Factors dominating 3-dimensional ozone distribution during high tropospheric ozone period.

Data from an in situ monitoring network and five ozone sondes are analysed during August of 2012, and a high tropospheric ozone episode is observed around the 8th of AUG. The Community Multi-scale Air Quality (CMAQ) model and its process analysis tool were used to study factors and mechanisms for high ozone mixing ratio at different levels of ozone vertical profiles. A sensitive scenario without chemical initial and boundary conditions (ICBCs) from MOZART4-GEOS5 was applied to study the impact of stratosphere-troposphere exchange (STE) on vertical ozone. The simulation results indicated that the first high ozone peak near the tropopause was dominated by STE. Results from process analysis showed that: in the urban area, the second peak at approximately 2 km above ground height was mainly caused by local photochemical production. The third peak (near surface) was mainly caused by the upwind transportation from the suburban/rural areas; in the suburban/rural areas, local photochemical production of ozone dominated the high ozone mixing ratio from the surface to approximately 3 km height. Furthermore, the capability of indicators to distinguish O-precursor sensitivity along the vertical O profiles was investigated. Two sensitive scenarios, which had cut 30% anthropogenic NO or VOC emissions, showed that O-precursor indicators, specifically the ratios of O/NOy, HO/HNO or HO/NO, could partly distinguish the O-precursor sensitivity between VOCs-sensitive and NOx-sensitive along the vertical profiles. In urban area, the O-precursor relationship transferred from VOCs-sensitive within the boundary layer to NOx-sensitive at approximately 1-3 km above ground height, further confirming the dominant roles of transportation and photochemical production in high O peaks at the near-ground layer and 2 km above ground height, respectively.