Calculation of maximum incremental reactivity scales based on typical megacities in China

Abstract

Ozone pollution is becoming increasingly serious in China, the accurate calculation of the volatile organic compounds (VOCs) contributions to ozone formation potential is the precondition for scientific and effective ozone control strategies. Maximum incremental reactivity (MIR) is an effective tool to evaluate VOCs' reactivities, which may vary significantly because of different atmospheric conditions, VOCs compositions, and the relative abundances of VOCs and NOx. Nevertheless, the most widely used MIR calculated by Carter based on 39 urban areas in the United States reflected the ozone formation potential of each VOCs species under atmospheric conditions in US. Whether their MIR values are appropriate under atmospheric conditions in China or not is still under discussion. In order to calculate the real contribution of VOCs to ozone formation, we firstly calculated the indigenized MIR values for Chinese megacities. Box model based on the second Regional Atmospheric Chemistry Mechanism (RACM2) was used to calculate the indigenized MIR for China. Observed data of four megacities (Beijing-Tianjin-Hebei, Yangtze River Delta, Pearl River Delta and Chengdu-Chongqing) which represent the typical atmospheric conditions in China were used as input parameters in calculation. We set two scenarios (Base scenarios and MIR scenarios) for MIR calculation, where Base scenarios were established by selecting the fourth highest ozone day during three years' observed data, MIR scenarios were set by adjusting the NOx availability to obtain the scenarios where ozone formation was most sensitive to VOCs. MIR values were calculated via the ratio of the change in ozone concentration due to the change in VOCs concentration divided by the change of VOCs concentrations in MIR scenarios. MIR_CHN (MIR values for four megacities) was obtained by averaging the normalized MIR values of all sites, with an uncertainty of 25%. Compared to the MIR calculated by Carter (MIR_USA in this study), our results showed a larger range values between the most active VOCs and the most inert VOCs. The MIR_CHN values of highly reactive VOCs (such as internal alkenes, butadiene, anthropogenic dienes, terminal alkenes, isoprene and o/m/p-xylene) were higher than MIR_USA values. Whereas MIR_CHN values of low reactive VOCs (HC5 and acetylene) were lower than MIR_USA values. Values of MIR_CHN and values of MIR_UAS were comparable for the moderate reactive VOCs. The differences between MIR_CHN and MIR_USA may be caused by differences in simulation time, reaction mechanism, and proportion of ozone precursors, NOx availability, and VOCs compositions. Ozone formation potential of four megacities calculated by the MIR_CHN were much higher than that calculated by the MIR_USA, which meant that using MIR_USA may not only underestimate ozone formation potential, but also miss the key VOCs species in ozone formation in China. The key VOCs were isoprene, internal alkenes and m/p-xylene in ozone formation in China based on MIR_CHN calculation.