P. Perros

Regional levels of ozone in the troposphere over eastern Mediterranean

During the last 5 years, information on the regional tropospheric ozone levels over the eastern Mediterranean has become available but was confined to measurements at ground level. Here available information is expanded with measurements at two more rural ground level sites spanning 8° latitude, ozonesonde ascents, lidar observations, ship cruises, and aircraft flights. During wintertime the ozone monthly values are 10-20 ppbv higher than values at other European sites, while during summertime the values are comparable. This results in regional ozone background levels in the area that are above the European Union (EU) 32 ppb/24 hours phytotoxicity limit during the entire year. Late spring lidar observations show that south and southwestern synoptic flows which are associated with Saharan dust events result in lower ozone above the planetary boundary layer (PBL) by 20-35 ppbv as compared to these during northerly flows, which transport air from continental Europe. These lidar observations along with ship measurements during July show that ozone is enhanced 1.5-2.2 times in the continental outflow, when compared to aged maritime air or air from the African continent. These results along with ozonesonde observations suggest that ozone abatement in the area is largely beyond the control of regional emissions and can be controlled only with emission reductions on a European scale. During September, measured background NO and NO 2 levels between 3 and 4.5 km above sea level ranged from 1 to 221 pptv and from 102 to 580 pptv, respectively. The troposphere contains around 40-70 Dobson units of ozone during summer and around 20-30 Dobson units during winter.

Influence of Continental Sources on Oceanic Air Composition at the Eastern Edge of the North Atlantic Ocean, TOR 1992-1995

The atmospheric concentrations of O3, NMHCs, PAN and NOx were measured at the TOR station in Porspoder (Brittany, France) from 1992 to 1995. The results from April 1992 to December 1994 are reported here. In order to study the behaviour of trace gases in the air masses coming from the Atlantic Ocean, the origin of these air masses was characterised by the analysis of the five-day back-trajectories reaching 950 mbar at Porspoder. Three kinds of oceanic air masses are considered, depending on their origin: North Atlantic northern and southern latitudes and North American continent. It appears that the oceanic high-latitude troposphere was strongly influenced by continental emissions, which in turn impacted the median latitude troposphere during polar outflow events. A greater accumulation of anthropogenic compounds in the Arctic sector was calculated and may be at the origin of the high oxidant levels measured in spring in this same sector. The influence of north American emissions on the European coast was also studied and seems to reach a maximum in spring. Long-range transport of long- lived species was observed throughout the year, but evidence for winter transport of more reactive species, such as ethene, is also shown. Meteorological parameters and local photochemistry appear to play an important role in the distribution of the pollutants over the North Atlantic Ocean throughout the year. Further investigations are needed to give more complete information about the origin, formation and destruction of the secondary pollutants.

Large‐scale distribution of peroxyacetylnitrate from aircraft measurements during the TROPOZ II experiment

Aircraft measurements of peroxyacetylnitrate (PAN) were performed during the TROPOZ II meridional campaign. A specially designed automatic PAN analyzer on board the airplane allowed us to obtain 422 PAN measurements. The data set covers a latitude range from 60 °N to 60 °S and altitudes up to 11 km. PAN was found to be an important reactive nitrogen species with a mixing ratio range of 4 part per trillion (ppt) (percentile 5)–1060 ppt (percentile 95). PAN mixing ratio showed a high degree of variability in both hemispheres. In the southern hemisphere, the higher troposphere exhibited a mixing ratio generally below 20 ppt. The highest mixing ratios were observed in the boundary layer. In polluted areas, PAN levels, higher than 100 ppt, were measured in the lower layers (0–5 km above sea level). In the northern hemisphere, concentrations of PAN ranged from 100–250 ppt in the lower troposphere to 20–40 ppt above 5 km. As a matter of fact, we have frequently measured high PAN mixing ratio (> 200 ppt) over West Africa. Since very few anthropogenic PAN precursors are normally released in this area, these high PAN levels were probably due to bush fires.

One-year measurements of PAN in the Paris basin: Effect of meteorological parameters

Abstract PAN has been measured continuously for 1 year in Creteil, a southeastern suburb of Paris Correspondence analysis was used to analyse the important PAN database obtained during this period, in relation to associated meteorological parameters and the location of precursors sources. This analysis revealed a complex relation (U-shaped curve) between PAN and temperatures, with a minimum in the range of 4–12°C and an increase in the lower and higher temperatures, arising from the competition between photochemical production and thermal decomposition. Long-range transport of PAN is associated with easterly winds and occurs under anticyclonic conditions in all seasons, particularly in winter. The highest PAN values (maximum 33.6 ppb) are observed in summer. They have a local origin and are associated with northwesterly to southwesterly winds, revealing the net impact, in summer, of nearby sources such as the urban conglomeration of Paris on PAN levels recorded in the surrounding areas.

Large-scale distribution of hydrogen peroxide from aircraft measurements during the TROPOZ II experiment

Abstract Aircraft measurements of hydrogen peroxide (H2O2) were performed during the TROPOZ II meridional campaign. A specially designed automatic H2O2 analyser on board the airplane allowed us to obtain more than 43,000 H2O2 measurements. The data set covers a latitude range from 60°N to 60°S and altitudes up to 11 km. Concentration of H2O2 ranged from 0.42 ppb (percentile 5) to 6.31 ppb (percentile 95) and exhibited a variability factor of as much as 15. Hydrogen peroxide concentration was typically low ( 3 ppb) could be noticed. Some episodes of high hydrogen peroxide mixing ratio were encountered in the vicinity of cloud systems probably by exchange of H2O2 from the liquid phase to the gas phase.

Pan measurements in a forested area (Donon, France)

Abstract Continuous PAN measurements using a fully automated gas chromatograph have been performed since September 1986 in a forested area (Donon) in the east of France. The results showed a marked seasonal variation with a maximum observed in spring. A PAN concentration of 0.2 ppb can be considered as the background level of this region, while substantial PAN levels (1–2 ppb) were recorded during several transport episodes. One of them is analysed and discussed in detail.

Continuous atmospheric measurements of peroxyacetyl nitrate (Pan) in a Mediterranean site (Athens, greece)

Abstract Photochemical pollution episodes are becoming an important problem for many major Mediterranean cities with a hot, sunny climate. Peroxyacetyl nitrate (PAN) can serve as a very good tracer of photochemical activity in the atmosphere, and was measured automatically for the first time in Athens, Greece, during the period 18 September‐10 October 1985. The observed overall PAN concentrations were generally lower than expected for a city with frequent formation of photochemical smog. It appears, however, that elevated PAN and ozone concentrations occur mainly by sea‐breeze circulation, which is often observed at coastal sites.