PARTITIONING AND BUDGET OF NOY SPECIES DURING THE MAUNA-LOA-OBSERVATORY PHOTOCHEMISTRY EXPERIMENT

During the Mauna Loa Observatory Photochemistry Experiment (MLOPEX), measurements were made of total odd nitrogen (NO(y)) and the known individual daytime odd-nitrogen species. The individual species measured were NO, NO2, HNO3, particulate NO3-, peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), methyl nitrate, and >C3 alkyl nitrates. The most abundant component of NO(y) was nitric acid; its median contribution to NO(y) in free tropospheric samples was 43%. The large fraction of HNO3 is consistent with the long transport times and photochemical processing of air masses reaching the mid-Pacific site as well as possible stratospheric input of NO(y). The median contribution of NO(x) to NO(y) in the free troposphere near 3.4 km was almost-equal-to 14%. PAN and other measured organic nitrates contributed < 7% to NO(y). The median sum of the individually measured species was 102% of NO(y) in upslope periods which consist of a mixture of island-modified marine boundary layer and free tropospheric air. This total was 75% of NO(y) during downslope periods representative of the free troposphere. This shortfall in the odd-nitrogen budget in die free troposphere corresponds to 72 pptv of reactive nitrogen, which is over 2 times median NO(x). The NO(y) shortfall and the composition of NO(y) appeared to have a regular variation in the free troposphere during the experiment which was related to air mass origin, recycling of odd nitrogen, and loss processes during transport. The presence of an odd-nitrogen deficit in the remote free troposphere suggests that our understanding of the NO(y) system is incomplete. Unidentified odd-nitrogen species, such as organic nitrates, may be present, but sampling limitations and analytical uncertainties in NO(y) and individual (NO(y))i measurements still restrict our ability to accurately define an NO(y) budget, especially in remote regions.