Reply on RC2

Abstract

With the rapid advance in urbanization, land-surface forcing related to the urban expansion and anthropogenic heat (AH) release from human activities significantly affect the urban climate and in turn the air quality. Focusing on the Yangtze River Delta (YRD) region, a highly urbanized place with sever ozone (O3) pollution and complex geography, we estimate the impacts of land-surface forcing and AH on meteorology (meteorological factors and local circulations) and O3 using the WRF-chem model, which can enhance our understanding about the formation of O3 pollution in those rapidly developing regions with unique geographical features as most of our results can be supported by previous studies conducted in other regions in the world. Regional O3 pollution episodes occur frequently (26 times per year) in the YRD in recent years. These O3 pollution episodes are usually under calm conditions characterized by high temperature (over 20 °C), low relative humidity (less than 80 %), light wind (less than 3 m s−1) and shallow cloud cover (less than 5). In this case, high O3 mainly appears during the daytime influenced by the local circulations (the sea and the lake breezes). The change in land-surface forcing can cause an increase in 2-m temperature (T2) by maximum 3 °C, an increase in planetary boundary layer height (PBLH) by maximum 500 m and a decrease in 10-m wind speed (WS10) by maximum 1.5 m s−1, and surface O3 can increase by maximum 20 μg m−3 eventually. Furthermore, the expansion of coastal cities enhances the sea-breeze below 500 m. During the advance of the sea-breeze front inland, the upward air flow induced by the front makes well vertical mixing of O3. However, once the sea-breeze is fully formed, further progression inland is stalled, thus the O3 removal by the low sea-breeze will be weakened and surface O3 can be 10 μg m−3 higher in the case with cities than no-cities. The expansion of lakeside cities can extend the lifetime of the lake-breeze from the noon to the afternoon. Since the net effect of the lake-breeze is to accelerate the vertical mixing in the boundary layer, the surface O3 can increase as much as 30 μg m−3 in lakeside cities. Compared with the effects from land-surface forcing, the impacts of AH are relatively small. And the changes mainly appear in and around cities where AH emission is large. There are increases in T2, PBLH, WS10 and surface O3 when AH are taken into account, with the increment about 0.2 °C, 75 m, 0.3 m s−1 and 4 μg m−3, respectively. Additionally, AH can affect the urban-breeze circulations, meteorological factors and O3 concentration, but its effect on local circulations, such as the sea and the lake breezes, seems to be limited.