Table 2. Chemical Tracers Tracers Composition NOx NO + NO2 + NO3 + HNO2 Ox O3 + O + NO2 + 2 x NO3 PAN peroxyacetyl nitrate CO ALK4 lumped >= C4 alkanes ISOP isoprene HNO3 H2O2 ACET acetone MEKa lumped >= C4 ketones ALD2 acetaldehyde RCHOb lumped > C2 aldehydes MVK methy vinyl ketone MACR methacrolein PMN peroxymethacryloyl nitrate PPN peroxypropionylnitrate R4N2 lumped >= C4 alkylnitrates PRPE lumped >= C3 alkenes C3H8 CH2O C2H6 N2O5 HNO4 MP CH3OOH DMS SO2 SO42-c SO42-(s) sulfate associated with sea salt MSA NH3 NH4+ NO3-d NO3-(s) nitrate associated with sea salt SALA accumulation mode sea salt aerosol SALC coarse mode sea salt hydrophobic BC hydrophilic BC hydrophobic POA hydrophilic POA hydrocarbon class Ie alpha-pinene, beta-pinene, sabinene, Delta3-carene, terpenoid ketones hydrocarbon class IIe limonene hydrocarbon class IVe myrcene, terpenoid alcohols, ocimene SOG1 lumped 9 gas phase products from oxidation of hydrocarbon classes I, II, and IIIe,f SOG2 lumped 3 gas phase products from oxidation of hydrocarbon class IV SOG3 lumped 2 gas phase products from oxidation of hydrocarbon class Ve,g SOG4 lumped 2 gas phase products from oxidation of isoprene SOA1 lumped 9 aerosol phase products from oxidation of hydrocarbon classes I, II, and III SOA2 lumped 3 aerosol phase products from oxidation of hydrocarbon class IV SOA3 lumped 2 aerosol phase products from oxidation of hydrocarbon class V SOA4 lumped 2 aerosol phase products from oxidation of isoprene aMEK does not include MVK. bRCHO does not include MACR. cSO42- is all sulfate, excluding the mass associated with sea salt. dNO3- is all nitrate, excluding the mass associated with sea salt. eParent hydrocarbons are grouped into five classes according to the values of their experimentally measured aerosol yield parameters [Griffin et al., 1999a]. Each of the hydrocarbon classes I, II, and IV is treated as a tracer, while classes III and V are diagnosed at each time step based on emissions. Hydrocarbon classes III and V are not transported in the model because of their high reactivity. fHydrocarbon class III includes alpha-terpinene, gamma-terpinene, and terpinolene. gHydrocarbon class V includes sesquiterpenes.