Loss of isoprene and sources of nighttime OH radicals at a rural site in the United States: Results from photochemical models

A one‐dimensional Lagrangian model for atmospheric transport andphotochemistry has been developed and used to interpret measurements made atPellston, Michigan, during the summer of 1998. The model represents a movingvertical column of air with vertical resolution of 25 m near the ground.Calculations have been performed for a series of trajectories, with representationof emissions, vertical mixing, and photochemistry for a 3‐day period ending with thearrival of the air column at Pellston. Results have been used to identify causes ofthe observed decrease in isoprene at night, to investigate causes of high nighttimeOH. Significant OH can be generated at night by terpenes if it is assumed that somefast‐reacting monoterpenes are emitted at rates comparable to inventory emissionsfor terpenes. However, this nighttime OH is confined to a shallow surface layer(0–25 m) and has little impact on nighttime chemistry. The observeddecrease in isoprene at night can be reproduced in models with low OH, and isattributed primarily to vertical dilution. There is also evidence that transportfrom Lake Michigan contributes to low nighttime isoprene at Pellston. Model resultscompare well with measured isoprene, NO x , and with isoprenevertical profiles. Significant model‐measurement discrepancies are found for OH,HO 2 , methylvinylketone, and formaldehyde.