TRACE-P OH and HO₂ measurements with the Airborne Tropospheric Hydrogen Oxides Sensor
(ATHOS) on the DC-8

Principal Investigator: William H. Brune, Pennsylvania State University, University Park, PA
Co-Investigators: Monica Martinez-Harder, Hartwig Harder

                The Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) measures OH and HO₂ from the NASA DC-8. This instrument detects OH by laser induced fluorescence (LIF) in detection chambers at low pressure and detects HO₂ by chemical conversion with NO followed by LIF detection. The demonstrated detection limit (S/N=2, 5 min.) for OH is about 0.005 pptv (1X10⁵ cm^-3 at 2 km altitude) and for HO₂ is 0.05 pptv (1X10⁶ cm^-3 at 2 km altitude). We will use ATHOS to measure OH, HO₂, and HO₂/OH during TRACE-P, analyze these results by comparing them against fundamental relationships and computer models, and publish the analyses. As participants in a tower-based study this summer, we are developing an attachment to ATHOS to detect RO₂+ HO₂-.  It may be tested well enough to include as an attachment for TRACE-P.  TRACE-P HO, measurements will help develop a clearer picture of the atmospheric oxidation and O₃ production that occur as Asian pollution spreads across the Pacific Ocean.

                We anticipate both new results and confirmation of previous results. Besides contributing to the science goals outlined in the TRACE-P NRA, HO_x, measurements will be combined with simultaneous measurements of environmental factors and other chemical species to test the following hypotheses:

• Asian outflow contains a unique mixture of HO_x sources; these, in combination with outflow and lightning NO_x can shift large regions of the Pacific troposphere from being ozone-destroying toward being ozone producing.

• As Asian outflow mixes with cleaner Pacific air, ozone production that is calculated from HO₂ and NO measurements is greater than that calculated by chemical transport models.

• Atmospheric oxidation over the western tropical Pacific is affected by the evolution of Asian outflow, but whether it is increased or decreased depends on the composition of the outflow and the influence of convection on that composition.  

• Asian aerosols influence HO. and atmospheric oxidation; they must be considered when calculating the influence of the Asian outflow on the photochemistry of the Pacific troposphere.  

• The outflow of HO_x sources into the Pacific free troposphere has greater influence on increasing ozone production than does the NO_x outflow because of the greater horizontal range of HO. sources compared to that of NO_x and the availability of NO_x from lightning.

                  Testing these hypotheses relies heavily on the simultaneous measurements of HO_x, all the chemical species that interact with HO_x, and the environmental conditions, particularly the photolysis frequencies. It is important to obtain this entire measurement suite from the planetary boundary layer to the ceiling of the DC-8 at 12 km, as well as in the presence of clouds and heavy pollution nearer the Asian coast. Measurements while flying out of and into airports are important photochemical kinetics tests because of the different amounts of NO that are mixed into the continental air. In addition, measurements must be made at different times of the day and night.

                TRACE-P will add to a growing body of HO. data that we have collected with ATHOS and GTHOS, the towered-based configuration of ATHOS that we use for near-surface studies. ATHOS has participated in SUCCESS (1996), SONEX (1997), PEM Tropics B (1999), and SOLVE (1999-2000). GTHOS has participated in PROPHET, at a semi-rural heavily forested site in northern lower Michigan in summer, 1998 and in Nashville SOS, in the urban plume of Nashville in summer, 1999. These measurements are giving us a view of HO. photochemistry for a wide range of HO_x sources, NO_x levels, and hydrocarbon loadings. TRACE-P allows us to sample the influence of urban-level, processed pollution on a low-hydrocarbon, pristine environment, a condition that we have not previously studied extensively.

                An exciting aspect of TRACE-P is the connection between the aircraft and satellite observations. While instruments on the Terra and ENVISAT satellites cannot measure OH and HO₂, they can measure chemical species that have great influence on HO_x, such as CO, O₃, and NO₂ (a measure of NO). By comparing the satellite and aircraft measurements for a wide range of environments, we should be able to construct parameterizations of atmospheric oxidation and ozone production.