Evidence of impact of aerosols on surface ozone concentration in Tianjin, China

In this study, we will present evidence that aerosol particles have strong effects on the surface ozone concentration in a highly polluted city in China. The measured aerosol (PM10), UV flux, and O3 concentrations were analyzed from 1 November (1 Nov) to 7 November (7 Nov) 2005 in Tianjin, China. During this period, the aerosol concentration had a strong day-by-day variation, ranging from 0.2 to 0.6 mg m−3. The ozone concentration also shows a strong variability in correlation with the aerosol concentration. During 1 Nov, 2 Nov, 6 Nov, and 7 Nov, the ozone concentration was relatively high (about 30–35 ppbv; defined as a high-ozone period), and during 3 Nov to 5 Nov, the ozone concentration was relatively low (about 5–20 ppbv; defined as a low-ozone period). The analysis of the measurement shows that the ozone concentration is strongly correlated to the measured UV flux. Because there were near cloud-free conditions between 1 Nov and 7 Nov, the variation of the UV flux mainly resulted from the variation of aerosol concentration. The result shows that higher aerosol concentrations produce a lower UV flux and lower ozone concentrations. By contrast, the lower aerosol concentration leads to a higher UV flux and higher ozone concentrations. A chemical mechanism model (NCAR MM) is applied to interpret the measurement. The model result shows that the extremely high aerosol concentration in this polluted city has a very strong impact on photochemical activities and ozone formation. The correlation between aerosol and ozone concentrations appears in a non-linear feature. The O3 concentration is very sensitive to aerosol loading when aerosol loading is high, and this sensitivity is reduced when aerosol loading is low. For example, the ratio of Δ[O3]/Δ[AOD] is about −16 ppbv AOD−1 when AOD is less than 2, and is only −4 ppbv AOD−1 when AOD is between 2 and 5. This result implies that a future decrease in aerosol loading could lead to a rapid increase in the O3 concentration in this region.