Ozone and Nitrogen Dioxide Pollution in a Coastal Urban Environment: The Role of Sea Breezes, and Implications of Their Representation for Remote Sensing of Local Air Quality.

We present an analysis of sea breeze conditions for the Boston region and examine their impact on the concentration of local air pollutants over the past decade. Sea breezes occur about one-third of the days during the summer and play an important role in the spatial distribution and temporal evolution of NO 2 and O 3 across the urban area. Mornings preceding a sea breeze are characterized by low horizontal wind speeds, low background O 3 , and an accumulation of local primary emissions. Air pollution is recirculated inland during sea breezes, frequently coinciding with the highest O 3 measured at the urban center. We use "Ox" (= NO 2  + O 3 ) to account for temporary O 3 suppression by NO and find large horizontal gradients (differences in Ox greater than 30 ppb across less than 15 km), which are not observed on otherwise westerly or easterly prevailing days. This implies a challenge in surface monitoring networks to adequately represent the spatial variability of secondary air pollution in coastal urban areas. We investigate satellite-based climatologies of tropospheric NO 2 , and find evidence of selection biases due to cloud conditions, but show that sea breeze days are well observed due to the fair weather conditions generally associated with these events. The fine scale of the sea breeze in Boston is not reliably represented by meteorological reanalyses products commonly used in chemical transport models required to provide inputs for the satellite-based retrievals. This implies a higher systematic error in the operational retrievals on sea breeze days compared to other days.