One longstanding issue in tropospheric chemistry is the observation that the ratio of HNO@sub 3@/NO@sub x@ often shows atmospheric values two to ten times lower than those modeled. This implies either a missing sink of HNO@sub 3@, a missing source of NO@sub x@ species, or a missing conversion from HNO@sub 3@ to NO@sub x@ in the models. There are several dominant particle types in the free troposphere that may provide surfaces for loss of HNO@sub 3@. In this paper, we present laboratory studies of HNO@sub 3@ uptake on two systems modeling global tropospheric particulate: ice surfaces representative of cirrus clouds and mineral particles representative of atmospheric dust. The results from current laboratory experiments will be discussed and, where possible, comparisons with recent atmospheric data will be presented.