The purpose of this work was to provide a scientific evaluation of the atmospheric fate of aromatic hydrocarbons, in particular, 1,2-dihydroxybenzenes and nitrophenols, which are important products in the oxidation of BTX. The data obtained within the present work will help to improve the knowledge on the atmospheric degradation of aromatic hydrocarbons.
The rate coefficients of the OH and NO3 radical initiated oxidation of some nitro/hydroxy substituted monoaromatic hydrocarbons improve the kinetic data base required to model the degradation mechanisms of aromatic compounds and to develop structure reactivity relationships for OH and NO3 radical with VOCs.
Relative rate coefficients have been measured for the first time for the reactions of NO3 radicals with 1,2-dihydroxybenzene, 3-methyl-1,2-dihydroxybenzene and 4-methyl-1,2-dihydroxybenzene. The investigations were performed in two chambers: the 1080 l quartz glass reactor in Wuppertal and in the EUPHORE chambers in Valencia at 1000 mbar total pressure and 298 ± 3K. The following rate coefficients were obtained: reactant kNO3 (cm3 s-1) 1,2- dihydroxybenzene (9.80 ± 5.0) × 10-11 1,2-dihydroxy-3-methylbenzene (17.2 ± 5.6) × 10-11 1,2-dihydroxy-4-methylbenzene (14.7 ± 6.5) × 10-11 This work has provided rate coefficients for the gas phase reactions of the OH radical with a series of methylated 2-nitrophenols (2-nitrocresols). The following rate coefficients were obtained at 1000 mbar total pressure and 298 ± 3K in the 1080 l quartz glass reactor: reactant kOH (cm3 s-1) 3-methyl-2-nitrophenol (3.69±0.16) × 10-12 4-methyl-2-nitrophenol (3.46±0.18) × 10-12 5-methyl-2-nitrophenol (7.34±0.52) × 10-12 6-methyl-2-nitrophenol (2.70±0.17) × 10-12 Photolysis rates for the 2-nitrocresol isomers have also been determined. All the nitrocresols under investigation have a photolysis lifetime of less than 1 h. The results from this work have shown that photolysis will be the dominant gas-phase loss process for the 2-nitrocresols.
The photolysis of nitrophenols was found to be a new gas-phase source of HONO. Previously, no observations of nitrous acid formation from the gas-phase photolysis of nitrophenols had been reported in the literature. A series of detailed experiments were performed to ensure that the gas phase photolysis of the nitrophenols was the source of HONO, these included: variation of the S/V ratio of the reactors, variation of the light intensity, testing the influence of the buffer gas and tests for a possible mechanism involing reactions of gaseous NO2. All the results were in line with gas-phase photolytic production of HONO from the nitrophenols.
Since a linear dependence of the HONO yield on the nitrophenol concentration was observed, the results obtained here have been extrapolated linearly to atmospheric concentrations. Based on the experimental data obtained for 3-methyl-2-nitrophenol a photolytic HONO formation rate in the atmosphere of 100 pptV h-1 is estimated for a maximum J(NO2) value of 10-2 s-1 in the presence of 1 ppbV of nitrophenols. Based on the HONO formation yields, a general photolysis mechanism has been proposed.
The formation of secondary organic aerosol (SOA) from the photolysis of a series of nitrophenols was investigated for the first time. The effect of NOx and the presence of an OH radical scavenger on the aerosol formation were also investigated. Significant aerosol formation was observed for the nitrophenols investigated. For 2-nitrophenol, the aerosol formation yield in the absence of an OH radical scavenger and NOx varied between 18 - 24%. The gas-phase/aerosol partitioning was fitted assuming the presence of only one compound in both phases. A possible mechanism to explain the aerosol formation observed in the photolysis of nitrophenols is proposed.