Lake Michigan air quality: The 1994–2003 LADCO Aircraft Project (LAP)

The goal of the 1994 to 2003 LADCO Airplane Project (LAP) was to study ozone formation over Lake Michigan so that equitable regional control strategies could be devised. This paper for the first time documents LAP in the peer-reviewed literature.Dye et al. (1995) found that the atmosphere over Lake Michigan is stable in the summer due to the airwater temperature difference, which creates an efficient reaction chamber for ozone formation. They also hypothesize that the southwest winds characteristic of ozone-conducive conditions transport ozone further north over the lake before it crosses the shoreline onto land. This statistical analysis of LAP data support the hypothesis of Dye et al.Below 200 m above the lake, ozone formation is VOC-limited in the morning and becomes NOx limited in the afternoon. Above 200 m, ozone formation is NOx-limited throughout the day. The onshore NOx and VOC diurnal cycles peak during the early morning rush hour and are clearly linked to traffic patterns. Over the lake, VOC and NOy concentrations peak during the mid-morning rather than the early morning, supporting the hypothesis that the land breeze transports VOC and NOy over the lake. The diurnal NOx pattern over Lake Michigan is less clearly defined than the VOC pattern possibly as a result of emissions from five coal-burning power plants located on the Lake Michigan shoreline.Using a “photochemical clock” model, we estimate the climatological average hydroxyl radical concentration over the lake to be (9.43 ± 5.88) × 106 molecule cm−3 near Chicago and (8.43 ± 3.68) × 106 molecule cm−3 near Milwaukee.

► First report of a decade-long airplane study of ozone episodes over Lake Michigan.
► O3 formation is first VOC-limited then NOx-limited at low altitudes over the lake.
► Average HO
• concentration was estimated using a “photochemical clock”.