HNO3, HO2NO2, and SO2

G. Huey and D. Tanner
Georgia Institute of Technology

The measurements of HNO3, HO2NO2, and SO2 by the GIT CIMS group are crucial to all the major science objectives of ANTCI. HNO3, HO2NO2 , SO2 measurements on the ground during the first year (2003) will provide important insight into the photochemical interactions between HOx and NOx as well as the budget of NOy and provide a reference point for the levels of reactive sulfur. These measurements will be continued on the C-130 platform in year 3 (2005). During the latter study the SO2 measurements will play a key role in our gaining a better understanding of how and where the oxidation of DMS and DMSO occur in the Antarctic environment.
HO2NO2, HNO3, and SO2 will be measured with the same basic chemical ionization method used during ISCAT 2000. This technique utilizes SF6- ion chemistry and the following reactions.

SF6^- + HNO3 → NO3^-•HF + SF5
SF6^- + HO2NO2 → NO4^-•HF + SF5
SF6^- + SO2 → F2SO2^- + SF5

This chemistry allows selective and sensitive detection of these species in an environment with low absolute humidity such as polar regions (dew points < -10 ^oC). The CIMS instrument is calibrated for both SO2and HNO3 by periodic standard additions of known quantities of both compounds. The sensitivity for HO2NO2 has been derived from laboratory calibrations of the instrument. The background signal for both HNO3 and HO2NO2 is periodically measured removing these gases from the sampled air with a nylon wool scrubber. The limit of detection for both HO2NO2 and HNO3 during the 2000 South Pole campaign was less than 5 pptv for a one minute integration period as defined by a signal to noise ratio of 3 where the noise is the standard deviation of the background. Similar limits are expected for ANTCI. The SO2 background during ANTCI was not measured because it could not be scrubbed using nylon wool. However, the total SO2 signal (signal + background) was typically less than 25 pptv on average which places an upper limit on SO2 at the SP. More importantly, it demonstrates that there are no large interferences to the SO2 measurement in the polar environment. In addition to the ISCAT measurements, we have used CIMS to successfully measure HNO3 on two different aircraft campaigns.
We plan to improve the SF6^- CIMS technique used previously at the South Pole by adding a collisional dissociation chamber (CDC) and an SO2 denuder. The CDC is a region of high collision energy that provides a mechanism for dissociating weakly bound ion molecule clusters. This basic technique has been used in numerous chemical ionization methods to reduce interferences due to water clustering. Preliminary laboratory results using the CDC have demonstrated that the viability of the SF6^- chemistry for detection of HNO3 and HO2NO2 can be extended up to a dew point of 10 ^oC. Either a potassium hydroxide (KOH) or a calcium carbonate (CaCO3) denuder will be used to periodically scrub SO2 from the ambient air stream to provide a zeroing mechanism for the SO2 measurement. Preliminary results with a potassium hydroxide denuder indicate that this zeroing mechanism is very promising and a limit of detection (LOD) of less than 20 pptv for a one minute integration period in ambient Atlanta air has been obtained. The LOD obtained in Atlanta should represent an absolute worst-case scenario. A LOD of less than 5 pptv for a one minute integration period is expected in the Antarctic environment given the background signal measured during ISCAT 2000.