Co-agglomeration, technological innovation and haze pollution: An empirical research based on the middle reaches of the Yangtze River urban agglomeration

•The co-agglomeration accentuates haze pollution.•Co-agglomeration has obvious spatial effects on haze pollution.•The pollution effect of co-agglomeration is weakened with the improvement of the economic level.•Technological innovation has an inhibitory effect on the pollution effect of co-agglomeration.