Flugzeugmessungen von NO, NOy, CO und O3 in der freien Troposphäre zwischen 60 °N und 60 °S - Nord-Süd-Kontrast und Verteilung in den Tropen

Trace gas measurements of NO, NOy, CO and O3 performed on board the
DLR aircraft Falcon are used to examine the interhemispheric
differences for midlatitudes during local autumn. The measurements
were part of the INCA campaign (Interhemispheric differences in
cirrus properties due to anthropogenic emissions). Major deployment
sites were Punta Arenas, Chile (53°S) and Prestwick, Scotland
(55°N). In addition, the meridional distribution of trace gases
measured during the transfer flights between Germany and Chile were
analysed for latitudes between 60°N and 60°S. Median mixing ratios
of NO, NOy, CO and O3 in the upper troposphere in the investigation
area near Prestwick exceed the observed concentrations in Punta
Arenas by factors of 4.5, 3, 1.5 and 1.2, respectively, due to the
influence of convective vertical transport of polluted boundary
layer air and aircraft emissions. The INCA measurements at northern
midlatitudes agree well with the data of previous campaigns such as
NOXAR and POLINAT 2. Based on selected case studies net ozone
production was calculated from O3, NO, CO and H2O measurements. For
these estimates the chemical reactions with hydrocarbons were
neglected and model results were used for the ozone photolysis
frequency and the OH concentrations. The results for Punta Arenas
show net ozone production for the upper troposphere, but net ozone
destruction for the middle troposphere due to the very low NO
levels observed there. For northern midlatitudes a significant net
ozone production was calculated for the middle and upper
troposphere caused by the higher concentration levels of ozone
precursors in the region of the North Atlantic flight corridor. The
meridional trace gas distributions in the upper troposphere show
strongly enhanced CO and NOy concentrations over tropical South
America. These air masses encountered deep convection over the
Amazon basin and are distributed over a large area due to an upper
level anticyclonic flow over South America, which is typical for
the wet season. Long-distance transport of CO emissions from West
African biomass burning sites partly contributes to the high CO
concentrations over the tropics. Comparisons of the INCA
measurements for the Southern midlatitudes with the chemical
transport model MOZART show that CO and NOy concentrations over
Punta Arenas are overestimated by the model. Probably the
assumptions for the local surface emissions and the long-distance
transport of biomass burning emissions are too high. In addition
the tropopause height simulated by the model is too low. Except for
the tropopause region the model simulates ozone and nitrogen oxides
in good agreement with the observations. The net chemical ozone
production calculated by the model agrees well with the estimates
based on the measurements. For comparison with the INCA data for
the northern midlatitudes model simulations of MOZART and NILU were
used. The measured CO profile over Prestwick was well represented
by the regional NILU simulation. In contrast, the MOZART model
sometimes showed more than twice the amount of CO observed. This
can be caused by the rough resolution of the model and the
assumption of too high surface emissions in Europe. The observed
trace gas distributions of NO, NOy and O3 are well reproduced by
the model. For the upper troposphere the net ozone productions
calculated by the models agree well with the results of this study
calculated directly from the observations. Only in the middle
troposphere lower net ozone production rates than simulated by the
CTM were calculated from the observations due to the neglection of
the hydrocarbon chemistry.