| Non Methane hydrocarbons (NMHCs) are oxidized by the OH radical in the atmosphere, leading to secondary pollutants such as ozone and aldehydes, which play an important role in the creation of photochemical smog and secondary organic aerosols. Therefore it is important to understand the detailed sources and behavior of NMHCs. The study of the stable carbon isotope ratio (delta 13C) can be useful to understand the history of an air parcel, including sources, mixing and photochemical processing.; The Mesoscale Compressible Community and Air Quality (MC2AQ) (3-D) model was modified to include isotope information for propene, toluene, propane, benzene, oxylenes, and isoprene, and run for the full month of July 1999. The NMHCs (both with 12C and 13C) were included as tracers in the model, reacting only with OH, with no feedback on the main model chemistry. This model structure can help to constrain the OH concentration using the stable carbon isotope ratio of NMHCs. The stable carbon isotope ratio could also be a good parameter to evaluate the loss and production of OH radical in the model.; The results obtained from the model, are summarized here: (1) The inverse correlation of the stable carbon isotope ratio with the emissions and mixing ratios of NMHCs was clearly demonstrated. These inverse correlations were observed in the measurement studies by Rudolph et al. (2000, 2001, 2003). This dependence is the consequence of the kinetic isotope effect for the reaction of NMHC with OH-radicals. (2) A diurnal pattern in the stable carbon isotope ratio is demonstrated, which is an indication of the effect of the processing by OH. The carbon isotope ratio is higher (compared with source values) with distance from the source due to the reaction with OH. The NMH 12Cs react faster with OH compared to NMH13Cs. Therefore, studying the carbon isotope ratio of NMHCs is a means to fingerprint the sources of compounds and their processing in the atmosphere. (3) The average day and night differences in stable carbon isotope ratio compared with the source values for some of the NMHCs are significant, and are different for different compounds. The mean day and night variation of the stable carbon isotope ratios in general correlates with lifetimes of NMHCs. (4) The stable carbon isotope ratios increase with increasing altitude. The steepness of the vertical gradient of the stable carbon isotope ratio depends on the lifetime of hydrocarbons. (5) Back trajectories of the stable isotope ratio were determined to study the history of each hydrocarbon independently using the average photochemical age. The results can help in the determination of the possible sources of individual hydrocarbons and the effects of mixing and dilution during parcel advection. This provides information to identify likely locations of the sources of the compounds being investigated. We also study integrated and average photochemical age along the back trajectories of air parcel to quantify the chemical processing of NMHCs. (6) The model was also set up to study the effect of the different emission types (area sources or point sources) of NMHCs on the stable carbon isotope ratio. This method can help us to identify the fractionation and location of these two source types. (7) There are limited measurements of the stable isotope ratios available in the model domain. We use those measurements and compare them with the model results. The results of modeling of the stable carbon isotope ratios of NMHCs show the diurnal behaviour follows the same pattern as the measurements. However, still the model should be developed for better results and more comparison with measurements. |
