While wet deposition makes up the majority of total reactive nitrogen deposition in the eastern U.S., dry deposition accounts for 20-50%, and thus can be a significant source of reactive nitrogen to ecosystems.  While many studies have measured dry N deposition in rural areas, there are few measurements of dry N deposition in urban areas. When combined with national monitoring (e.g., NADP-NTN and CASTNET) sited in rural locations, dry N deposition in urban areas is poorly characterized. However, these measurements are especially important in areas with high traffic volumes because previous studies have shown that dry N deposition deposits close to the source, especially from automobiles.  For example, previous research suggests that N deposition observed by NADP-NTN and CASTNET reflects NO­_y­ derived primarily from regionally-transported emissions from stationary sources (e.g., power plant smoke stacks) rather than mobile sources (e.g. automobiles).  Furthermore, CASTNET does not measure NO₂; 80% of dry N deposition measured by CASTNET is from HNO₃.  However, previous studies have documented that NO₂ is a large component of total dry N deposition, especially near roadways. 

     This study characterized the amount and sources of dry N deposition (NO₂ and HNO₃) along two urban to rural gradients, one in Baltimore, MD and the other in Pittsburgh, PA, in order to better understand dry N deposition dynamics in urban areas.  Passive samplers were deployed at urban, suburban and rural sites for five months to collect NO₂ and HNO₃ for calculation of N fluxes. This method provides a straightforward and inexpensive approach for monitoring N deposition and sources across large spatial gradients for extended time periods. 

            Results showed that the Pittsburgh gradient urban site had 1.8 times higher N flux than the corresponding rural site and 2.3 times higher N flux than the suburban site.  The Maryland gradient urban site had 1.5 times greater total N flux than the rural site. Further, N deposition at urban sampling locations was greater than that measured at nearby CASTNET sites, which may lead to an underestimation of total N deposited in urban areas.   59-71% of the total nitrogen was NO₂, which is not measured by CASTNET.  NO₂ flux was strongly correlated with traffic volume at each site, indicating that NO₂ flux may be derived primarily from automobiles and deposits locally.  In contrast, HNO₃ flux correlated with stationary source NO_x emissions, indicating that HNO₃ may be transported regionally due to its longer atmospheric lifetime.