Abstract
Abstract. Air quality forecast models typically predict large summertime ozone abundances over water relative to land in the Great Lakes region. While each state bordering Lake Michigan has dedicated monitoring systems, offshore measurements have been sparse, mainly executed through specific short-term campaigns. This study examines ozone abundances over Lake Michigan as measured on the Lake Express ferry, by shoreline differential optical absorption spectroscopy (DOAS) observations in southeastern Wisconsin and as predicted by the Community Multiscale Air Quality (CMAQ) model. From 2008 to 2009 measurements of O3, SO2, NO2 and formaldehyde were made in the summertime by DOAS at a shoreline site in Kenosha, WI. From 2008 to 2010 measurements of ambient ozone were conducted on the Lake Express, a high-speed ferry that travels between Milwaukee, WI, and Muskegon, MI, up to six times daily from spring to fall. Ferry ozone observations over Lake Michigan were an average of 3.8 ppb higher than those measured at shoreline in Kenosha, with little dependence on position of the ferry or temperature and with greatest differences during evening and night. Concurrent 1–48 h forecasts from the CMAQ model in the upper Midwestern region surrounding Lake Michigan were compared to ferry ozone measurements, shoreline DOAS measurements and Environmental Protection Agency (EPA) station measurements. The bias of the model O3 forecast was computed and evaluated with respect to ferry-based measurements. Trends in the bias with respect to location and time of day were explored showing non-uniformity in model bias over the lake. Model ozone bias was consistently high over the lake in comparison to land-based measurements, with highest biases for 25–48 h after initialization.
Highlights
IntroductionThe shoreline air quality has gone from a highly impacted environment for surface ozone in the 1970s–80s to persistent non-attainment status in the 2008 ground-level ozone standards for counties near to Lake Michigan in Wisconsin (Sheboygan and Kenosha), Illinois (Cook, Lake, Grundy, Kane, Kendall, McHenry, Will) and Indiana (Lake, Porter)
Air quality near Lake Michigan has been under study for more than 30 years
The highest median ozone and SO2 mixing ratios observed at the Kenosha harbor location arise from air masses flowing from the lake (0–180◦ are from offshore), whereas the highest NO2 and formaldehyde observations arise from air masses originating on land
Summary
The shoreline air quality has gone from a highly impacted environment for surface ozone in the 1970s–80s to persistent non-attainment status in the 2008 ground-level ozone standards for counties near to Lake Michigan in Wisconsin (Sheboygan and Kenosha), Illinois (Cook, Lake, Grundy, Kane, Kendall, McHenry, Will) and Indiana (Lake, Porter). The proposed rule as of 26 November 2014 is to reduce the 8 h primary standard to between 65 and 70 ppb ozone, which has the possibility of maintaining the non-attainment status for these counties in the future (EPA, 2014). These Wisconsin counties in non-attainment are unique in that they are both suburban Lake Michigan shoreline counties as opposed to urban or rural counties. We evaluate the Lake Michigan ozone mixing ratios offshore with those onshore, including agreement with ozone forecast models over water and at the shoreline
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