VOC into four categories: anthropogenic, biogenic,aldehyde, and other oxygenated VOC. Anthropogenic andbiogenic VOC consist of primary emissions, while aldehy-des and other oxygenated VOC include both primary an-thropogenic emissions and secondary products from primaryVOC oxidation. To evaluate the model treatment of VOCchemistry, we compare calculated and modeled OH and VOCreactivities using the following metrics: 1) cumulative distri-bution functions of NOxconcentration and VOC reactivity(ROH,VOC), 2) the relationship between ROH,VOCand NOx,3) total OH reactivity (ROH,total) and speciated contributions,and 4) the relationship between speciated ROH,VOCand NOx.We find that the model predicts ROH,totalto within 25–40% atthree sites representing urban (Sacramento), suburban (Gran-ite Bay) and rural (Blodgett Forest) chemistry. However inthe urban area of Fresno, the model under predicts NOxandVOC emissions by a factor of 2–3. At all locations the modelis consistent with observations of the relative contributionsof total VOC. In urban areas, anthropogenic and biogenicROH,VOCare predicted fairly well over a range of NOxcon-ditions. In suburban and rural locations, anthropogenic andother oxygenated ROH,VOCrelationships are reproduced, butcalculated biogenic and aldehyde ROH,VOCare often poorlycharacterized by measurements, making evaluation of themodel with available data unreliable. In central California,Correspondence to: A. L. Steiner([email protected])30–50% of the modeled urban VOC reactivity is due to alde-hydes and other oxygenated species, and the total oxygenatedROH,VOCis nearly equivalent to anthropogenic VOC reactiv-ity. In rural vegetated regions, biogenic and aldehyde reac-tivity dominates. This indicates that more attention needsto be paid to the accuracy of models and measurements ofboth primary emissions of oxygenated VOC and secondaryproduction of oxygenates, especially formaldehyde and otheraldehydes, and that a more comprehensive set of oxygenatedVOC measurements is required to include all of the impor-tant contributions to atmospheric reactivity.1 IntroductionAtmospheric concentrations of volatile organic compounds(VOC) depend on primary anthropogenic and biogenic emis-sions, chemical formation and loss in the atmosphere (dueto gaseous, aqueous and heterogeneous chemical reactions),and removal from the atmosphere due to dry and wet depo-sition. A wide suite of VOC with varying chemical prop-erties is present in the atmosphere, and the chemical struc-tures of these compounds affect reactivity (Atkinson, 2000).Once in the atmosphere, VOC oxidation leads to the forma-tion of tropospheric ozone when in combination with suffi-cient concentrations of nitrogen oxides (NOx=NO+NO2) andsunlight. VOC oxidation also leads to the formation of sec-ondary organic aerosol via gas-to-particle conversion (e.g.,Odum et al., 1997), aqueous phase chemistry (e.g., Ervenset al., 2004), and/or oligomerization (e.g., Kalberer et al.,2004).Published by Copernicus Publications on behalf of the European Geosciences Union.352 A. L. Steiner et al.: VOC reactivity in central CaliforniaReducing anthropogenic VOC and/or NOxprecursoremissions has been the main method of controlling ozone inthe United States over the past several decades (National Re-search Council, 1991). In some regions, such as the Los An-geles area in southern California, control of anthropogenicVOC from automobiles and industrial processes has proveneffective at reducing ozone exceedances over the past threedecades, indicating that VOC were the limiting factor inozone production in this region (Milford et al., 1989; Mar-tien and Harley, 2006). In other regions with high emissionsof biogenic VOC, NOxcontrol has been more efficient atreducing ozone (Trainer et al., 1988; Sillman et al., 1990;Cardelino and Chameides, 1990; Han et al., 2005). In cen-tral California, differences between weekday and weekendozone concentrations indicate that ozone production in ur-ban regions are NOx-saturated while more remote areas suchas the southern San Joaquin Valley and the Sierra Nevada aremore NOx-sensitive (Blanchard and Fairley, 2001; Marr andHarley, 2002; Murphy et al., 2006a, b).Certain VOC such as alkenes are more reactive than alka-nes and, in the hours immediately after emission, they cancontribute more to ozone formation per unit mass. As a re-sult, VOC have been ranked according to their ozone forma-tion potential and incremental contributions to ozone produc-tion (e.g., Carter, 1994). One element of the overall controlstrategy has been to use modeling studies to identify and re-duce VOC with high ozone formation potential (Russell etal., 1995; Avery, 2006).This approach requires an assessment of regional VOCreactivity and applicable control strategies, which are oftenperformed using air quality models. Recent studies have usedground-based VOC measurements to assess VOC reactivityand validate emission inventories. These results vary by loca-tion, but studies suggest that urban alkanes and alkenes tendto be under estimated by


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