Airborne observations of dust aerosol over the North Atlantic Ocean during ACE 2: Indications for heterogeneous ozone destruction

Abstract

Abstract. Aerosol size distribution measurements have been performed in the free troposphere during the Second Aerosol Characterization Experiment (ACE 2) near Tenerife, Canary Islands, in July 1997. During one measurement flight, on July 8, a uniform aerosol layer was encountered between 2.5 and 5.5 km altitude, characterized by a relatively low Aitken mode particle number concentration and high concentrations of accumulation and coarse mode particles, resulting in a relatively large aerosol surface area and mass, which is estimated to be about 400/xg m -3. Five-day backward trajectories indicate that the aerosol in this layer was mineral dust originating from arid regions on the North African continent. The dust layer was associated with reduced ozone mixing ratios. Model simulations have been performed with a photochemical box model including different heterogeneous removal reactions to study the interaction between gas phase chemistry and mineral aerosol. The best agreement between the observed and modeled ozone concentrations was obtained when heterogeneous removal of ozone and precursor gases on dust aerosol were taken into account. Heterogeneous 0 3 loss is estimated at 4 ppbv 0 3 per day. Although uncertainties concerning heterogeneous ozone removal remain, in particular related to the reactive uptake coefficient of 0 3 , it is likely that the loss of 0 3 and precursor gases on mineral dust aerosol significantly reduces the 0 3 abundance in large-scale dust plumes.

Assessment of atmospheric processes driving ozone variations in the subtropical North Atlantic free troposphere

“…In summer we obtain a logarithmic fit with a good correlation which resembles the O 3 -PM 10 fit in the same season. Prospero et al (1995) (Cuevas, 1995), might be due to a sum of all or some of the following causes: (1) air masses come from lower levels within the Saharan CBL where O 3 concentrations are generally low (Güsten et al, 1996); (2) air masses originate from similar or lower latitudes which are not enriched with the latitudinal O 3 gradient existing in spring and summer in the Northern Hemisphere (London and Liu, 1992); (3) O 3 might be destroyed by dry deposition on suspended dust particles because of a larger surface area to dry deposition (Güsten et al, 1996); and (4) heterogeneous removal processes on dust aerosol by nitric acid (de Reus et al, 2000). records at IZO are lower than 10 µg m −3 and monthly means of PM 10 P75th are below 10 µg m −3 through the year, except in summertime when significant PM 10 concentrations are observed above P80th (26 µg m −3 ).…”

“…Chem. Phys., 13, 1973-1998, 2013 www.atmos-chem-phys.net/13/1973/2013/ 10 E. Cuevas et al: Surface O 3 characterization in the subtropical North Atlantic troposphere The low O 3 values associated with African trajectories which, in turn, cause a systematic rise in relative humidity records at IZO (Cuevas, 1995), might be due to a sum of all or some of the following causes: (1) air masses come from lower levels within the Saharan CBL where O 3 concentrations are generally low (Güsten et al, 1996); (2) air masses originate from similar or lower latitudes which are not enriched with the latitudinal O 3 gradient existing in spring and summer in the Northern Hemisphere (London and Liu, 1992); (3) O 3 might be destroyed by dry deposition by suspended dust particles because of a larger surface area to dry deposition (Güsten et al, 1996); and (4) heterogeneous removal processes on dust aerosol by nitric acid (de Reus et al, 2000). • and 55…”

Atmospheric radiative effects of an in situ measured Saharan dust plume and the role of large particles

“…The aerosol size distributions presented in this work have been composed using the measurements of five instruments onboard the Cessna Citation aircraft during ACE-2, which are summarised in Table 1. A detailed description of the instrumentation can be found in de de Reus et al (2000a) andde Reus et al (2000b).…”

“…The dominant volume size distribution mode in these layers is the coarse mode. A more detailed description of the aerosol size distributions observed in the Saharan dust layer is given by de Reus et al (2000a).…”

Aerosol-ozone correlations during dust transport episodes

“…On the contrary, under southerly winds coming from the desert, very low O 3 concentrations with a diurnal variation between 25 and 45 ppbv were measured (Gusten et al, 1996). Outside the African deserts, but close to the west coast of north Africa, trace gas measurements, carried out at the Izaña baseline mountain station (Canary Islands), showed marked surface ozone depletions during Saharan dust events (Prospero et al, 1995;Schmitt et al, 1998), which significantly influenced the annual mean ozone concentrations (Schmitt et al, 1998;de Reus et al, 2000).…”