R. O. Jakoubek

Airborne intercomparison of vacuum ultraviolet fluorescence and tunable diode laser absorption measurements of tropospheric carbon monoxide

During the fall 1997 North Atlantic Regional Experiment (NARE 97), two separate intercomparisons of aircraft-based carbon monoxide measurement instrumentation were conducted. On September 2, CO measurements were simultaneously made aboard the National Oceanic and Atmospheric Administration (NOAA) WP-3 by vacuum ultraviolet (VUV) fluorescence and by tunable diode laser absorption spectroscopy (TDLAS). On September 18, an intercomparison flight was conducted between two separate instruments, both employing the VUV fluorescence method, on the NOAA WP-3 and the U.K. Meteorological Office C-130 Hercules. The results indicate that both of the VUV fluorescence instruments and the TDLAS system are capable of measuring ambient CO accurately and precisely with no apparent interferences in 5 s. The accuracy of the measurements, based upon three independent calibration systems, is indicated by the agreement to within 11% with systematic offsets of less than 1 ppbv. In addition, one of the groups participated in the Measurement of Air Pollution From Satellite (MAPS) intercomparison [Novelli et al., 1998] with a different measurement technique but very similar calibration system, and agreed with the accepted analysis to within 5%. The precision of the measurements is indicated by the variability of the ratio of simultaneous measurements from the separate instruments. This variability is consistent with the estimated precisions of 1.5 ppbv and 2.2 ppbv for the 5 s average results of the C-130 and the WP-3 instruments, respectively, and indicates a precision of approximately 3.6% for the TDLAS instrument. The excellent agreement of the instruments in both intercomparisons demonstrates that significant interferences in the measurements are absent in air masses that ranged from 7 km in the midtroposphere to boundary layer conditions including subtropical marine air and continental outflow with embedded urban plumes. The intercomparison of the two VUV instruments that differed widely in their design indicates that the VUV fluorescence technique for CO measurements is not particularly sensitive to the details of its implementation. These intercomparisons help to establish the reliability of ambient CO measurements by the VUV fluorescence technique.

Intercomparison of UV/visible spectrometers for measurements of stratospheric NO2 for the Network for the Detection of Stratospheric Change

During the period May 12–23, 1992, seven groups from seven countries met in Lauder, New Zealand, to intercompare their remote sensing instruments for the measurement of atmospheric column NO2 from the surface. The purpose of the intercomparison was to determine the degree of intercomparability and to qualify instruments for use in the Network for the Detection of Stratospheric Change (NDSC). Three of the instruments which took part in the intercomparison are slated for deployment at primary NDSC sites. All instruments were successful in obtaining slant column NO2 amounts at sunrise and sunset on most of the 12 days of the intercomparison. The group as a whole was able to make measurements of the 90° solar zenith angle slant path NO2 column amount that agreed to about ±10% most of the time; however, the sensitivity of the individual measurements varied considerably. Part of the sensitivity problem for these measurements is the result of instrumentation, and part is related to the data analysis algorithms used. All groups learned a great deal from the intercomparison and improved their results considerably as a result of this exercise.

Mixing of anthropogenic pollution with stratospheric ozone: A case study from the North Atlantic wintertime troposphere

As part of the North Atlantic Regional Experiment (NARE), instrumentation for the measurement of O3 and CO was included on research flights conducted by the National Oceanic and Atmospheric Administration WP-3D Orion aircraft from St. Johns, Newfoundland, Canada, and Keflavik, Iceland, from February 2 to 25, 1999. These flights sampled the lower troposphere over the western North Atlantic Ocean. One significant feature observed during these flights was the close proximity of air masses with contrasting source signatures: high levels of anthropogenic pollution immediately adjacent to elevated O3 of stratospheric origin. Here we present a case study showing the most pronounced example of this proximity, which was associated with a frontal passage across North America and out into the North Atlantic region. Trajectory analyses and satellite imagery are used to investigate the transport mechanisms that create the interleaving of air masses from the different sources. One important chemical feature was noted: in air masses with differing amounts of anthropogenic pollution admixed, O3 was negatively correlated with CO, which indicates that emissions from surface anthropogenic sources had reduced O3 in this wintertime period, even in air masses transported into the free troposphere.

Multiscale simulations of tropospheric chemistry in the eastern Pacific and on the U.S. West Coast during spring 2002

[ 1] Regional modeling analysis for the Intercontinental Transport and Chemical Transformation 2002 (ITCT 2K2) experiment over the eastern Pacific and U. S. West Coast is performed using a multiscale modeling system, including the regional tracer model Chemical Weather Forecasting System (CFORS), the Sulfur Transport and Emissions Model 2003 (STEM-2K3) regional chemical transport model, and an off-line coupling with the Model of Ozone and Related Chemical Tracers ( MOZART) global chemical transport model. CO regional tracers calculated online in the CFORS model are used to identify aircraft measurement periods with Asian influences. Asian-influenced air masses measured by the National Oceanic and Atmospheric Administration (NOAA) WP-3 aircraft in this experiment are found to have lower DeltaAcetone/DeltaCO, DeltaMethanol/DeltaCO, and DeltaPropane/DeltaEthyne ratios than air masses influenced by U. S. emissions, reflecting differences in regional emission signals. The Asian air masses in the eastern Pacific are found to usually be well aged (> 5 days), to be highly diffused, and to have low NOy levels. Chemical budget analysis is performed for two flights, and the O-3 net chemical budgets are found to be negative ( net destructive) in the places dominated by Asian influences or clear sites and positive in polluted American air masses. During the trans-Pacific transport, part of gaseous HNO3 was converted to nitrate particle, and this conversion was attributed to NOy decline. Without the aerosol consideration, the model tends to overestimate HNO3 background concentration along the coast region. At the measurement site of Trinidad Head, northern California, high-concentration pollutants are usually associated with calm wind scenarios, implying that the accumulation of local pollutants leads to the high concentration. Seasonal variations are also discussed from April to May for this site. A high-resolution nesting simulation with 12-km horizontal resolution is used to study the WP-3 flight over Los Angeles and surrounding areas. This nested simulation significantly improved the predictions for emitted and secondary generated species. The difference of photochemical behavior between the coarse (60-km) and nesting simulations is discussed and compared with the observation.

Measurement of tropospheric OH by long-path laser absorption at Fritz Peak Observatory, Colorado, during the OH Photochemistry Experiment, fall 1993

The determination of the concentration of hydroxyl (OH) in the Earths troposphere is of fundamental importance to an understanding of the chemistry of the lower atmosphere. This paper describes the results from the laser long-path spectroscopic OH experiment used in the Tropospheric OH Photochemistry Experiment (TOHPE) held at Fritz Peak, Colorado, in fall 1993. A primary goal of TOHPE was to compare the OH concentrations measured using a variety of different techniques: a long-path spectroscopic instrument [Mount, 1992], an in situ ion-assisted chemical conversion instrument (Eisele and Tanner, 1991, 1993), a laser resonance fluorescence instrument [Stevens et al., 1994), and a liquid scrubber instrument (X. Chen and K. Mopper, unpublished data,; 1996), all with sensitivities at or below 1×106 molecules cm−3. In addition to the OH measurements, a nearly complete suite of trace gas species that affect the OH concentration were measured simultaneously, using both in situ and/or long-path techniques, to provide the information necessary to understand the OH variation and concentration differences observed. Measurements of OH, NO2, CH2O, SO2, H2O, and O3 were made using long-path spectroscopic absorption of white light or laser light and OH, NO, NO2, NOy, O3, CO, SO2, CH2O, j(O3), j(NO2), RO2/HO2, HO2, H2O, SO2, PAN, PPN, HNO3, and aerosols (size and composition) and ozone and nitrogen dioxide j-values were measured using in situ instruments. Meteorological parameters at each end of the long path and at the Idaho Hill in situ site were also measured. The comparison of the long-path and in situ species from this set of complementary measurements provides an effective way of interpreting air masses over the long path with those at the in situ site; this is a critical issue since the long-path spectroscopic OH determinations provide a nonchemical and well-calibrated measurement of OH which must be compared in a meaningful manner with the in situ determinations. Over the period of the TOHPE experiment, OH concentrations were typically low during periods of clean and clear airflow, averaging about 4×106 molecules cm−3 at noon. In contrast, during the well-defined pollution episodes which occurred during the campaign, OH concentrations rose as high as 15×106 molecules cm−3.

Measurement of tropospheric trace gases by long‐path differential absorption spectroscopy during the 1993 OH Photochemistry Experiment

In August and September 1993 a comparative study of tropospheric long-path absorption techniques with in situ methods was performed for both the hydroxyl radical and the other important trace species. Long-path measurements were made over the 10.3 km path between Fritz Peak Observatory and Caribou Mine in the mountains 17 km west of Boulder, Colorado. At Caribou Mine, a 121 element, 1 m2 retroreflector array folds the optical path to give a total path of 20.6 km. The in situ instruments were located at Idaho Hill 0.5 km northwest of Caribou Mine. The optical design and analysis techniques used to obtain the path-integrated concentrations of O3, CH2O, SO2, and NO2 will be presented. The spectrograph used in this study is a 1/4 m double, crossed Czerny-Turner that employs a diode array detector allowing the acquisition of 40 nm spectral bands in the near UV and visible spectral regions. This system also utilizes automatic alignment and self-adjusting time integration so the system will acquire data in an unattended mode. The spectral bands selected for this study permit the simultaneous measurement of O3, NO2, CH2O, and SO2; NO2 and H2O; and NO3 and H2O. The data analysis uses a nonlinear least squares regression procedure to deduce the concentration of each of the species present in the atmosphere and also provides an effective method for removing the influence of scattered solar light for daytime measurements. An estimate of the measurement precision can be found by comparing atmospheric spectra analyzed with two different IO spectra; one measured through the atmosphere and the other a direct arc lamp spectrum.