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Dimethylphenols – Why? • They are the major products of the xylenes oxidation (benzene, toluene and the xylenes (BTX) are the most important categories of atmospheric pollutants with a remarkable impact on air quality. • And are possible precursors of organic aerosol OH CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 o-xylene m-xylene p-xylene OH CH 3 OH OH OH OH OH CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 2,3 DMP 2,4 DMP 2,5 DMP 2,6 DMP 3,4 DMP 3,5 DMP OH + OH CH 3 CH 3 OH OH CH 3 CH 3 Data Analysis Different reaction pathway: • It is Known that OH reacts with aromatics compounds via an addition at the aromatic ring • NO 3 could reacts via H-atom abstraction. A highly reactive phenoxy radical is formed; amongst its poducts there could be nitrophenols, involved in the aerosol formation. Products (Including nitrophenols) NO 3 + OH CH 3 CH 3 O CH 3 CH 3 HNO 3+ Further Work • Identity products and determine yields • Develop the reaction mechanism • Study the aerosol formation Summary So far we managed to: • Determine rate coefficients & lifetimes • Understand chemical reactivity • Understand the importance of NO 3 reaction More work has to be done… Chemistry in the Troposphere The troposphere acts as a giant reactor where gases (organic, inorganic), aerosols, etc. interact with reactive species (O 3 ; OH; NO 3 ) that initiate their degradation. Day - time O 3 +hυ (λ<310 nm) O 2 +O O+H 2 O (g) 2 OH OH has a global average daytime concentration a of 1.6x10 6 molecule cm -3 NO 3 +hυ(λ<630 nm)NO 2 +O NO+ O 2 NO 3 has a diurn lifetime of ca. 5 s Night - time Overnight O 3 is not photolised so OH can’t be formed NO 2 +O 3 NO 3 +O 2 NO 3 has a global average nighttime concentration b of ca. 5x10 8 molecule cm -3 Two types of chemistry Aims of this project • Determine rate coefficients for the reaction NO 3 + DMPs Products (Reaction with OH already studied) • Identify and quantify the products • Develop chemical mechanisms • Determine the secondary organic aerosol formation All this is done in order to evaluate the environmental impact of DMPs The Atmospheric Smog Chamber • 4 m length 1,2 m diameter V 4750 L • Chemically inert • High transparency • Low wall loss • FEP-Teflon • Dry purified air at 1 atm Chemical analysis by GC, FTIR spectroscopy and particle counter Hydrocarbon concentration 1-20 ppm To reproduce the atmospheric conditions an atmospheric smog chamber is used. Here are reported its characteristics: [ ] [ ] [ ] [ ] = t t t t reference reference k k DMP DMP 0 0 ln ln 2 1 [DMP] t0 , [reference] t0 = concentration at the time t 0 [DMP] t , [reference] t = concentration at the time t k 1 , k 2 = rate coefficients for the reaction (1) and (2) (1) NO 3 +DMP products (2) NO 3 + reference products k o-cresol = 1.6 x 10 -11 cm 3 molecule -1 s -1 c The Relative Rate Method The decay rates of the DMPs and of a reference organic are monitored in presence of NO 3 . Providing that the DMPs and the reference react only with NO 3 , then, Experimental Data 2,6 DMP 3,4 DMP 3,5 DMP -0.2 0 0.2 0.4 0.6 0.8 1 1.2 0 0.1 0.2 0.3 0.4 0.5 0.6 ln([o-cres] t0 /[o-cres] t )-wl*t ln([DMP] t 0 /[DMP] t )-wl*t 2,3 DMP 2,5 DMP 2,4 DMP -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 ln([o-cres] t 0 /[o-cres] t )-wl*t ln([DMP] t 0 /[DMP] t )-wl*t Plotting the ln{[DMP] t0 /[DMP] t } against the ln{[Ref] t0 /[Ref] t } we obtain straight lines trough zero. Their slope moltiplied by k o-cresol gives k DMP. 2,6 DMP and 3,5 DMP present rates of reaction very different fron all the other DMPs. 2,6 DMP 2,4 DMP 2,5 DMP 2,3 DMP 3,4 DMP 3,5 DMP Compound Rate Coefficients & Lifetimes 5.72 3.57 3.48 3.39 2.91 1.28 k NO3 x 10 -11 (cm 3 molecule -1 s -1 ) 6.59 7.15 8.00 8.02 8.14 11.3 k OH x 10 -11 (cm 3 molecule -1 s -1 ) d 9484 35 8741 56 7812 57 7793 59 7678 69 5530 156 τ OH a (day-time s) τ NO3 b (nigh-time s) • The DMPs’ lifetime result shorter for the reaction with NO 3 rather than with OH considering average global concentrations for both oxidants. • NO 3 and OH rates of reaction have opposite values: it indicates different reaction pathway Aerosol formation 3,5 DMP - particle formation (ln) -5 -4 -3 -2 -1 0 1 2 0 20000 40000 60000 80000 100000 time in sec ln(Xt/X0) 0 1 2 3 4 5 6 7 8 9 ln(Pt/P0) 3,5 DMP product A product B particles Preliminary study on the aerosol formation have been done. It is evident that the particle concentration is linked to the DMP’s and products presence. Product’s aerosols result bigger than DMP’s ones. Kinetics of the gas - phase reactions of nitrate radicals with dimethylphenols Perla Bardini and John Wenger Centre for Research into Atmospheric Chemistry (CRAC) - Department of Chemistry - University College Cork Cork, Ireland Photochemical Smog - Definition The photochemical smog is a mixture of gases (NO x ,O 3 , CO, RH and VOC) and particles originated by the emmision of primary pollutants into the troposphere. Troughout photochemical reactions secondary pollutants (O 3 and highly oxidised compounds) are produced. These coumpounds are carcenogenic so it is important to understand their chemistry and yield. O 3 h (<400 nm) PHOTOCHEMICAL SMOG NO 2 Gases Products Organic Aerosol HYDROCARBONS (RH) ALKYL PEROXY RADICALS (RO 2 ) O 2 RADICALS (R ) OH NO ALKOXY RADICALS (RO ) O 3 h<400 nm a [OH] = 1.6x10 6 molecule cm -3 – R. Prinn et al., Science, 269, 187 (1995) b [NO 3 ] = 5x10 8 molecule cm -3 - R. Atkinson et al., J. Geophys. Res, 100, 7275 (1995) c R. Atkinson (1991) d R.Atkinson et al., Int. J. Chem. Kinet., 22,59-67 (1990) We wish to thank EPA and NDP for supporting this research and the Crac lab crew . References
