Diurnal sea breeze worsens coastal air quality and complicates monitoring of background North Atlantic air

Abstract. In coastal environments, land and sea absorb and release heat at different rates due to their differing thermal properties. The resultant regular fluctuation in winds from onshore during the day to offshore at night, termed the diurnal sea breeze effect, can have a strong but uncertain impact on coastal air quality. In this study, from 10 years of observations from the Penlee Point Atmospheric Observatory on the northeast Atlantic coast, we identified 428 diurnal sea breeze events. Such events were most prevalent in spring and summer, when sea temperature is cooler than air temperature over land, wind speeds are relatively low, and the solar irradiance is strong. Observed surface concentrations of trace gases (O₃, NOx, CH₄, CO₂) as well as aerosols (total aerosol number, PM2.5, PM10) were all elevated in the daytime during sea breeze events, increasing air quality regulation exceedance. Sea breeze generally coincided with the highest Ox (O₃+NOx) levels in this environment (mean daytime mixing ratio around 45 ppb), likely due to poor pollutant dispersion at night and inflow of air with high O₃ during the day from the marine atmosphere. The occurrence of diurnal sea breeze also confounds the representativeness of coastal observations for background North Atlantic atmosphere – excluding sea breeze events reduces the diurnal amplitudes in O₃ and CH₄ and also modifies their seasonal variations during southwesterly wind conditions.