Isoprene is the most common plant-emitted biogenic hydrocarbon. Ambient isoprene concentrations ordinarily exhibit strong diurnal patterns in response to emissions driven by sunlight and temperature. Immediately after sunset, isoprene concentrations are known to precipitously decline with time. Such temporal isoprene changes are well known in rural and urban environments. Little is known about the isoprene temporal patterns in semi-urban environments, at the edge of urban and rural areas. Therefore, the objective of this presentation is to report on the processes governing the precipitous isoprene declines in a semi-urban environment. Ambient levels of isoprene and other hydrocarbons were investigated with a Proton-Transfer-Reaction Mass Spectrometer in Beltsville, MD during an intensive field campaign in July 2011. In addition, ozone, nitric oxide, and nitrogen dioxide, as well as wind speed, temperature, pressure, relative humidity, and net radiation were measured. Results indicated that nocturnal decreases in isoprene levels did not always exhibit close correlations with associated reaction products, but also depended on the levels of nitrogen oxides and turbulence. As revealed from isoprene vertical profiles, advection of anthropogenic and biogenic hydrocarbons aloft influenced the amounts of hydrocarbons during the nighttime.