Gas-particle reactions of reactive nitrogen represent an important but variable atmospheric chemical cycle.听听For example, uptake of N2O5to aerosol regulates the lifetime of NOx(=NO+NO2) and oxidant burdens in the troposphere.听听Uptake of N2O5to chloride-containing aerosol to produce ClNO2is an efficient halogen activation mechanism that may represent a substantial fraction of tropospheric halogen budgets.听听Our group has developed instrumentation and led major ground, ship and aircraft based field intensives in听,听,听,听,听听补苍诲听.听听The recent听听field campaign on the NSF C-130 aircraft on the U.S. East Coast, collaborative with the听听at the University of Washington, specifically targeted heterogeneous nitrogen chemistry.