比较关系中that的用法疑问

CRX1230

The carbon emissions kilogram weight from cars is higher in summer than that of winter.

出于简洁形式考虑， 改成了The weight of the carbon emissions from cars is higher in summer than in winter.

可是结尾这里than 为什么后边不用加that？ 也就是than that in winter.    that指代第一个句子

或者说that什么时候可以省略什么时候不可以？
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