NO and NOy

D. Davis, and M. Buhr
Georgia Institute of Technology

Measurements of NO and NOy are fundamental to the understanding of fast photochemistry and the nitrogen budget. It is currently anticipated that the NO sensor will also be present at SP during field deployment 2005. The aircraft-based system used in the first deployment will also include measurements of ozone, temperature, dew point, pressure, and 3-dimensional sampling position. NO and NOy will be measured using custom built chemiluminescence instruments which will be equipped with heated molybdenum catalytic converters. The chemiluminescence method of measuring NO is a well characterized technique that has been intercompared with other techniques (e.g., TP-LIF) on several occasions. The heated molybdenum catalytic converter for measurement of NOy is also a well characterized technique that has been thoroughly tested in several formal intercomparisons.

The measurement frequency for NO and NOy using this instrument will be 2 minutes, which will include 45 seconds of ambient measurements and a 15 second zero level determination each for NO and NOy. The limit of detection for the NO channel is ~2 pptv for a one-minute integration of one-second data. The expected limit of detection for the NOy channel is about 50 pptv for a one minute integration. In both cases, the limit of detection is controlled by the level of the artifact signal, which is the apparent signal measured while the instrument is sampling zero air. Measurement of the zero-air artifact will be performed on a daily basis. The instrument will be calibrated daily using the standard addition technique. The calibration gas used for the NO channel will be a NIST-traceable NO in N2 gas mixture. The NOy channel will be calibrated with the NO standard and with NO2 produced via gas phase titration using a low-pressure mercury lamp. The conversion efficiency of the NOy channel will also be measured using an HNO3 standard generated using a permeation tube-based calibration source. The NOy converter will be mounted close to the sample port inside the ARO building and will be connected to the ambient air with a short, heated (~30 ^oC), Teflon tube. The heated tube will maximize transmission of species like HNO3 to the catalytic converter. NO will be sampled through a short, un-heated Teflon tube. The instrument includes a laptop computer-based data acquisition and control system.

For the Twin Otter NO/NOy system, a zero level determination will be performed every minute for 10 seconds. Calibration of the system will be performed using standard addition of NO, NO2, and HNO3 to ambient air at least twice per flight. Like the ground-based system, it is expected that the limit of detection will be controlled by artifact level and zero air tests will be performed pre- and post-flight to determine the NO and NOy artifact. The expected limit of detection is about 10 and 100 pptv for NO and NOy, respectively, for a 5 second integration. Similar to the ground-based system, the NOy converter will be mounted close to the sampling port and will be connected to the outside air via a short, heated, Teflon tube. The NO sample inlet will be an unheated Teflon tube located close to the NOy inlet. In both cases, the inlet will be forward facing and will be vented to the aircraft cabin, providing ram-air flow to which the instrument inlets will be attached at right angles. Both inlets will be mounted on a window blank that will be installed on the aircraft prior to its deployment to Antarctica.

As noted above the aircraft system will also include measurements of ozone, ambient temperature, pressure, dew point, and aircraft position. Ozone will be measured using a Dasibi UV photometer operated at a high flow rate (ca. 3 slpm). This instrument’s response will be compared with a certified ozone transfer standard photometer before deployment to Antarctica. Temperature will be measured with a platinum resistance thermometer. The thermometer will be mounted in an aircraft-standard shrouded inlet (Rosemount) attached to the window blank mentioned above. The temperature measurement will have a resolution of 0.1 ^oC and a response time of one second. Dew point will be measured with an aircraft-standard cooled mirror hygrometer that includes a window blank-mounted sensor and rack-mounted control electronics. The system will include a GPS with a window-blank mounted aircraft-standard patch antenna. The GPS will provide 3-D position with a resolution of at least 10 meters in each direction. The aircraft instrument package will all be mounted in one rack plus an external vacuum pump and the window-mounted inlet system. One laptop-computer based data acquisition and control system will be integrated with the system.