Impacts Of Non-traditional Factors On Summertime Near-surface Ozone Pollution In The North China Plain

Tropospheric ozone（O₃）pollution frequently occurs in China during summer.High O₃ concentrations exert detrimental impacts on the health of human beings and ecosystems.O₃ is formed by a series of complex photochemical reactions of nitrogen oxides（NO_X）、volatile organic compounds（VOCs）,and carbon monoxide（CO）in the presence of sunlight with no direct emissions.O₃ concentrations are influenced by the intricacies of atmospheric chemistry and meteorology.An increasing trend was shown by long-term observations of near-surface O₃ concentrations in China.Particulate matter（PM）concentrations have significantly decreased since the implementation of the“Air Pollution Prevention and Control Action Plan”during 2013–2017.Near-surface O₃ concentrations have dramatically increased and O₃ pollution becomes a primary environmental challenge in China recently.The North China Plain（NCP）is featured by the heaviest pollution of O₃ among megacity clusters.Previous studies about O₃ concentration attributions and drivers of O₃ enhancement are traditionally focused on anthropogenic precursor sources（including NO_X and VOCs emissions from industrial,power plant,traffic,and residential sources）and meteorological factors（including air temperature,relative humidity,wind direction,and wind speed）.However,the mechanism of deteriorated O₃ pollution remains controversial currently.Therefore,to explore summertime near-surface O₃ pollution in the NCP,combining observations and model simulations,the effects of weather conditions,precursor emissions,and chemical processes on O₃ pollution and its deterioration were investigated in this paper.Studies included no-traditional effects of（1）extreme weather conditions——heat waves,（2）cropland NO_X emission,（3）vessel emission,and（4）HO₂·heterogeneous uptake on wet aerosol surfaces on near-surface O₃ concentrations in summer in the NCP.Detailed information and main outcomes are represented as follows:（1）Effects of heat waves on O₃ pollution:Remarkably elevated surface air temperature and intensified UV radiation were observed in summer in the Beijing-Tianjin-Hebei region（BTH）during 2013–2017.The main part of the WPSH extended considerably westward and northward in the summer of 2017,inducing a warmer and drier climate in the BTH.Compared to those in the 2013–2016 mean,the frequencies,duration,and intensity of heat waves remarkably increased by 11%,31%,and 102%,respectively,in the BTH in JJA 2017,providing weather conditions that facilitate O₃formation and accumulation.The frequency of O₃ exceedances in JJA 2017 was 1.5-times the average frequency of O₃ exceedances during JJA 2013–2016,and O₃exceedances lasting more than six days significantly increased.However,the contribution of weather shifts to O₃ concentration variability owing to climate change remains elusive.The WRF-Chem model was used to investigate the impacts of the anthropogenic emission mitigation and weather condition shifts on summertime O₃concentration variability in the BTH region.Results showed that anthropogenic emission changes contributed approximately 23%of the increase in maximum daily average 8-hour（MDA8）O₃ concentrations in the BTH in JJA 2017 compared with that in 2013.The consecutive occurrences of unfavorable weather conditions（heat wave events）substantially contributed 15.5μg m^-3（approximately 50%）of the MDA8 O₃concentration enhancements,constituting one of the most important factors worsening summertime O₃ pollution over the BTH region in JJA 2017.（2）Effects of cropland NO_X emissions on O₃ pollution:Soil NO_X emissions were measured and estimated to be the second most significant contributor to the NO_X burden following the fossil fuel combustion source globally.NO_X emissions from croplands were subject to being underestimated or overlooked in air pollution simulations of regional atmospheric chemistry models.With constraints of ground and space observations of NO₂,the WRF-Chem model was used to explore the cropland NO_Xemission and its contribution to the near-surface O₃ pollution in NCP during a growing season.Model simulations revealed that the cropland NO_X emissions were underestimated by around 80%without constraints of satellite measured NO₂ column densities.The biogenic NO_X source was estimated to account for 49%of the anthropogenic NO_X emissions in the NCP during the growing season.Additionally,the cropland NO_X source contributed around 5.0%of MDA8 O₃ concentration and 27.7%of NO₂ concentration in the NCP.Our results suggest that agriculture NO_X emission exerts non-negligible impacts on the summertime air quality and needs to be considered when designing emission abatement strategies.（3）Effects of vessel emissions on O₃ pollution:The NCP region is adjacent to the Bohai and Yellow Sea.However,the role of vessel emissions in near-surface O₃pollution in the NCP remains elusive.Using the WRF-Chem model,this study quantitatively evaluated the effects of vessel emissions and the differences in ultraviolet（UV）albedo between the ocean and land surfaces on near-surface MDA8 O₃concentrations over the ocean,the whole region,and coastal cities of the NCP in June2018.Results showed that vessel emissions contributed 34.0%,7.7%,and 10.2%to the mean NO₂,O₃,and MDA8 O₃ concentrations over the ocean,respectively,and ship-induced NO_X emissions dominated the contributions.Daytime（6:00–18:00）photolysis rate coefficient for NO₂ was averaged higher over the ocean than that over the land,resulting in approximately 12%of NO₂ being photolyzed over the ocean and 4%of MDA8 O₃ increases.Vessel emissions played an important role in MDA8 O₃concentrations over the ocean,and the higher surface UV albedo for the ocean further amplified the ship-induced effects,with a combined contribution of 12.6%to MDA8O₃ concentrations over the ocean.The ship-induced near-surface MDA8 O₃concentrations over the whole region and coastal cities of the NCP are 0.6%and 3.0%,respectively.And the contributions increased to 0.9%and 5.2%,respectively,when combined with the effect of higher surface UV albedo for the ocean.Vessel emissions played an insignificant role in the mean near-surface O₃ concentrations in the NCP but a non-negotiable role in O₃ pollution in the coastal cities of the NCP.Given the growing NO_X emissions from vessels,emission abetment policies for the shipping industry should be carried out.（4）Effects of HO₂·heterogenous loss on the O₃ pollution:Hydroperoxy radical（HO₂）is a crucial oxidant participating in the oxidation of nitrogen oxide to nitrogen dioxide which constitutes one of the most important pathways for the O₃ photochemical formation in the troposphere.Laboratory experiments and field observations have revealed efficient HO₂ heterogeneous uptake on wet aerosols,but its impact on the O₃formation remains controversial.The WRF-Chem model was employed to evaluate the impacts of the HO₂ heterogeneous loss on the O₃ formation in the NCP in the summer of 2018.Results showed that the HO₂ heterogeneous loss decreased the daytime HO₂and MDA8 O₃ concentration by about 5%and 1%,respectively,in the NCP with an effective uptake coefficient（!"!）of 0.2.Emission mitigation from 2013 to 2018 led to a 2.1μg m^-3（5%）increase in the MDA8 O₃ concentration due to decreased aerosol sink for the HO₂ heterogeneous loss in the NCP.Our results revealed that decreased HO₂ heterogeneous uptake does not constitute an important factor driving the O₃ trend since 2013 in the NCP.