Chemistry of OH during ARCTAS

The NASA ARCTAS study presented a very unique opportunity to investigate the tropospheric chemistry of the Arctic environment, both rural and urban areas of California, and plumes and outflows from boreal fires in northern Canada. Here OH data obtained from the NCAR OH, Sulfuric, and MSA CIMS instrument aboard the NASA DC-8 will be presented together with model simulations. OH showed a large variation depending upon the type of environment sampled with values as low as 2-3 x 105 molecule cm-3 in the clean Arctic to well over 107 molecule cm-3 in urban areas or fire plumes. Measurements during ozone depletion events yielded OH concentrations of 3x105 molecule cm-3, significantly above zero indicating a significant secondary source of OH. Simulations using a box model tend to agree well with observations at low NO concentrations, but tended to under estimate OH at high NO concentrations. An intercomparison of OH values from the CIMS with OH values from a LIF technique (ATHOS) aboard the aircraft will be presented.