Observations of Total Peroxy Nitrates during TOPSE. J. A. Thornton 1 , P. J. Wooldridge 1 , D. A. Day 1 , R. S. Rosen 1 , R. C. Cohen 1 , F. Flocke 2 , A. Weinheimer 2 , B. A. Ridley 2 Email: [email protected]
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J. A. Thornton1, P. J. Wooldridge1, D. A. Day1, R. S. Rosen1, R. C. Cohen1, F. Flocke2, A. Weinheimer2, B. A. Ridley2
Email: [email protected]
1 Department of Chemistry; University of California, Berkeley; Berkeley, CA 94720; 2 Atmospheric Chemistry Division, NCAR; Boulder CO
IV. PAN Measurements
V. PAN and the NOy Budget
II. Thermal-Dissociation Laser-Induced Fluorescence
The above figures demonstrate the TD-LIF technique’s capabilities. On the left, 12-second average PAN (triangles) and NO2 (open squares) concentrations measured by the TD-LIF instrument are plotted versus time for a local research flight based out of Churchill, Canada April 11, 2000. Arrows indicate when the thermal dissociation oven was either turned on or off. When the oven is off, the two channels measure the same NO2 concentrations, and thus the difference in signal between the two channels is zero on average. When the oven is on, the difference in signal between the two channels is proportional to the sum total peroxynitrate concentration which ranged from near zero to 750 ppt during this flight. To the right, we demonstrate the high time resolution by showing the the 12-second average data from a narrow time window (56500 sec – 62500 sec) together with the NCAR (PAN+PPN) data (solid black squares) which was measured on a slower time interval by Gas Chromatography ECD.
XNO2 + heat X + NO2
The two plots above show the ratio of NOyi/NOy versus ambient pressure. On the left, NOyi = NO + NO2 + PAN + HNO3, where the NO and NO2 measurements were made by NCAR, HNO3 by the UNH mist chamber instrument, and PAN by the TD-LIF instrument. The ratio of the sum of these NOy species to the total is scattered about the value of 1 indicating closure in the NOy budget. On the right, NOyi = NO + NO2 + GC_(PAN+PPN) + HNO3, where the NCAR GC measurements of PAN and PPN are used in place of the TD-LIF PAN measurements. The ratio of the sum of these NOy species to the total exhibits a similar trend versus pressure as that observed in the comparison to the TD-LIF instrument.
The apparent calibration offset between the three instruments (TD-LIF, GC, total NOy) affects our ability to assess the abundance of other peroxy nitrates such as HNO4.
The NO2 yield vs. temperature curve for a combination of different classes of nitrate compounds can be predicted and measured.
T< 325 K NO2
T~ 500 K PAN + PPN + HNO4 + N2O5 + RO2NO2 + NO2
T~ 650 K RONO2 + RO2NO2 + NO2
VI. O3 and PAN Correlations
1/4”diameter, 25 cm long pyrex tube wrapped in
wire for heating.
~ 3m of tubing
Pinholes to reduce pressure
(increased flow rate)
The two figures above show PAN (black squares) and GC_(PAN+PPN) (red circles) measurements plotted versus time for two consecutive flights. Up to three 12-second PAN measurements were averaged to each GC_(PAN+PPN) measurement. The flights are the same as those shown in section III NO2 Measurements. Flight 40, on the left, shows periods where the TD-LIF PAN measurements are higher than the GC_(PAN+PPN) measurements as well as periods when they are lower. During Flight 41, on the right, the measurements agree well during periods when concentrations were changing rapidly.
III. NO2 Measurements
The results of a point-by-point comparison of all TD-LIF PAN and NCAR GC_(PAN+PPN) measurements are shown above. 12-second average TD-LIF data that fell within 36 seconds of a GC_(PAN+PPN) measurement were averaged for the comparison. A linear fit to the comparison data suggests the two data sets agree to with 4% on average with a 1.6 ppt offset and good correlation (R2 = 0.77). However, at the lower concentrations the TD-LIF measurements are biased low relative to the GC measurements. This bias is also evident in the plot to the left. The ratio of LIF_ PAN/GC_(PAN+PPN) is plotted versus ambient pressure (mbar). At the lowest pressures (highest altitudes), the TD-LIF instrument measures slightly higher (~10-15%) total peroxynitrates than the GC_(PAN+PPN) measurements on average, and at the highest pressures (lowest altitudes) the TD-LIF measurements are ~30% less than the GC measurements. Because PAN concentrations are correlated with ambient pressure (altitude), it is difficult to determine the cause of this trend. The trend maybe due to a relative calibration offset between the two instruments and to the presence of peroxynitrates other than PAN and PPN such as HNO4.
The TD-LIF measurements of PAN are in good general agreement with the GC_(PAN+PPN) measurements. These two
data sets provide evidence that the major NOy species in the Arctic troposphere is PAN. Calibration offsets between the
two data sets (10-30%) make estimates of the abundance of other peroxynitrates difficult. However, future analysis will be aimed at constraining the abundance of HNO4 in the Arctic troposphere using the TD-LIF PAN measurements.
The altitude dependence to the correlation of PAN with O3 demonstrates the importance of separating the roles of mixing and chemistry.
The observed seasonal increase of both PAN and O3 concentrations provides a strong constraint for models aimed at describing the roles of both transport and chemistry.
Examples of the LIF NO2 measurements (red circles) made on the C-130 are shown above for two consecutive flights along with the NO2 measurements made by NCAR using Photo-Fragment Chemiluminescence. The measurements shown here are 1-minute averages. The LIF and NCAR NO2 are in reasonable agreement above 10 ppt, and the comparison is noisy below that. Both channels in the TD-LIF instrument measured the same NO2 concentrations.