Investigation On The O₃-precursor Relationship Over Spatio-temporal Scales In An Industrial City Of The North China Plain

Ambient ozone（O₃）has become one of the dominant environmental issues in China,and the non-linear O₃-precursor relationship also shows significant spatial-temporal differences over a large geographical scale in a city.Therefore,it is essential and imperative to comprehensively investigate the variability of O₃ formation chemistry and its influencing mechanism for formulating precise and effective O₃control strategies at high-resolution spatio-temporal scale in a specific city.Through 3-site VOC field campaigns and 117-site environmental observation network dataset from May to September 2019 in Zibo City of the North China Plain,this study comprehensively explored and quantified the spatial-temporal distribution and evolution of O₃-precursors relationships at the spatial scale of this city by serval approaches（i.e.,0-D box model（based on MCMv3.3.1 chemical mechanism）,smog production model（SPM）and geostatistical indicators）.This study mainly comprises three parts,including the variability of O₃ formation chemistry over multiple patterns of time scale,the driving mechanism of the changing of O₃-VOC-NO_x sensitivity on O₃ weekend effect,and the spatial characteristic and influencing factors of photochemical regime in a city scale.These results are helpful to provide policy-relevant guidance to formulate long-term and accurate O₃ precursor reduction strategies.The main results are followed:1.Based on the dataset from 3 VOC observation sites in the summertime of 2019,the O₃ formation chemistry over multiple patterns of time scale showed overall consistent but non-negligible variability.It was found that the relative incremental reactivity（RIR）of major precursor groups（e.g.,anthropogenic volatile organic compound（AVOC）and NO_x）was overall consistent in the sign along with time scales changed from wider to narrower（four patterns:five-month,monthly,weekly,and daily）at each site,though the magnitudes of RIR varied at different sites.The time series of the photochemical regime(using RIR_NOx/RIR_AVOC as indicator)in weekly or daily patterns further showed a synchronous temporal trend among the three sites,while the magnitude of RIR_NOx/RIR_AVOC was site-to-site dependent.The derived RIR ranking（top 10）of individual AVOC species showed consistency at three patterns（i.e.,five-month,monthly,and weekly）.It was further found that the campaign-averaging photochemical regimes showed overall consistency in the sign but non-negligible variability among the four patterns of time scale,which was mainly due to the embedded uncertainty in model input dataset when averaging individual daily pattern into different timescales.This implies that utilizing narrower time scale（i.e.,daily pattern）is useful to derive reliable and robust O₃-precursor relationship.Our results highlight the importance of quantifying the impact of different time scales to constrain the photochemical regime,which can formulate more accurate policy-relevant guidance for long-term O₃ pollution control.2.Based on the observation dataset in Zibo City,the changing of O₃-NO_x-VOC sensitivity at weekend drove the variations of O₃ weekend effect over spatial scale of Zibo City.During the whole campaign,the 3 VOC observational sites showed diverse characteristics upon O₃ weekend effect.Compared with the weekday,both enhancement of NO_x-O₃ sensitivity and reduction of VOC-O₃ sensitivity on weekends jointly drove the occurrence of O₃ weekend effect at the 3 sites.Also,the changing of O₃ precursors,meteorological conditions and atmospheric radical concentrations on the weekend to varying degrees had an impact on the distribution of O₃ weekend effect.Additionally,according to high-density environmental observation network dataset,it was found that this city mainly presented a spatial reversal of the O₃ weekend effect.The meteorological conditions with high temperature and low humidity,increase of CO concentration,decrease of NO_x concentration and enhancement of atmospheric oxidation capacity were all conducive to promoting the intensity of O₃weekend effect at the spatial scale of this city.In addition,the RIR_NOx/RIR_TVOC and extent（E（t））at the3 VOC observational sites showed significantly correlated over the daily and hourly patterns of time scale,suggesting that employing E（t）indicator was capable of quantifying the O₃-precursor relationship for the observational sites in the city.Compared with weekdays,along with the O₃ formation chemistry changed from VOC-O₃ sensitivity to NO_x-O₃ sensitivity（indicated by E（t）indicator）at the spatial scale of this city on weekend,the O₃weekend effect would be synchronously enhanced.Meanwhile,the increase of VOC concentration and atmospheric oxidation as well as the decline of NO_x concentration at weekend would enhance the weekend E（t）values,consequently leading to the a higher O₃ concentrations on the weekend.Therefore,our result highlights the importance of formulating long-term O₃ precursors reduction strategie at both the spatial and weekly scales of this city.3.Based on the 117-site dataset from the environmental observation network,the spatial variability of O₃ formation chemistry is essential to investigate and formulate precise O₃ control strategies of Zibo City.During the whole campaign,the high-O₃ pollution areas were mainly distributed in the central and northern regions,while low-O₃ pollution regions were mainly located at the southern region.According to the results from various geostatistical methods,it was found that the emission characteristics of O₃ precursors at diversified sites showed significant differences in this city,which also obviously affected the spatial distribution pattern of O₃ formation chemistry.Meanwhile,the southern and northern regions were mainly characterized by high-E（t）agglomeration,while the central areas presented low-E（t）agglomeration,and the O₃-VOC-NO_x sensitivity showed more significant non-aggregation distribution during the pollution period,which also challenged the local O₃precise prevention at the spatial scale of this city to some extent.During high-O₃ hours,the high-E（t）diurnal slopes primarily lied in the northern region,while the low-E（t）mainly distributed in the central area,implying that the northern diurnal variations of the extent for O₃ formation chemistry were stronger,which may be driven by the spatial discrepancies of diurnal variations from O₃ precursors in this city.During high-O₃ hours,the photochemical regimes were mainly considered as transitional（in the central region）and NO_x-limited（in the southern and northern region）in this City.Therefore,the central region in this city should emphasize the importance of synergistic reduction of NO_x and VOC,while the southern and northern regions should focus on NO_x reduction.Along with the NO_x-O₃ sensitivity gradually enhanced at the spatial scale,the summertime O₃pollution significantly increased in this city.Hence,when the O₃ pollution is found in the growth stage in this city,this city should focus on synergistic reduction especially for NO_xemission to alleviate the growth process of O₃ pollution.This study investigated the variability and influencing mechanism for O₃formation chemistry in diverse spatial-temporal scales（i.e.,pattern of time scale,weekly cycle and spatial scale）in Zibo City,which is conducive to providing the precise control strategies for the similar cities in China.