Ozone pollution is a serious air quality problem in Hong Kong (HK) and the Pearl River Delta (PRD) region. Controlling the high ozone levels is a hot topic. The ozone formation regime (NOx-limit or VOC-limit) and source apportionment are not only important scientific questions but also important for making effective ozone control strategies. The NOx or VOC reduction has been a controversial issue for ozone control in China in recent years, and it has been a contentious controversial topic in US since 1790s.
To answer these questions, we update and validate our modeling system with the latest available local emission inventory and recent available air quality monitoring data from the PRD. Model evaluation of ozone chemical simulation over the PRD region is performed for the first time. Good agreement between the simulation and observation demonstrates the reliability of latest PRD local emission inventory integrated in the modeling system and the capability of our modeling system.
This well validated modeling system is applied to distinguish contributions by different source regions and source categories at city scales during high ozone episode in the PRD region (ozone source apportionment) via the Ozone Source Apportionment Technology (OSAT) module hosted in CAMx. Based on this method, the local, regional, and super-regional (beyond the PRD region) source contributions from different source categories to the surface ozone of Hong Kong and the PRD are examined and quantified. On monthly average, the super-regional ozone transported into PRD areas is about 70% (40 ppb) in autumn and about 55% (20 ppb) in summer. The super-regional contribution does not change much between the episode and non-episodic days, while the local and PRD regional contributions account for about 50% ozone of episode days in autumn, and up to about 70% of episode days in summer. Increased local and PRD regional contributions (emission sources within the PRD region) are identified as the major cause of high ozone episode of Hong Kong and the PRD region.
Subsequently, the temporal and spatial variation of the ozone formation regime is examined by multiple numerical approaches. Unlike previous results on this issue that suggest that the ozone formation of HK or PRD was VOC-limited and a NOx reduction would make the ozone pollution worse, we find that the ozone concentration are VOCs-limited dominated only in urban and downwind suburbs in the morning and night during ozone non-peak hours, and becomes NOx-limited condition at most areas of HK and the PRD region during peak ozone hours, except some location that we define as “NOx-titration” area of extremely high NOx and low VOC/NOx ratio emissions (e.g. Power Plants). It can be inferred that NOx control could be important for reducing peak ozone concentrations in “non NOx-titration” areas.
Based on our results, a set of control strategies are designed and evaluated through our modeling system. The results are consistent with our above findings. The NOx emission reductions from vehicular source causes the peak ozone decrease in NOx-limited dominated cities of the PRD region, while the VOC emission controls generally lead to ozone reduction all the time.
Moreover, though NOx emission reduction from large point sources could increases ozone at NOx-titration areas, we find that the ozone increase generally happens at the locations with low ozone concentration, whereas at locations with high ozone levels, the ozone generally decreases. Furthermore, this increased ozone does not come from new ozone formation, but is caused by the weakening of the NOx-titration effect.
According to our studies, we believe that NOx control from point sources is also a feasible strategy as well as reducing NOx and VOC emission from vehicular sources. Generally speaking, the more NOx and/or VOC emission are controlled, the larger the decrease in peak ozone. However, to avoid the possible ozone increase make high ozone levels, the NOx emission control of large point sources is better implemented after or together with the effective control of other major sources with less NOx-titration effect.
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