Measurements of ultrafine particles carrying different number of charges in on- and near-freeway environments

► Electrical charges on ultrafine particles (UFPs) were measured on- and near-freeways. ► The fractions of charged UFPs were significantly higher on the freeways than in the background. ► Charged particles decayed faster than total particles but slower than ion downwind from the freeway. ► Strong net positive charges were found on nucleation mode particles near freeways.