What is the role of と in this article title?
花粉 この春東京都内で飛ぶ量は去年とほぼ同じ
Together with 同じ it is how you phrase "to be the same as". ほぼ means "roughly". So in this case, it means that there is roughly the same amount of pollen as there was last year.
と here means "to compare with". "XXとほぼ(about)同じ(same)" = "about the same with XX". と plays the role of "with".