Ozone nighttime recovery in the marine boundary layer: Measurement and simulation of the ozone diurnal cycle at Reunion Island

Abstract

Abstract. We describe the diurnal cycle of ozone in the marine boundary layer measured at Reunion Island (21øS, 55øE) in the western part of the Indian Ocean in August-September 1995. Results from a box chemistry model are compared with ozone measurements at Reunion Island. We focus on the peak-to-peak amplitude of ozone concentration, since our measurements show a variation of about 4 parts per billion by volume, which is close to the value obtained by Johnson et al. [1990] during the SovietAmerican Gases and Aerosols (SAGA) 1987 Indian Ocean cruise. Different dynamical mechanisms are examined in order to reproduce such a variation. We conclude that the most important one is the exchange between the ozone-rich free troposphere and the ozone-poor boundary layer. This exchange is supposed to be more important during the night than during the day, allowing ozone nighttime recovery. This is the key point of the observed diurnal cycle, since daytime ozone photochemistry is well described by the model. Then we assume an entrainment velocity equal to 1 mm s -• during the day and 14 mm s -• during the night to closely match our measurements. Topography influences, together with clouds, are presumed to be responsible for this difference between nighttime and daytime entrainment velocities of free tropospheric air into the boundary layer at Reunion Island. Over the open ocean the difference of the turbulent flux of sensible heat between the day and the night explains the strong ozone nighttime recovery observed by us and by Johnson et al. [1990].

Spatial and temporal variability of tropospheric ozone (O3) in the boundary layer above the Aegean Sea (eastern Mediterranean)

“…The very low NO/ NO x ’ ratio indicates (1) influence of our station by aged air masses and (2) that the high concentrations of O 3 are likely due to region‐wide pollution and long‐range transport. Note that the amplitude (in percentage) of the diurnal variation of ozone observed at Finokalia is in agreement with that observed in remote marine locations [see, e.g., Johnson et al , 1990; Oltmans and Levy , 1994; Ayers et al , 1997; Bremaud et al , 1998]. However, the pattern of the diurnal O 3 cycle at Finokalia, although in agreement with model calculations (maximum amplitude between 0900 and 1400 LT calculated by De Laat and Lelieveld [2000]), is quite different from that reported for the remote marine locations.…”

“…Indeed, all campaigns conducted in these areas reported that maximum ozone concentrations over the oceans occur late at night and minimum in the late afternoon [ De Laat and Lelieveld , 2000, and references therein]. Several authors tried to simulate and understand the factors controlling the diurnal pattern of ozone and concluded that apart from photochemistry, changes in boundary layer (BL) depth during the day can also be responsible for the observed diurnal variability of ozone [ Thompson et al , 1993; Dickerson et al , 1999; Bremaud et al , 1998; Oltmans and Levy , 1994; Ayers et al , 1997]. Data on the diurnal variation of the marine BL do not exist for our area.…”

Atmospheric pollutant outflow from southern Asia: a review

“…Chand et al (2003) observed that the diurnal variations also persisted in regions where there were no horizontal gradients, concluding that downward mixing from the free troposphere can also contribute to producing the diurnal cycles, especially near the equator. Bremaud et al (1998) observed a similar phenomenon on Reunion Island in the southwestern Indian Ocean, concluding from their data that the entrainment velocity would need to be an order of magnitude greater at night than during the day in order to match their observations; some of this difference was hypothesized to be due to Atmos. Chem.…”

Estimating the climate significance of halogen-driven ozone loss in the tropical marine troposphere

“…Saiz-Lopez et al: Estimating the climate significance of halogen-driven ozone loss the tropical regions of the Atlantic Ocean (Oltmans and Levy II, 1992;Dickerson et al, 1999), Pacific Ocean (Johnson et al, 1990;Kley et al, 1996;Nagao et al, 1999;Shiotani et al, 2002;Takashima et al, 2008), and Indian Ocean (Johnson et al, 1990;Bremaud et al, 1998;Dickerson et al, 1999;de Laat et al, 1999;Burket et al, 2003). Tropical ozonesonde data also show events of substantially reduced ozone levels in the upper troposphere at different locations throughout the tropics (Solomon et al, 2005), although Vömel and Diaz (2010) claim that such events might be caused by artefacts in the measurement procedures.…”