Xuewei Hou

Impact of Shanghai urban land surface forcing on downstream city ozone chemistry

The urban land surface has a significant impact on local urban heat island effects and air quality. In addition, it influences the atmospheric conditions and air quality in the downwind cities. In this study, the impact of Shanghai urban land surface forcing on weather conditions and air quality over Kunshan was investigated using the Weather Research and Forecasting model coupled with a multilayer urban canopy model and the Community Multiscale Air Quality model. Two simulations were conducted to identify the impact of upstream effects with and without upstream urban land surfaces in control and sensitivity experiments. The results show that the near-surface temperature and boundary layer height over Kunshan increased significantly with the appearance of the upstream urban land surface. Horizontal transport of O3 and its precursors, from Shanghai to Kunshan, are suppressed in the lower boundary layer but are strengthened in the upper boundary layer because of strong urban heat island circulation. As a result, O3 chemical production is decreased in the lower boundary layer of Kunshan but is increased in the upper boundary layer. On average, daytime O3 concentrations over Kunshan are decreased by approximately 2 ppbv in the lower boundary layer but are increased by as much as 40 ppbv in the upper air.

A case study of surface ozone source apportionment during a high concentration episode, under frequent shifting wind conditions over the Yangtze River Delta, China

Surface ozone is an environmental issue occurring at several scales, ranging from local to continental. One of the most developed regions in China, the Yangtze River Delta (YRD), experiences severe tropospheric ozone problem. Hence, quantifying the contributions from various geographical source regions is helpful for better understanding the regional ozone problem. Ozone source apportionment studies can provide relevant information for designing suitable air pollution protection strategies. In the present work, the WRF-Chem model coupled with an online ozone tagging method is applied to a case study, with the objective of exploring the ozone contributions to the surface ozone from different source regions over the YRD region, during a frequent wind-shifting period. Our results show that the YRD was highly affected by the upwind source regions bearing high values both ozone and its precursors. The contribution from the source region outside the main air pollution zones in the Central Eastern China (super regional contribution) was also important, accounting for more than 30 ppb of daytime maximum mean ozone concentrations. Ozone arising from increased local and regional emissions during high-concentration events was more significant than super regional contribution. It reveals that the ozone from Anhui region was transported through vertical mixing and horizontal advection to receptor areas in the YRD during the study time focus. Chemical process contributed significantly at ground and high altitude levels of 500 and 1000 m. However, most of the ozone from the remote regions of Henan and Hubei provinces was transported to the receptor of Nanjing through physical processes. The vertical mixing process played a crucial positive role at super regional scales, with regard to the formation of surface ozone over the YRD region during the addressed time interval.

Analysis of the seasonal ozone budget and the impact of the summer monsoon on the northeastern Qinghai‐Tibetan Plateau

Seasonal variations in ozone (O3) and the impact of the East Asian summer monsoon at Mount Waliguan (WLG) in the northeastern Qinghai-Tibetan Plateau (TP) and in the surrounding regions were analyzed for 1997–2007 using a global chemical transport model coupled with O3 tagging simulations. The model-simulated O3 and its precursors agreed well with observed values. An O3 budget analysis combined with O3 tagging results implied that photochemistry over the TP and long-range transport of O3 from East Asia, Europe, and Africa were responsible for the surface O3 summer maximum at WLG. In June, the contribution of O3 from the TP was 11.8 ppbv, and the total contribution of O3 transport from eastern China, Japan, Korean Peninsula, Europe, and Africa was 22.7 ppbv. At 400mb, the O3 exports from the stratosphere, Europe, Africa, and the Americas seemed to be the main sources of O3 at WLG. The contributions to surface O3 from deep convection process and lightning-induced photochemistry at WLG were both low in summer and are unlikely to be the key processes or contributors for the O3 peak. At several mountain sites in southeast East Asia, the increasing summer monsoon index was related to a decreasing trend for O3 from spring onward at Mount Tai and Mount Huang. At Mount Hua and WLG, regional O3 accumulated over the monsoon’s northernmost marginal zone under the influence of the East Asian summer monsoon and TP thermal circulation; this is most likely a key reason for the O3 summer maxima.

Diurnal variations and source apportionment of ozone at the summit of Mount Huang, a rural site in Eastern China.

Comprehensive measurements were conducted at the summit of Mount (Mt.) Huang, a rural site located in eastern China during the summer of 2011. They observed that ozone showed pronounced diurnal variations with high concentrations at night and low values during daytime. The Weather Research and Forecasting with Chemistry (WRF-Chem) model was applied to simulate the ozone concentrations at Mt. Huang in June 2011. With processes analysis and online ozone tagging method we coupled into the model system, the causes of this diurnal pattern and the contributions from different source regions were investigated. Our results showed that boundary layer diurnal cycle played an important role in driving the ozone diurnal variation. Further analysis showed that the negative contribution of vertical mixing was significant, resulting in the ozone decrease during the daytime. In contrast, ozone increased at night owing to the significant positive contribution of advection. This shifting of major factor between vertical mixing and advection formed this diurnal variation. Ozone source apportionment results indicated that approximately half was provided by inflow effect of ozone from outside the model domain (O3-INFLOW) and the other half was formed by ozone precursors (O3-PBL) emitted in eastern, central, and southern China. In the O3-PBL, 3.0% of the ozone was from Mt. Huang reflecting the small local contribution (O3-LOC) and the non-local contributions (O3-NLOC) accounted for 41.6%, in which ozone from the southerly regions contributed significantly, for example, 9.9% of the ozone originating from Jiangxi, representing the highest geographical contributor. Because the origin and variation of O3-NLOC was highly related to the diurnal movements in boundary layer, the similar diurnal patterns between O3-NLOC and total ozone both indicated the direct influence of O3-NLOC and the importance of boundary layer diurnal variations in the formation of such distinct diurnal ozone variations at Mt. Huang.

Simulation of tropical tropospheric ozone variation from 1982 to 2010: The meteorological impact of two types of ENSO event

The effects of two types of ENSO events on tropical ozone (O3) variations from 1982 to 2010, and the mechanisms underlying these effects, were analyzed using observations and model simulations. Tropospheric column O3 anomalies (TCOA) during canonical El Nino were different from El Nino Modoki. Absolute TCOA values are larger during canonical El Nino than during El Nino Modoki in most regions. La Nina events were not separated into the different types because of their similarity in terms of sea surface temperature patterns. TCOA in La Nina showed a reversed dipole from canonical El Nino. During canonical El Nino, anomalous downward motion together with suppressed convection weakened O3 outflow from the troposphere, causing an increase in tropospheric O3 over western Pacific. Over central and eastern Pacific, decreased O3 concentrations resulted primarily from a change in net chemical production of O3. The change in net O3 chemical production relates to increased levels of HOx under wetter condition. During El Nino Modoki, transport and chemical fluxes were similar but weaker than during canonical El Nino. During La Nina, O3 anomalies and transport fluxes were the opposite of those during the El Nino Modoki. Stratospheric O3 played a key role in the development of O3 anomaly above 250 hPa during ENSO events, contributing >30% to the O3 anomalies. The change in free tropospheric O3 affected the O3 anomaly from 850 hPa to 200 hPa (60% of O3 anomaly). The contribution of O3 from planetary boundary layer was concentrated at the surface, with a contribution of <15%.

Quantifying Arctic lower stratospheric ozone sources in winter and spring

The dynamical and chemical characteristics of unusually low Arctic ozone events in 2005 and 2011 have been well-studied. However, the quantitative identification of Arctic ozone sources is lacking. Here, we use tagged ozone tracers in a numerical simulation to quantify the contributions to Arctic lower stratospheric ozone (ARCLS_O3) at diverse latitudes in winter and spring from 2005–2011. We demonstrate that the northern mid-latitudinal stratosphere steadily contributes approximately half of ARCLS_O3. The absolute contributions during February have evident variations, which are smaller in cold years (151.3 ± 7.0 Dobson units (DU) in 2005 and 139.0 ± 7.4 DU in 2011) and greater in warm years (182.6 ± 7.3 DU in 2006 and 164.6 ± 7.4 DU in 2009). The tropical stratosphere is also an important source. During February, its absolute contributions are 66.5 ± 11.5 DU (2005), 73.1 ± 4.7 DU (2011), 146.0 ± 9.0 DU (2006), and 153.7 ± 7.0 DU (2009). Before and after stratospheric warming, variations in the tropical components of ARCLS_O3 (51.8 DU in 2006 and 77.0 DU in 2009) are significantly larger than those in the mid-latitudinal components (17.6 DU in 2006 and 18.1 DU in 2009). These results imply that although the mid-latitudinal components of ARCLS_O3 are larger, the tropical components control stratospheric temperature-induced ARCLS_O3 anomalies in winter and spring.

Source Apportionment of Tropospheric Ozone by Chemical Transport Model: From Global to City Cluster

Quantifying the ozone contributions from all sources to a given location can help understanding ozone source-receptor relationships and interpreting the mechanism of distributions and variations of ozone and its precursors. In this Chapter, we introduce the methods of ozone source apportionment, with a special focus on the source apportionment of ozone by tracer tagging methods in chemical transport model (air quality model). To some extent, the tagging methods can decrease the error caused by the chemical nonlinearity in comparison to traditional perturbing emission method. Two source apportionment case studies, one with a modified global scale chemical transport model (MOZART4) used in the East Asia Pacific rim and another with a modified regional air quality model (WRF-Chem) applied to the Yangtze River Delta, china. Two tagging methods, tagging ozone production regions and tagging ozone precursors (NOx and VOCs), will be compared and their feasibility in global and regional model will be discussed.