Export of NOy from the North American boundary layer during 1996 and 1997 North Atlantic Regional Experiments

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This paper studies the removal timescales of the NOy originating from surface emissions of nitrogen oxides (NOx) and studies what fraction of the emissions reaches the free troposphere. These are important questions because NOx, which limits ozone production in the free troposphere, can be recycled from NOy after the primary NOx has been exhausted. We present an analysis of aircraft measurement data of odd nitrogen (NOy) and carbon monoxide (CO) taken during the North Atlantic Regional Experiment (NARE) in spring 1996 and fall 1997. Because CO has a relatively long chemical lifetime in the atmosphere and because it has similar emission patterns to NOx, CO concentrations can serve as a reference against which NOy removal can be studied. Correlating NOy and CO for different heights, we show that for the NARE 1997 (and NARE 1996) measurement campaigns, 78–91% (33–76%) of the surface NOy emissions were lost within the boundary layer or during transport from it, and only 3% (5%) of the emissions reached altitudes above 3 km. This accounted for ∼35% (∼41%) of the total NOy present above 3 km, the remainder coming from in situ emissions. In order to establish the timescales over which NOy is removed from the atmosphere, we determine the time elapsed between emission and measurement using a Lagrangian tracer transport model. We obtain approximate, effective mean e‐folding lifetimes of NOy of 1.7 (1.8) days for the two intensives. On average, only 4% (9%) of the NOy remains in the atmosphere after 4–5 days. After 5–6 days, NOy from anthropogenic surface sources has been further depleted and accounts for <50% of the observed NOy. The rest of the observed NOy must come from in situ emissions, not from anthropogenic surface emissions.