ALBERT

Description: [1]  Heterogeneous N 2 O 5 uptake onto aerosol is the primary nocturnal path for removal of NO x (= NO + NO 2 ) from the atmosphere and can also result in halogen activation through production of ClNO 2 . The N 2 O 5 uptake coefficient has been the subject of numerous laboratory studies; however, only a few studies have determined the uptake coefficient from ambient measurements, and none have been focused on winter conditions, when the portion of NO x removed by N 2 O 5 uptake is the largest. In this work, N 2 O 5 uptake coefficients are determined from ambient wintertime measurements of N 2 O 5 and related species at the Boulder Atmospheric Observatory in Weld County, CO, a location that is highly impacted by urban pollution from Denver, as well as emissions from agricultural activities and oil and gas extraction. A box model is used to analyze the nocturnal nitrate radical chemistry and predict the N 2 O 5 concentration. The uptake coefficient in the model is iterated until the predicted N 2 O 5 concentration matches the measured concentration. The results suggest that during winter, the most important influence that might suppress N 2 O 5 uptake is aerosol nitrate, but that this effect does not suppress uptake coefficients enough to limit the rate of NO x loss through N 2 O 5 hydrolysis. N 2 O 5 hydrolysis was found to dominate the nocturnal chemistry during this study consuming ~80% of nocturnal gas-phase nitrate radical production. Typically, less than 15% of the total nitrate radical production remained in the form of nocturnal species at sunrise when they are photolyzed and reform NO 2 .