Peroxy Radicals Chemically Amplified Theoretical Studies And Numerical Simulation Of The Water-effect Mechanism

Peroxy radical chemical amplification (PERCA) is an important measurement method to measure atmospheric radicals, which is widely used in determination of peroxy radicals in troposphere for its feature of low cost, high detection sensitivity and facilitated operation. Previous research showed that there is a decrease of the chain length (CL) with the presence of water, known as the water effect in PERCA, which was attributed to the impaction of water on the reactions of radicals (OH, HO2). Early study mainly focused on the reaction of HO2 with NO. Therefore, this work will study other three reactions:OH+NO2(+H2O), OH+CO(+H2O) and OH+CO(+H2O).In this work, we use Gauss-Assisted Software (GAS) to list all possible configurations of the three reaction system and screened out the final configurations. And then, theoretical research of water effect under the chemical amplification process is carried out at the CCSD(T)/6-311G(d, p)//B3LYP/6-311G(d, p) level using the software Gaussian03.The results show that the main channel of the reaction OH+NO2(+H2O) is the formation of HNO3(+H2O). With the presence of water, the trap energy decreases about 135.38 kJ/mol. Thus the presence of water will increase the rate constant of this reaction. The main products of the OH+CO(+H2O) system are H, CO2 and H2O. By comparing the potential energy surface in the condition of with water and no water, we find out that the main channel barrier with water is 135.76 kJ/mol, which is 33.46 kJ/mol higher than it in the condition of no water, indicating a decrease of the rate constant. For the reaction CO+HO2(+H2O), it is discovered that the potential barrier will decrease at the presence of water. The main reaction channel barrier has increased from 96.27 kJ/mol to 101.97 kJ/mol, which imply an decrease of the rate constant. Using the improved Eyring transition state theory, we calculate the rate constant of OH+CO(+H2O) reaction, which is 1.6×10-16 cm3·molecule-1·s-1 with water and 8.0×10-10 cm3·molecule-1·s-1 with no water.Meanwhile, using improved self-adaptive simulated annealing algorithm and the bee genetic algorithm, we use the CMCP procedures to optimize the rate constant of the 4 reactions. The optimized results are: k276=1.85×10-25, k277=2.87×10-26, k278=1.3×10-33, k279=4.0×10-52. The optimal results and Gaussian calculations are exactly consistent with each other.Based on the optimization results with the Gaussian results, we use CMCP to simulate the water effect. The simulation results are consistent with experimental results. To compare simulation results with the experimental results, we introduce three statistics:R2 (The Coefficient of Determination), FB (The Fractional Bias) and IA (The index of Agreement). The results show that the determination coefficient R2 of simulation data with experimental is 0.9947, close to 1, indicating simulation results and experiment data with good correlation. Calculated FB is about-0.35% which is very close to zero. Those results show that the simulation results and the mean of experimental results are consistent. IA value of simulated chain length and experimental data is 0.9982, indicating that simulated data accurately describe the experimental data changes.This work shows that the water effect owns to these four reactions. However, the increase of the rate constant of the two reactions, NO2+OH and HO2+NO, will lead to an increase of radicals loss and decrease the chain length with the present of water. For the other two reaction, there is no net consumption for radicals, thus it need further studies.