Ozone distributions over southern Lake Michigan: comparisons between ferry-based observations, shoreline-based DOAS observations and model forecasts

摘要

Air quality forecast models typically predict large summertime ozoneabundances over water relative to land in the Great Lakes region. While eachstate bordering Lake Michigan has dedicated monitoring systems, offshoremeasurements have been sparse, mainly executed through specific short-termcampaigns. This study examines ozone abundances over Lake Michigan asmeasured on the ferry, by shoreline differential opticalabsorption spectroscopy (DOAS) observations in southeastern Wisconsin andas predicted by the Community Multiscale Air Quality (CMAQ) model. From2008 to 2009 measurements of O, SO, NO and formaldehyde weremade in the summertime by DOAS at a shoreline site in Kenosha, WI. From2008 to 2010 measurements of ambient ozone were conducted on the , ahigh-speed ferry that travels between Milwaukee, WI, and Muskegon, MI, up tosix times daily from spring to fall. Ferry ozone observations over Lake Michiganwere an average of 3.8 ppb higher than those measured at shoreline inKenosha, with little dependence on position of the ferry or temperature andwith greatest differences during evening and night. Concurrent 1–48 hforecasts from the CMAQ model in the upper Midwestern region surroundingLake Michigan were compared to ferry ozone measurements, shoreline DOASmeasurements and Environmental Protection Agency (EPA) station measurements. The bias of the model Oforecast was computed and evaluated with respect to ferry-basedmeasurements. Trends in the bias with respect to location and time of daywere explored showing non-uniformity in model bias over the lake. Modelozone bias was consistently high over the lake in comparison to land-basedmeasurements, with highest biases for 25–48 h after initialization.