SOURCE CONTRIBUTIONS OF VOLATILE ORGANIC COMPOUNDS TO OZONE FORMATION IN SOUTHEAST TEXAS

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1 Presented at the 8 th Annual CMAS Conference, Chapel Hll, NC, ctober 19-21, 2009 SURCE CNTRIBUTINS F VLATILE RGANIC CMPUNDS T ZNE FRMATIN IN SUTHEAST TEAS Q Yng* Zachry Department of Cvl Engneerng, Texas A&M Unversty, College Staton, T, USA 1. INTRDUCTIN The Houston-Galveston-Brazora (HGB) and Beaumont-Port Arthur (BPA) areas n the southeast Texas are respectvely n severe and moderate non-attanment status of the Natonal Ambent Ar Qualty Standards for ozone ( 3 ) (EPA 2004). Measures must be taken to control and further mprove the 3 ar qualty to better protect publc health. To desgn an effcent emsson control strategy, t s mportant to quanttatvely determne whch sources are responsble for 3 formaton n the polluted areas. The major role of the VCs n the 3 formaton process s n the formaton of peroxy radcals that convert N back to N 2 and regenerate H to allow sustaned net 3 formaton (Fnlayson-Ptts and Ptts 1997; Atknson 2000). The objectves of ths study are to 1) develop a source-orented model to determne the contrbutons of VCs from major emsson sources to regonal 3 formaton and 2) apply the model n Southeast Texas to determne source contrbutons to 3 formaton durng a severe 3 ar polluton epsode. Ths s the frst tme that the regonal source contrbutons of VCs to 3 formaton have been quantfed usng a threedmensonal source-orented model. 2. METHD The net ozone formaton can be attrbuted to responsble VC sources based on the contrbutons of the VCs and ther ntermedate oxdaton products to the N to N 2 converson process. In ths study, the sources of the drectly emtted VCs, the reactve ntermedates and the R 2 and H 2 radcals formed from ther oxdaton are properly tracked n the smulaton of the photochemstry as well as transport and removal processes usng a revsed Communty Multscale Ar Qualty (CMAQ) model (verson 4.6) wth a source-orented SAPRC-99 photochemcal mechansm. The source-orented technque *Correspondng author: Q Yng, Texas A&M Unversty, College Staton, T ; e- mal: qyng@cvl.tamu.edu ntroduces addtonal chemcal speces to represent contrbutons from dfferent sources. For example, f superscrpt represents VC emssons from an explct source (for example, gasolne powered hghway vehcles) and superscrpt represents emssons from all other sources, then the reactons of a general reactve hydrocarbon (RH) from sources and wth H can be wrtten as: RH H R 2 H2 (R1) RH H R H 2 In ths way, the contrbuton of source to the peroxy radcal budget n the model can be drectly determned. The amount of N 2 that s regenerated through reactons of R 2 and N at each chemstry tme step can be determned by keepng track of the extent of reactons (R2) usng the process analyss (PA) tool ncluded n the standard CMAQ dstrbuton. R 2 N N 2 R (R2) R 2 N N 2 R Smlarly, by followng the reactons of R and R wth 2, contrbutons to H 2 for the two sources can be determned and the amount of N 2 regenerated from H 2 and N reacton can be quantfed. The above dscusson llustrates a method to track VC from an explct source () wth emssons from other sources lumped nto a sngle group (). Theoretcally, t s possble to expand the reactons so that emssons from more than one explct source can be drectly tracked smultaneously n a sngle model smulaton. However the number of reactons n the expanded photochemcal mechansm wll ncrease almost exponentally due to the large amount of radcal/radcal reactons. In order to reduce the unnecessary complexty n expandng the mechansm and to make the approach more flexble to resolve a large number of sources, we decded to track only one source explctly at a tme. The fnal revsed condensed SAPRC-99 mechansm ncludes 127 speces and 489 reactons. 2 1

2 Presented at the 8 th Annual CMAS Conference, Chapel Hll, NC, ctober 19-21, MDEL APPLICATIN The CMAQ model wth source-orented SAPRC-99 extenson was appled to study 3 formaton durng the Texas Ar Qualty Study 2000 (TexAQS 2000) epsode, from August 16 to September 6, A three-level nested doman was used n ths study: the eastern Unted States was modeled wth 36 km horzontal grd resoluton; the east Texas and surroundng states were modeled wth 12 km grd resoluton and the HGB and BPA areas n southeast Texas were modeled wth 4 km grd resoluton (referred as the 4 km model doman hereafter). Meteorology nputs needed to drve the CMAQ model were generated by the Meteorology- Chemstry Interface Processor (MCIP) usng the output from the PSU/NCAR mesoscale model (MM5) smulaton conducted by the Texas Commsson of Envronmental Qualty. The EPA s 2001 Clean Ar Interstate Rules (CAIR) emsson nventory was used as a base to generate emsson nputs for the TexAQS 2000 epsode. The emsson nventory was enhanced by usng the epsode-specfc wldfre emssons from the Center for Energy and Envronmental Resources at the Unversty of Texas at Austn (Junquera, Russell et al. 2005). Emssons of olefns from petrochemcal ndustral pont sources were ncreased by a factor of 5 from the reported values based on recent emsson measurements. A revsed Sparse Matrx perator Kernel Emssons (SMKE) emsson processng model (verson 2.4) from US EPA was used to process the updated CAIR emsson nventory to generate grdded, specated and temporally allocated VC emsson rates from each emsson source for the CMAQ model. Bogenc emssons were generated usng the Bogenc Emssons Inventory System, Verson 3 (BEIS3) ncluded n the SMKE dstrbuton. The 1-km resoluton BELD3 land cover data wth 230 dfferent cover types (Vukovch and Perce 2002) were used to estmate emsson from vegetaton. The emssons were grouped nto eght major source categores: bogenc sources, desel vehcles, hghway gasolne vehcles, off-hghway gasolne vehcles, petroleum ndustres, other ndustres, solvent utlzaton and wldfres. ther VC sources do not belong to these explct source categores were lumped nto the other source category. To generate emsson for a specfc source category, a Source Classfcaton Code (SCC) flter was added to the SMKE code so that only emssons from a set of predefned SCC codes would be processed. 4. RESULTS AND DISCUSSIN 4.1 Predcted and bserved 3 Concentratons The predcted concentratons were compared wth all avalable surface observaton data. In general, results show that the predcted concentratons of 3 and Nx agree well wth observatons. Peak 3 concentratons n some days are under-predcted at several stes. Ths under-predcton of peak 3 n the Houston area has also been prevously reported n other modelng studes (Byun, Km et al. 2006). Fg. 1 shows the comparson of the observed and predcted maxmum 8-hour 3 concentratons at all surface statons wthn the 4 km model doman. Except for several extreme events, the predctons are generally wthn % of the observed values. Fg. 1 Predcted and observed daly maxmum 8- hour 3 concentratons at ndvdual observaton staton durng the entre modelng epsode. 4.2 Relatonshp between Net 3 Formaton and N to N 2 Converson Rates Fg. 2 shows the predcted N to N 2 converson rates due to VCs (the sum of the contrbutons from R 2 and H 2 radcals) for all grd cells and the correspondng net photochemcal 3 formaton rates n the 4 km model doman. The net 3 formaton rate s the dfference between the 3 producton rate and the 3 removal rate n a gas-phase chemstry step. The results are for Central Standard Tme (CST), August 31, 2000 n the surface model layer (0-42 m from the surface). Results n upper layers are smlar and thus are not shown here. 2

3 Presented at the 8 th Annual CMAS Conference, Chapel Hll, NC, ctober 19-21, 2009 nly postve net 3 formaton rates are ncluded n the plot. Grd cells where Nx concentratons are hgh actually show negatve 3 formaton rates and these negatve rates are not ncluded n the plot. The data ponts are grouped nto fve dfferent categores based on the VC (ppbc) to Nx (ppb) rato (VC/Nx rato) at each grd cell. In general, the net 3 formaton rate appears to be lnearly correlated wth the N to N 2 converson rates. Data ponts devated from ths lnear relatonshp usually have low VC/Nx rato of 0-5. The relatvely hgher Nx concentratons n these grd cells enhance the N ttraton reacton wth 3 and lead to lower net 3 formaton rates (Klenman, Daum et al. 2005). To acheve the hghest 3 formaton rate, the VC/Nx rato has to be between 5 and 15. The excellent lnear relatonshp between the N to N 2 converson rate and the net 3 formaton rate for most of the data ponts wthn the doman at dfferent VC/Nx ratos suggests that t s reasonable to attrbute the contrbutons of dfferent VC sources to net 3 formaton based on ther contrbutons to the N to N 2 converson processes. Fg. 2 Net ozone formaton rate as a functon of N to N 2 converson rate due to peroxy radcals at CST, August 31, 2000 for the surface model layer. overall net 3 formaton rate (ppb hr -1 ); s 3, z the apportoned contrbuton of VC source to the overall net 3 formaton rate (ppb hr -1 ) and R s the N to N 2 converson rate (ppb hr -1 ); f N 2 s the relatve contrbuton of source to the N to N 2 converson. The delta functon takes a value of 1 when the net 3 formaton rate s greater than zero otherwse t takes a value of 0. K s the total number of VC sources. 3 formaton n the layers above surface layer contrbutes to the surface ozone concentraton through rapd vertcal turbulent dffuson wthn the mxng layer (Byun, Km et al. 2007; Song, Vzuete et al. 2008). Thus, t s useful to defne a columnaveraged source contrbuton expresson as llustrated n Equaton (3): where Z pbl 3, avg w z) z 1 3,avg ( (3) 3, z s the column-averaged contrbuton of source to net ozone formaton, Z pbl s the layer where mxng layer heght s located and the weghtng functon w(z) s the rato of the thckness of the z th layer to the mxng layer heght. Fg. 3 shows the regonal dstrbuton of column-averaged net 3 formaton rate between CST from August 16 to September 6, Hghest 3 formaton rate of 16 ppb hr -1 s located n the downwnd area of the urban and ndustral regons n the model doman. In general, areas north of the Houston Shp Channel area show most sgnfcant 3 formaton rate at ths hour. Several grd cells near the Shp Channel regon shows low net 3 formaton rate. Further analyss of the results ndcates that VC/Nx ratos n these source areas are close to zero due to hgh Nx concentratons durng daytme hours. 4.3 Source Contrbutons of VC to Net 3 Formaton The contrbutons of the th VC source to the net 3 formaton rate at a grd cell n the z th model layer s calculated usng Equaton (1) and (2): f ) (1) 3, z z 3, z ( 3, z R f, 1,2... K N2, z z K k R k 1 N2, z where superscrpt s the source type ndex; z s the model layer ndex; s the predcted 3,z (2) Fg. 3 Net ozone formaton rate below the mxng heght at CST averaged from August 16 September 6, Unts are ppb hr -1. 3

4 Presented at the 8 th Annual CMAS Conference, Chapel Hll, NC, ctober 19-21, 2009 Fg. 4 shows the regonal dstrbuton of VC source contrbutons to the column-averaged net 3 formaton rate at CST averaged from 16 August to September 6, 2000 (see Fg. 3) based on equatons (1-3). Fg. 4(a) shows that contrbuton from bogenc sources s hghest n areas north of the Houston metropoltan area wth a maxmum net 3 formaton rate of approxmately 12.2 ppb hr -1. In the urban areas, the net 3 formaton rate due to bogenc sources s approxmately 6 ppb hr -1. The bogenc VC concentratons are hghest n areas near the northern boundary of the 4km doman but the 3 formaton rates are low due to low Nx concentratons (VC/Nx rato typcally exceeds 500 n these areas). Fg. 4(b) shows that VCs emtted from desel vehcles do not contrbute sgnfcantly to the net 3 formaton process. The hghest contrbuton s approxmately 0.1 ppb hr -1 n the urban Houston area. Fg. 4(c) shows that the spatal dstrbuton of contrbutons from freeway gasolne vehcles s smlar to that of desel vehcle wth hghest contrbuton n the urban Houston regon. The maxmum net 3 formaton rate s approxmately 2.5 ppb hr -1. Fg. 4(d) shows that off-hghway gasolne vehcles contrbute to the net 3 formaton process wth a maxmum rate of approxmately 1.4 ppb hr -1 n the Galveston and Port Arthur area. An nspecton of the emsson nventory ndcates that emssons from pleasure marne crafts (boats powered by 2- and 4- stroke gasolne engnes) are sgnfcant n these regons. Fg. 4(e) shows that petroleum ndustres contrbute to a maxmum of approxmately 1.8 ppb hr -1 to the net photochemcal 3 formaton process. The nfluence of the petroleum ndustres s obvous n the HGB and BPA areas as well as over the waters of the Galveston Bay and the Gulf of Mexco. Fg. 4(f) shows that other ndustral sources contrbute sgnfcantly to the 3 formaton processes wth a peak value of 6.6 ppb -1 n the Shp Channel regon. Fg. 4(g) shows that the contrbuton from solvent utlzaton s manly located west part of the urban Houston regon, wth a maxmum converson rate of 1.4 ppb hr -1. Fg. 4(h) shows that the nfluence of wldfres averaged over the entre model epsode s only lmted n a few areas wth a hghest contrbuton of approxmately 1.2 ppb hr -1. The actual contrbuton of a wldfre near the source s much hgher than the epsode average contrbuton shown here. Fg. 4() shows that sources unaccounted for by the above eght source categores contrbute to the 3 formaton process manly over the off-shore drllng areas n the Gulf of Mexco where the peak contrbuton can be as hgh as 3.1 ppb hr -1. An nspecton of the emsson nventory ndcates that emssons from recprocatng engnes powered by natural gas are the most sgnfcant contrbutors to the other source categores n ths area. Fg. 4 Regonal dstrbutons of net photochemcal ozone formaton rates below the mxng heght for CST averaged from August 16 September 6, The number on each panel s the maxmum converson rate for each source category n unts of ppb hr -1. The color bar on the rght bottom can be used to estmate the rates on each sub-panel. The color scale s lnear, startng from zero to the maxmum value noted on the fgure. Fg. 5 shows the averaged hourly 3 formaton rate from August 16 to September 6, 2000 attrbuted to each VC emsson source. Top 10% of the ar columns wth hghest column-averaged 3 formaton rate were ncluded n the calculaton. The hghest column 3 formaton rate occurs at early afternoon hours. Contrbutons from anthropogenc sources are hghest n the afternoon hours. Fg. 5 Hourly VC source contrbutons to net column ozone formaton rate averaged from August 16 to September 7,

5 Presented at the 8 th Annual CMAS Conference, Chapel Hll, NC, ctober 19-21, Back-trajectory Analyss of Hgh 3 Events Although regonal analyss dscussed n the prevous sectons could determne VC source contrbutons to 3 formaton rate at any gven locaton at any gven tme, t does not drectly determne whch sources contrbute to the observed 3 concentraton because 3 bulds up over tme and the ar parcel arrved at a specfc receptor locaton passes through dfferent regons wth dfferent emsson sgnatures. Backtrajectores of ar parcels arrvng at locatons when hgh 3 concentratons occur can be used to better understand the sources that contrbute to the elevated 3 concentratons at specfc locatons. By nvestgatng the 3 concentratons, net 3 formaton rates and the source contrbutons of N to N 2 converson along an ar parcel trajectory, the contrbutons from major emsson sources to a hgh 3 event can be determned. The relatve contrbuton of VCs from source to surface 3 concentraton at the end of a trajectory can be calculated usng Equaton (4): F T d( t) 3, avg ( t) 1, 1,2,..., K (4) t K T k 1 t 1 d( t) 3, avg ( t) where F s the relatve contrbuton to 3 formaton due to source, ( ) s the contrbuton of 3, avg t source to the column-averaged net 3 formaton rate at tme t, as defned n Equaton (3), T s the number of tme steps used n the trajectory calculaton, d(t) s a decay functon that scales the contrbuton of 3 formaton at tme t to the fnal 3 concentraton and K s the total number of VC sources. In ths study, d(t) s taken as 1 to smplfy analyss. Analyss shows that ths s good frst approxmaton. Equaton (4) was appled to quantfy the source contrbutons to the maxmum 8-hour 3 concentratons n the HGB and BPA areas. All the grd cells on land wthn the Texas state boundary n the 4 km doman were ncluded n ths analyss. For each grd cell, f the daly maxmum 8-hour average 3 concentraton exceeds 90 ppb, 8 backtrajectores arrvng at ths grd cell durng the 8- hour span were calculated and the contrbutons of VCs to 3 formaton along each trajectory were determned. Although the 8-hour 3 standard has been revsed to 75 ppb, a cut concentraton of 90 ppb s selected to represent extreme hgh 3 exceedance cases. Most of the 3 exceedance cases occur on August and September 3-5, grd cells were predcted to have daly maxmum 8-hour 3 concentraton hgher than the 90 ppb lmt and thus a total number of 2400 trajectores were ncluded n the analyss. Fg. 6 shows the cumulatve frequency dstrbuton of the relatve contrbutons of bogenc sources, nternal combuston engnes (desel vehcles, hghway gasolne vehcles, and offhghway gasolne vehcles combned), ndustres (petroleum ndustres and other ndustres combned), solvent utlzaton, wldfres and other sources to the net 3 formaton rate based on the hour trajectores. Bogenc sources are the major contrbutors to hgh 3 concentratons for most of the 8-hour exceedance events predcted by the model. In approxmately 50% of the cases bogenc sources contrbute to over 55% of the net 3 formaton. Industral sources and nternal combuston engnes are both sgnfcant anthropogenc source of VC to 3 formaton. In approxmately 50% of the cases, ther respectve contrbutons exceed 10% and n approxmately 20% of the cases ther contrbutons exceed 30%. Solvent utlzaton s also a large anthropogenc source, contrbutng over 10% for 20% of the cases consdered. Maxmum relatve vehcle contrbuton to 3 formaton s approxmately 20%. Contrbutons from wldfres vary sgnfcantly. In some cases, ts contrbuton can be as hgh as 34%, exceedng the maxmum contrbuton from solvent utlzaton. Fg. 6 Cumulatve frequency dstrbuton of the relatve VC source contrbutons to net ozone formaton for surface grd cells where predcted daly maxmum 8-hour average ozone concentraton s greater than 90 ppb. 5. SUMMARY In ths paper, we llustrated that durng the TexAQS epsode, the net photochemcal 3 formaton rate was lnearly related wth the N 5

6 Presented at the 8 th Annual CMAS Conference, Chapel Hll, NC, ctober 19-21, 2009 to N 2 converson rate due to peroxy and hydroperoxy radcals generated from the photochemcal oxdaton of VCs. A sourceorented SAPRC-99 photochemcal mechansm was developed and ncorporated nto the CMAQ model to drectly determne the contrbutons of eght explct VC sources to the predcted net 3 formaton n the entre model doman based on ther contrbutons to the N to N 2 converson rate. f the eght sources studed, bogenc sources were found to be the largest contrbutor to regonal 3 formaton. Industral sources are the largest anthropogenc source to 3 formaton, followed by gasolne vehcles and solvent utlzaton. Wldfres could contrbute to fast ozone formaton near the fre locatons but ther nfluence s generally localzed. Vukovch JM and Perce T (2002) The Implementaton of BEIS3 wthn the SMKE modelng framework. 2002, / MCNC- Envronmental Modelng Center, Research Trangle Park and Natonal ceanc and Atmospherc Admnstraton, Cty. ACKNWLEDGEMENTS Ths research s supported by the Texas Ar Research Center (TARC) under the project 078ATM2080A. REFERENCES Atknson R (2000) Atmospherc chemstry of VCs and Nx. Atmospherc Envronment. 34(12-14): Byun DW, et al. (2006) Evaluaton of ar qualty models for the smulaton of a hgh ozone epsode n the Houston metropoltan area. Atmospherc Envronment. 41(4): Byun DW, et al. (2007) Evaluaton of ar qualty models for the smulaton of a hgh ozone epsode n the Houston metropoltan area. Atmospherc Envronment. 41(4): EPA: 2004, Regon 6: State Desgnatons for the Hour zone Standard ( andards/regons/regon6desg.htm) Fnlayson-Ptts BJ and Ptts JN (1997) Tropospherc ar polluton: zone, arborne toxcs, polycyclc aromatc hydrocarbons, and partcles. Scence. 276(5315): Junquera V, et al. (2005) Wldfres n eastern Texas n August and September 2000: Emssons, arcraft measurements, and mpact on photochemstry. Atmospherc Envronment. 39(27): Klenman LI, et al. (2005) A comparatve study of ozone producton n fve U.S. metropoltan areas. J. Geophys. Res.-Atmos. 110(D2):20. Song J, et al. (2008) Comparsons of modeled and observed soprene concentratons n southeast Texas. Atmospherc Envronment. 42(8):