Depletion of carbon monoxide from the nocturnal boundary layer

Measurements of carbon monoxide concentration were made at Bush Estate, Midlothian, Scotland, in May–June 1994. A diurnal pattern in CO concentration was observed, with a decrease in concentration during the night, increase during the early morning, and little change in the late morning and afternoon. The decrease in CO concentration at night was thought to be caused by deposition to the ground, and oxidation of CO by soil microorganisms. This was followed by an increase in the morning caused by mixing air from the free troposphere with that in the nocturnal boundary layer as it broke up, and by the start of photochemically driven CO production. Boundary layer heights and deposition velocities were calculated. The mean deposition velocity was 1.1 × 10−3 ms−1. This is in reasonable agreement with values obtained by other methods. Appreciable proportions of the total amount of CO in the marine boundary layer were removed when the air had passed over land for approximately 100 km.