The impact of isoprene emissions on air quality in China

Isoprene, primarily emitted by vegetation, is considered to be the most important biogenic volatile organic compound in the troposphere due to its strong rate of emission and high ozone (O3) formation potential. In model simulations, the impact of isoprene on the O3 distribution is highly affected by the chemical mechanism used, including the treatment of isoprene nitrates (INs) and their impact on NOX recycling. However, most of the chemical schemes included in the chemistry transport model have a rather simple representation of isoprene chemistry with intermediates lumped, which may not be able to reproduce the nonlinear chemistry of isoprene very well. This study aims to understand the detailed isoprene chemistry and its impact under various NOX-VOCs regimes in China using the WRF-Chem model. To do so, a more explicit isoprene chemical mechanism (M4) is developed and implemented into WRF-Chem, and its simulations are validated with the field observations in Beijing as part of the APHH-Beijing programme. The modelled diel patterns of INs agree well with the observations suggesting that the key chemical processes are captured reasonably well, although there are discrepancies in absolute concentrations. Compared to the simpler chemical mechanism (MOZART-4), the reaction between RO2 and HO2 and other RO2 are more competitive in M4 resulting in a slower NOX recycling and lower NOX concentrations over China, which then impact the O3 distribution (±4%). In very high NOX environments, 25% reduction of NOX emissions is simulated to result in up to 20% (10 ppb on average) O3 enhancement, which is due to the increased O3 production owing to reduced loss of HO2 and reduced O3 titration by NO. If isoprene emission increases in these areas, the photochemical pollution can become even more severe. Hence, different O3 control measures need to be designed according to the local NOX-VOCs regimes.