Nachshon ben Amminadav, and each of the eleven tribal leaders who followed him, offered "one young he-goat for a sin offering" as part of their gifts for the dedication of the Mishkan (Num. 7:16 passim). Rashi there, citing Sifri, states that this was
to atone for [uncleanness caused by] a grave in the depths [i.e., an unknown grave which may lie in the earth over which people unknowingly pass, rendering them unclean],which is a [case of] uncertain contamination.
But in his commentary to Lev. 10:17, he points out (citing Sifra) that the sin offering that Aharon's sons burned (instead of eating it) must have been the one brought in honor of Rosh Chodesh, since Moshe identifies it as an offering brought "to gain forgiveness for the sin of the community," and Nachshon's offering was not brought for purpose of atonement.
[The sin offering of Rosh Chodesh is meant to atone for cases where "there was no knowledge [of being impure] at either the beginning [before entering the Beis Hamikdash or eating sacred food] or at the end [after having done so]" (Rambam, Hil. Shegagos 11:9, from Shevuos 2a).]
So in that case, isn't Nachshon's offering indeed for purposes of atonement - indeed, it is largely similar to what's covered by the Rosh Chodesh offering?