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10.2 Carbon Bond Mechanism (CB-IV)

Chemistry mechanisms included in most Photochemical Air Quality Simulation Models (PAQSMS) use reduced versions of known chemical mechanisms. Most chemical mechanisms are too big to be implemented efficiently in PAQSMS, as is, due to high computational overhead involved with their solution at each time-step. These mechanisms are lumped, or approximated by a smaller number of reactions, by one of several strategies: mathematical lumping, molecular lumping, or structural lumping. The Carbon Bond-IV Mechanism was developed mainly for urban smog and regional atmospheric modeling[154]. This photochemistry mechanism is implemented in the Urban Airshed Model (UAM) [154], and the Reactive Plume Model (RPM)[195,153].

This mechanism is a hybrid of explicit chemistry, surrogate approximations, and lumped or generalized chemistry designed to simulate the features of urban smog chemistry. Explicit chemistry is used for the inorganic and Carbonyl species and the chemistries of Ethene, Isoprene, and Formaldehyde. The Ethene chemistry, for example, is treated explicitly because it reacts much slower than other alkenes, it is a large fraction of common hydrocarbon emissions, and that it yields a high fraction of Formaldehyde. Isoprene is an Alkene but it is treated explicitly also because its reactions with free radicals are much faster, and it forms a prominent part of biogenic emissions in rural areas. Many peroxy radicals have been lumped into a single XO$_2$ universal Peroxy radical. The lumping method of carbon-bonds is used mainly for paraffins and olefins. Molecular surrogates Toluene and Xylene are used for higher aromatic compounds. For instance, a complex molecule with both aromatic and alkene structures might be represented with a combination of TOL, OLE, and PAR surrogates. Also, some of the rates and the stoichiometries of some incorporated reactions depend upon the atmospheric composition of reacting hydrocarbons.

The chemical species and surrogates that are explicitly represented in CBM-IV are listed in Table C.1. The hierarchical relationship between the major species in the carbon bond mechanism is depicted in Figure C.1. The most complex species, in terms of their oxidation products (paraffins, olefins, isoprene and aromatic hydrocarbons), are placed at the highest levels. The simplest species, in terms of molecular complexity ( NOx, HOx, CO and formaldehyde), are placed at the lowest levels. Table C.2 presents the chemical reactions that constitute the CB-IV mechanism.


  
Figure C.1: Major species in the CBM and their hierarchical relationship (adapted from Wang [213])
\begin{figure} \centerline{\epsfig{figure=species.ps,width=5in}} \end{figure}



   
Table C.1: Chemical Species in the CBM-IV mechanism
Species Name representation
Nitric oxide NO
Nitrogen dioxide NO2
Nitrogen trioxide (nitrate radical) NO3
Dinitrogen pentoxide N2O5
Nitrous acid HONO
Nitric acid HNO3
Peroxy-nitric acid (HO$_2$NO$_2$) PNA
Oxygen atom (singlet) O1D
Oxygen atom (triplet) O
Hydroxyl radical OH
Water H2O
Ozone O3
Hydroperoxy radical HO2
Hydrogen peroxide H2O2
Carbon monoxide CO
Formaldehyde (CH$_2=$O) FORM
High molecular weight aldehydes ALD2
Peroxyacyl radical (CH$_3$C(O)OO$\cdot$) C2O3
Peroxyacyl nitrate (CH$_3$C(O)OONO$_2$) PAN
Paraffin carbon bond (C-C) PAR
Secondary organic oxy radical ROR
Olefinic carbon bond OLE
Ethene (CH$_2=$CH$_2$) ETH
Toluene (C$_6$H$_5$-CH$_3$) TOL
Cresol and higher molecular weight phenols CRES
Toluene-hydroxyl radical adduct TO2
Methylphenoxy radical CRO
High molecular weight aromatic oxidation ring fragment OPEN
Xylene (C$_6$H$_4$-(CH$_3$)$_2$) XYL
Methylglyoxal (CH$_3$C(O)C(O)H) MGLY
Isoprene ISOP
NO-to-NO$_2$ operation XO2
NO-to-nitrate operation XO2N
Total 33



  
Table C.2: Chemical Reactions in the Carbon Bond IV Mechanism
Reaction      
No.   Reaction  
       
R1 NO$_2$ + h$\nu$ $\longrightarrow$ NO + O
R2 O $\stackrel{O_2, M}{\longrightarrow}$ O$_3$
R3 O$_3$ + NO $\longrightarrow$ NO$_2$
R4 O + NO$_2$ $\longrightarrow$ NO
R5 O + NO$_2$ $\stackrel{M}{\longrightarrow}$ NO$_3$
R6 O + NO $\stackrel{M}{\longrightarrow}$ NO$_2$
R7 NO$_2$ + O$_3$ $\longrightarrow$ NO$_3$
R8 O$_3$ + h$\nu$ $\longrightarrow$ O
R9 O$_3$ + h$\nu$ $\longrightarrow$ O$^1$D
R10 O$^1$D $\stackrel{M}{\longrightarrow}$ O
R11 O$^1$D + H$_2$O $\longrightarrow$ 2 OH
R12 O$_3$ + OH $\longrightarrow$ HO$_2$
R13 O$_3$ + HO$_2$ $\longrightarrow$ OH
R14 NO$_3$ + h$\nu$ $\longrightarrow$ 0.89 NO$_2$ + 0.89 O + 0.11 NO
R15 NO$_3$ + NO $\longrightarrow$ 2 NO$_2$
R16 NO$_3$ + NO$_2$ $\longrightarrow$ NO + NO$_2$
R17 NO$_3$ + NO$_2$ $\stackrel{M}{\longrightarrow}$ N$_2$O$_5$
R18 N$_2$O$_5$ + H$_2$O $\longrightarrow$ 2 HNO$_3$
R19 N$_2$O$_5$ $\stackrel{M}{\longrightarrow}$ NO$_3$ + NO$_2$
R20 NO + NO $\stackrel{O_2}{\longrightarrow}$ 2 NO$_2$
R21 NO + NO$_2$ H$_2$O $\longrightarrow$ 2 HNO$_2$
R22 NO + OH $\stackrel{M}{\longrightarrow}$ HNO$_2$
R23 HNO$_2$ + h$\nu$ $\longrightarrow$ NO + OH
R24 OH + HNO$_2$ $\longrightarrow$ NO$_2$
R25 HNO$_2$ + HNO$_2$ $\longrightarrow$ NO + NO$_2$
R26 NO$_2$ + OH $\stackrel{M}{\longrightarrow}$ HNO$_3$
R27 OH + HNO$_3$ $\stackrel{M}{\longrightarrow}$ NO$_3$
R28 HO$_2$ + NO $\longrightarrow$ OH + NO$_2$
R29 HO$_2$ + NO$_2$ $\stackrel{M}{\longrightarrow}$ PNA
R30 PNA $\stackrel{M}{\longrightarrow}$ HO$_2$ + NO$_2$
R31 OH + PNA $\longrightarrow$ NO$_2$
R32 HO$_2$ + HO$_2$ $\longrightarrow$ H$_2$O$_2$
R33 HO$_2$ + HO$_2$ H$_2$O $\longrightarrow$ H$_2$O$_2$
R34 H$_2$O$_2$ + h$\nu$ $\longrightarrow$ 2 OH
R35 OH + H$_2$O$_2$ $\longrightarrow$ HO$_2$
R36 OH + CO $\stackrel{O_2}{\longrightarrow}$ HO$_2$
R37 FORM + OH $\stackrel{O_2}{\longrightarrow}$ HO$_2$ + CO
R38 FORM + h$\nu$ $\stackrel{2O_2}{\longrightarrow}$ 2 HO$_2$ + CO
R39 FORM + h$\nu$ $\longrightarrow$ CO
R40 FORM + O $\longrightarrow$ OH + HO$_2$ + CO
R41 FORM + NO$_3$ $\stackrel{O_2}{\longrightarrow}$ HNO$_3$ + HO$_2$ + CO
R42 ALD2 + O $\stackrel{O_2}{\longrightarrow}$ C$_2$O$_3$ + OH
R43 ALD2 + OH $\longrightarrow$ C$_2$O$_3$
R44 ALD2 + NO$_3$ $\stackrel{O_2}{\longrightarrow}$ C$_2$O$_3$ + HNO$_3$
R45 ALD2 + h$\nu$ $\longrightarrow$ FORM + 2 HO$_2$ + CO + XO2
R46 C$_2$O$_3$ + NO $\stackrel{O_2}{\longrightarrow}$ FORM + NO$_2$ + HO$_2$ + XO2
R47 C$_2$O$_3$ + NO$_2$ $\longrightarrow$ PAN
R48 PAN $\longrightarrow$ C$_2$O$_3$ + NO$_2$
R49 C$_2$O$_3$ + C$_2$O$_3$ $\longrightarrow$ 2 FORM + 2 XO2 + 2 HO$_2$
R50 C$_2$O$_3$ + HO$_2$ $\longrightarrow$ 0.79 FORM + 0.79 XO2 + 0.79 HO$_2$ + 0.79 OH
R51 OH $\longrightarrow$ FORM + XO2 + HO$_2$
R52 PAR + OH $\longrightarrow$ 0.87 XO2 + 0.13 XO2N + 0.11 HO$_2$ + 0.11 ALD2
      + 0.76 ROR - 0.11 PAR
R53 ROR $\longrightarrow$ 0.96 XO2 + 1.1 ALD2 + 0.94 HO$_2$ + 0.04 XO2N
      + 0.02 ROR - 2.1 PAR
R54 ROR $\longrightarrow$ HO$_2$
R55 ROR + NO$_2$ $\longrightarrow$  
R56 O + OLE $\longrightarrow$ 0.63 ALD2 + 0.38 HO$_2$ + 0.28 XO2 + 0.3 CO
      + 0.2 FORM + 0.02 XO2N + 0.22 PAR + 0.2 OH
R57 OH + OLE $\longrightarrow$ FORM + ALD2 - PAR + XO2 + HO$_2$
R58 O$_3$ + OLE $\longrightarrow$ 0.5 ALD2 + 0.74 FORM + 0.22 XO2 + 0.1 OH
      + 0.33 CO + 0.44 HO$_2$ - PAR
       
R59 NO$_3$ + OLE $\longrightarrow$ 0.91 XO2 + FORM + 0.09 XO2N + ALD2
      + NO$_2$ - PAR
R60 O + ETH $\longrightarrow$ FORM + 1.7 HO$_2$ + CO + 0.7 XO2 + 0.3 OH
R61 OH + ETH $\longrightarrow$ XO2 + 1.56 FORM + 0.22 ALD2 + HO$_2$
R62 O$_3$ + ETH $\longrightarrow$ FORM + 0.42 CO + 0.12 HO$_2$
R63 TOL + OH $\longrightarrow$ 0.44 HO$_2$ + 0.08 XO2 + 0.36 CRES + 0.56 TO2
R64 TO2 + NO $\longrightarrow$ 0.9 NO$_2$ + 0.9 HO$_2$ + 0.9 OPEN
R65 TO2 $\longrightarrow$ CRES + HO$_2$
R66 OH + CRES $\longrightarrow$ 0.4 CRO + 0.6 XO2 + 0.6 HO$_2$ + 0.3 OPEN
R67 CRES + NO$_3$ $\longrightarrow$ CRO + HNO$_3$
R68 CRO + NO$_2$ $\longrightarrow$  
R69 OH + XYL $\longrightarrow$ 0.7 HO$_2$ + XO2 + 0.2 CRES + 0.8 MGLY
      + 1.1 PAR + 0.3 TO2
R70 OPEN + OH $\longrightarrow$ XO2 + 2 CO + 2 HO$_2$ + C$_2$O$_3$ + FORM
R71 OPEN + h$\nu$ $\longrightarrow$ C$_2$O$_3$ + HO$_2$ + CO
R72 OPEN + O$_3$ $\longrightarrow$ 0.03 ALD2 + 0.62 C$_2$O$_3$ + 0.7 FORM + 0.03 XO2
      + 0.69 CO + 0.08 OH + 0.76 HO$_2$ + 0.2 MGLY
R73 OH + MGLY $\longrightarrow$ XO2 + C$_2$O$_3$
R74 MGLY + h$\nu$ $\longrightarrow$ C$_2$O$_3$ + HO$_2$ + CO
R75 O + ISOP $\longrightarrow$ 0.6 HO$_2$ + 0.8 ALD2 + 0.55 OLE + XO2
      + 0.5 CO + 0.45 ETH + 0.9 PAR
R76 OH + ISOP $\longrightarrow$ XO2 + FORM + 0.67 HO$_2$ + 0.13 XO2N
      + ETH + 0.4 MGLY + 0.2 C$_2$O$_3$ + 0.2 ALD2
R77 O$_3$ + ISOP $\longrightarrow$ FORM + 0.4 ALD2 + 0.55 ETH + 0.2 MGLY
      + 0.1 PAR + 0.06 CO + 0.44 HO$_2$ + 0.1 OH
R78 NO$_3$ + ISOP $\longrightarrow$ XO2N
R79 XO2 + NO $\longrightarrow$ NO$_2$
R80 XO2 + XO2 $\longrightarrow$  
R81 XO2N + NO $\longrightarrow$  
R82 XO2 + HO$_2$ $\longrightarrow$  

     

next up previous contents
Next: 11. THE EPACMTP MODEL Up: 10. URBAN AIRSHED MODEL (CB-IV).2 Previous: 10.1 The Urban Airshed
Sastry S. Isukapalli
1999-01-19