ALBERT

Description: Serious urban ozone (O3) pollution was observed during the campaign of 2016 G20 summit in Hangzhou, China, while other pollutants had been significantly reduced by the short-term emission control measures. To understand the underlying mechanism, the Weather Research Forecast with Chemistry (WRF-Chem) model is used to investigate the spatial and temporal O3 variations in Hangzhou from 24 August to 6 September 2016. The model is first successfully evaluated and validated for local and regional meteorological and chemical parameters by using the ground and upper-air level observed data. High ozone concentrations, temporally during most of the daytime emission control period and spatially from the surface to the top of the planetary boundary layer, are captured in Hangzhou and even the whole Yangtze River Delta region. Various atmospheric processes are further analyzed to determine the influential factors of local ozone formation through the integrated process rate method. Interesting horizontal and vertical advection circulations of O3 are observed during several short periods, and the effects of these processes are nearly canceled out. As a result, ozone pollution is mainly attributed to the local photochemical reactions that are not obviously influenced by the emission reduction measures. The ratio of reduction of Volatile Organic Compounds (VOCs) to that of NOx is a critical parameter that needs to be carefully considered for future alleviation of ozone formation. In addition, the vertical diffusion from the upper-air background O3 also plays an important role in shaping the surface ozone concentration. These results provide insight into urban O3 formation in Hangzhou and support the Model Intercomparison Study Asia Phase III (MICS-Asia Phase III).

Description: A multi-model ensemble of Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) simulations is used to study the atmospheric oxidized nitrogen (NOy) deposition over East Asia under climate and emission changes projected for the future. Both dry and wet NOy deposition show significant decreases in the 2100s under RCP4.5 and RCP8.5, primarily due to large anthropogenic emission reduction over both land and sea. However, in the near future of the 2030s, both dry and wet NOy deposition increase significantly due to continued increase in emissions. Marine primary production from both dry and wet NOy deposition increases by 19 %–34 % in the 2030s and decreases by 34 %–63 % in the 2100s over the East China Sea. The individual effect of climate or emission changes on dry and wet NOy deposition is also investigated. The impact of climate change on dry NOy deposition is relatively minor, but the effect on wet deposition, primarily caused by changes in precipitation, is much higher. For example, over the East China Sea, wet NOy deposition increases significantly in summer due to climate change by the end of this century under RCP8.5, which may subsequently enhance marine primary production. Over the coastal seas of China, as the transport of NOy from land becomes weaker due to the decrease in anthropogenic emissions, the effect of ship emissions and lightning emissions becomes more important. On average, the seasonal mean contribution of ship emissions to total NOy deposition is projected to be enhanced by 24 %–48 % and 3 %–37 % over the Yellow Sea and East China Sea, respectively, by the end of this century. Therefore, continued control of both anthropogenic emissions over land and ship emissions may reduce NOy deposition to the Chinese coastal seas.

Description: The Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) was used to study the effect of extreme weather events on ozone in the US for historical (2001–2010) and future (2046–2055) periods under the RCP8.5 scenario. During extreme weather events, including heat waves, atmospheric stagnation, and their compound events, ozone concentration is much higher compared to the non-extreme events period. A striking enhancement of effect during compound events is revealed when heat wave and stagnation occur simultaneously as both high temperature and low wind speed promote the production of high ozone concentrations. In regions with high emissions, compound extreme events can shift the high-end tails of the probability density functions (PDFs) of ozone to even higher values to generate extreme ozone episodes. In regions with low emissions, extreme events can still increase high-ozone frequency but the high-end tails of the PDFs are constrained by the low emissions. Despite the large anthropogenic emission reduction projected for the future, compound events increase ozone more than the single events by 10 to 13 %, comparable to the present, and high-ozone episodes with a maximum daily 8 h average (MDA8) ozone concentration over 70 ppbv are not eliminated. Using the CMIP5 multi-model ensemble, the frequency of compound events is found to increase more dominantly compared to the increased frequency of single events in the future over the US, Europe, and China. High-ozone episodes will likely continue in the future due to increases in both frequency and intensity of extreme events, despite reductions in anthropogenic emissions of its precursors. However, the latter could reduce or eliminate extreme ozone episodes; thus improving projections of compound events and their impacts on extreme ozone may better constrain future projections of extreme ozone episodes that have detrimental effects on human health.

Description: To elucidate the factors governing urban ozone (O3) pollution during the campaign of G20 summit in 2016 Hangzhou, China, the Weather Research Forecast with Chemistry (WRF-Chem) model was used to simulate the spatial and temporal O3 evolution in the Yangtze River Delta (YRD) region from 24 August to 6 September 2016. Various atmospheric processes were analyzed to determine the influential factors of ozone formation through integrated process rate method. The results indicated that both the vertical diffusion and the enhanced process of local chemical generation accounted for the increase of surface O3 concentration in Hangzhou. Local chemical generation was found to positively correlated with O3 concentrations, with correlation coefficient of 0.77. In accordance with the tropical weather cycle, subsidence air and stagnant weather were induced. Dynamic circulations of O3 through advection were associated with the urban heat island effect. All these factors intensified ozone pollution in Hangzhou, particularly on 25 August 2016 (O3-8 h: 98 ppb). These findings provide insight into urban O3 formation and dispersion during tropical cyclone events, and support the Model Intercomparison Study Asia Phase III (MICS-Asia Phase III).

Description: A multi-model ensemble of Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) simulations are used to study the atmospheric oxidized nitrogen (NOy) deposition over East Asia under climate and emission changes projected for the future. Both dry and wet NOy deposition shows significant decreases in the 2100s under RCP 4.5 and RCP 8.5, primarily due to large anthropogenic emission reduction over both land and sea. However, in the near future of the 2030s, both dry and wet NOy deposition increases significantly due to continued increase in emissions. The individual effect of climate or emission changes on dry and wet NOy deposition is also investigated. The impact of climate change on dry NOy deposition is relatively minor, but the effect on wet deposition, primarily caused by changes in precipitation, is much higher. For example, over the East China Sea, wet NOy deposition increases significantly in summer due to climate change by the end of this century under RCP 8.5, which may subsequently enhance marine primary production. Over the coastal seas of China, as the transport of NOy from land becomes weaker due to the decrease of anthropogenic emissions, the effect of ship emission and lightning emission becomes more important. On average, seasonal mean total NOy deposition is projected to be enhanced by 24–48% and 3%–37% over Yellow Sea and East China Sea, respectively, by the end of this century. Therefore, continued control of both anthropogenic emission over land and ship emissions may reduce NOy deposition to the Chinese coastal seas.

Description: The Weather Research and Forecasting model with Chemistry (WRF/Chem) was used to study the effect of extreme weather events on ozone in US for historical (2001–2010) and future (2046–2055) periods under RCP8.5 scenario. During extreme weather events, including heat waves, atmospheric stagnation, and their compound events, ozone concentration is much higher compared to non-extreme events period. A striking enhancement of effect during compound events is revealed when heat wave and stagnation occur simultaneously and both high temperature and low wind speed promote the production of high ozone concentrations. In regions with high emissions, compound extreme events can shift the high-end tails of the probability density functions (PDFs) of ozone to even higher values to generate extreme ozone episodes. In regions with low emissions, extreme events can still increase high ozone frequency but the high-end tails of the PDFs are constrained by the low emissions. Despite large anthropogenic emission reduction projected for the future, compound events increase ozone more than the single events by 10 % to 13 %, comparable to the present, and high ozone episodes are not eliminated. Using the CMIP5 multi-model ensemble, the frequency of compound events is found to increase more dominantly compared to the increased frequency of single events in the future over the US, Europe, and China. High ozone episodes will likely continue in the future due to increases in both frequency and intensity of extreme events, despite reductions in anthropogenic emissions of its precursors. However, the latter could reduce or eliminate extreme ozone episodes, so improving projections of compound events and their impacts on extreme ozone may better constrain future projections of extreme ozone episodes that have detrimental effects on human health.

Description: To elucidate the factors governing the urban O3 pollution during the campaign period of 2016 Group of Twenty (G20) summit in China, the Weather Research Forecast with Chemistry (WRF-Chem) model was used to simulate the spatial and temporal O3 evolution in the Yangtze River Delta (YRD) region from August 24 to September 06, 2016. A unique mechanism was found to modulate the high ozone episodic event. Before the tropical cyclone, a prevailing north wind component brought in emission sources which are favorable for ozone formation. With the invasion of tropical cycle, subsidence air and stagnant weather were induced. Together with local urban heat island effect, there factors intensify ozone pollution in the YRD region. Different atmospheric processes were further analyzed to investigate the control factors of ozone formation through the integrated process rate method. It was found that both the vertical diffusion and the enhancing process of local chemical generation accounted for the growth of surface O3 concentration in Hangzhou. Besides, dynamical circulations of O3 advection associated with urban heat island effect were observed during the high O3 episode (August 24–25, 2016), and low O3 episode on September 5–6, 2016 was mainly resulting from the local chemical consumption. This provides insight into urban O3 formation and dispersion in East China during the tropical cyclone events.