Photochemical smog and PM_2.5 are two dominant air pollution issues in the Pearl River Delta region. In order to formulate effective control strategy to tackle these pollution issues, we need to have more in-depth studies on chemical formation of key oxidants and secondary aerosols in this region since recent observational studies have highlighted that the atmosphere chemistry in the PRD region are more complex and unique than other regions.

We update the modeling system with the latest available local emission inventory and modified temporal profiles of NOx mobile source emission which consider the traffic control policy implemented in the urban area of PRD region. The model results are validated with recent available air quality monitoring data from the monitoring stations and supersi...[ Read more ]

Photochemical smog and PM_2.5 are two dominant air pollution issues in the Pearl River Delta region. In order to formulate effective control strategy to tackle these pollution issues, we need to have more in-depth studies on chemical formation of key oxidants and secondary aerosols in this region since recent observational studies have highlighted that the atmosphere chemistry in the PRD region are more complex and unique than other regions.

We update the modeling system with the latest available local emission inventory and modified temporal profiles of NOx mobile source emission which consider the traffic control policy implemented in the urban area of PRD region. The model results are validated with recent available air quality monitoring data from the monitoring stations and supersites over the PRD region. Good agreement between the simulation and observation demonstrates the capability of our modeling system to reproduce the ambient atmospheric environment from given emissions and meteorological conditions.

Recently reported model-measurement discrepancies of hydroxyl radical (OH) concentrations under NOx-limited high-isoprene conditions indicate possible deficiencies in OH chemistry currently employed in chemical transport models. With the CB05CL mechanism alone, the CMAQ model under-predicted the observed daytime OH concentrations at the Back Garden site by a factor of 3~4 at low NO levels (<1 ppb), which is consistent with previous studies. After including the Leuven Isoprene Mechanism (LIM) into CMAQ model, modeled OH concentrations increased, compared to base case simulation. LIM0 (the first version of Leuven Isoprene Mechanism) achieves the largest increase on simulated OH concentrations by a factor of 2. Lower but still significant OH increases (40%-55%) in the afternoon can be obtained with updated LIM0, which is based on the results of Müller et al. (2012). If using the 1,6 H-shift isomerization rate suggested by Crounse et al. (2011), only about 15% increase of OH concentrations are obtained. There is still a significant gap compared to the experimental data, even with the LIM mechanism. There remains the need for considering another unidentified OH production mechanism or expecting more OH production from secondary chemistry part in LIM in the future work. The updated LIM0 mechanism increases monthly mean O₃ concentrations by about 2~3 ppb in most PRD region. The enhancement of O₃ in rural and sub-rural areas is to due increased net O₃ production, whereas the enhancement of O₃ in the urban areas with high NOx emissions is mainly due to O₃ regional transport. Under favorable meteorological conditions, the maximum increase of hourly O₃ concentrations can reach up to 10 ppb. It also increases the monthly mean secondary organic aerosol levels by up to 20% in rural areas and about 10% in urban areas.

Potential importance of aqueous phase reaction S(IV) oxidation by NO₂ on sulfate concentrations in the PRD region as well as China is evaluated by using updated sulfate tracking model employed in CMAQ model. Our results show that the monthly mean sulfate in winter increases as high as 5-8% in the north part of the PRD region by including this new pathway into CMAQ model. For the whole China, the enhancement of sulfate due to including S(IV)-NO₂ reaction is more evident in the Yangtze River Delta, the peak value of sulfate formation from S(IV)-NO₂ reaction is up to 15 μg/m³. The relative importance of oxidation reaction of S(IV) with NO₂ shows distinct seasonal trends by increasing sulfate formation more in winter than in summer. Contributions from aqueous-phase, gas-phase, primary sources and long-range transport to sulfate concentrations in the PRD region are also quantified in this study. Model results suggest that in the PRD region, sulfate concentrations in summer is typically dominated by aqueous-phase oxidation of S(IV) by H₂O₂ (about 34%) and gas-phase reaction of SO₂ with OH (about 22%), whereas most of sulfate in winter are contributed by long-range transport (about 79%).