Seasonal variation of atmospheric nitric acid over the South Pole in 1992

Abstract

We conducted year‐round measurements of mid infrared atmospheric emission over the South Pole in 1992. We were able to observe a large seasonal change of the total column of nitric acid (HNO3) vapor. During the summer, the HNO3 column abundance was about 2×1016 molecules/cm². There was a small increase in the fall. A rapid decrease of 50% was observed in late June, soon after the stratospheric temperature reached the threshold for formation of type I polar stratospheric clouds (PSCs). By early July, the stratospheric temperature was cold enough to form type II PSCs, and the HNO3 column decreased to 7×1015 molecules/cm². Measured nitric acid values in the spring remained extremely low even after the stratosphere warmed well above PSC temperatures. This may indicate permanent removal of HNO3 by gravitational settling, or long‐term sequestering in large particles. Normal summer values were not observed until the vortex dissipated and allowed the lateral transport of HNO3, to the south polar region.

Photochemistry of Molecules Relevant to the Atmosphere: Photodissociation of Nitric Acid in the Gas Phase

“…Before discussing the results for HNO 3 , some general remarks on the energetics involved in a photodissociation process [23,24] may be helpful. The parent molecule carries initially only a small (under jet conditions even negligible) amount of internal energy E int .…”

“…They managed to create a cold molecular beam of HNO 3 and were able to determine the recoil distributions of the four photoproducts OH, NO 2 , O and HONO formed in the primary decay. Assigning these products to an OH channel and a HONO channel gave a branching ratio of 0.6:0.4, and since neither evidence for other primary dissociation channels nor fluorescence was found, the authors assumed this ratio to correspond to f(OH) = 0.6 AE 0.1 and f(HONO) = 0.4 AE 0.1.…”

“…This isomer, detected in an argon matrix [16,17] was calculated to lie about 138 kJ mol À1 higher in energy than HNO 3 . Based on their results, the authors concluded that HOONO is an unimportant byproduct of reaction (1) and therefore unlikely to be significant in atmospheric chemistry.…”

“…[11] The IR spectra of HNO 3 and some of its isotopomers were investigated by McGraw et al [10] Later, this study was complemented by recording high-resolution Fourier transformation (FT) spectra of the bands n 3 and n 4 , [12] n 6 , n 7 and n 8 , [13] and n 9 , [14] which were analyzed in detail with respect to the rovibrational structure. These accurate determinations of line positions and intensities serve to identify and quantitatively assess HNO 3 in the atmosphere by optical methods.…”

Atmospheric near‐surface nitrate at coastal Antarctic sites

“…from the winter depletion of the total HNO 3 column reported byVan Allen et al [1995] for south pole. Note that unlike the air mass exchange inputs, which are site specific since they are essentially based on mBe/7Be ground level observations, the PSC-related inputs are taken as distributed homogeneously over the Antarctic continent.In view of the crude estimates and the large Seasonal pattern of the arrival of stratospheric aerosol tracers at Neumayer Station, indicated by enhanced •øBe/7Be(1983)(1984)(1985) or 7Be/2•øpb (1983-1993) ratios.…”