We demonstrate a direct connection between anomalously high levels of NO3- in aerosols over the northeastern U.S. with transport of air masses from south of the Great Lakes during periods of unseasonably warm climatic conditions in that area. Previous studies document the predominant and pervasive role of SO42- and organic matter in regional aerosol composition over the Northeast. Here we characterize multi-day episodic events where NO3- exceeded SO42- by up to 3-fold at Mount Washington and increased from the seasonal median of about 1 nmol m-3 to 60--100 nmol m-3. These events take place in the colder seasons, with the three most significant ones in the last 6 years occurring in November 2001, January 2002, and March 2004. We propose that large amounts of NH3 are volatilized in the Midwest under these conditions, which exceeds available SO42- and allows high levels of NH4NO3 aerosol to form. The occurrence of these events, which completely changes the character of aerosols over the Northeast and may lead to enhanced NO3- deposition to ecosystems, could increase with continued warming over North America.