Bidyut K. Sen

Precision requirements for space-based XCO 2 data

Precision requirements are determined for space-based column-averaged CO_2 dry air mole fraction (X_(CO)_2) data. These requirements result from an assessment of spatial and temporal gradients in (X_(CO)_2) the relationship between (X_(CO)_2) precision and surface CO_2 flux uncertainties inferred from inversions of the (X_(CO)_2) data, and the effects of (X_(CO)_2) biases on the fidelity of CO_2 flux inversions. Observational system simulation experiments and synthesis inversion modeling demonstrate that the Orbiting Carbon Observatory mission design and sampling strategy provide the means to achieve these (X_(CO)_2) data precision requirements.

Measurements of reactive nitrogen in the stratosphere

We present volume mixing ratio profiles of NO, NO2, HNO3, HNO4, N2O5, and ClNO3 and their composite budget (NOy), from 20 to 39 km, measured remotely in solar occultation by the Jet Propulsion Laboratory MkIV Interferometer during a balloon flight from Fort Sumner, New Mexico (35°N), on September 25, 1993. In general, observed profiles agree well with values calculated using a photochemical steady state model constrained by simultaneous MkIV observations of long-lived precursors and aerosol surface area from the Stratospheric Aerosol and Gas Experiment II. The measured variation of concentrations of NOx (= NO + NO2) and N2O5 between sunrise and sunset reveals the expected ∼2:1 stoichiometry at all altitudes. Despite relatively good agreement between theory and observation for profiles of NO and HNO3 the observed concentration of NO2 becomes progressively higher than model values below 30 km, with the discrepancy reaching ∼30% at 22 km. This suggests an incomplete understanding of factors that regulate the NO/NO2 and NO2/HNO3 ratios below 30 km. Observations obtained during September 1990, prior to the June 1991 eruption of Mount Pinatubo, as well as during April 1993 and September 1993 provide a test of our understanding of the affect of aerosol surface area on the NOx/NOy ratio at midlatitudes. The observations reveal a decrease in the NOx/NOy ratio for increasing aerosol surface area that is consistent with the heterogeneous hydrolysis of N2O5 being the dominant sink of between altitudes of 18 and 24 km for the conditions encountered (e.g., surface areas as high as 14 μm2 cm−3 and temperatures from 209 to 219 K).

Ground‐based observations of Arctic O3 loss during spring and summer 1997

Ground-based solar absorption spectra were measured from Fairbanks, Alaska (65°N, 148°W) from March to September 1997 by the Jet Propulsion Laboratory (JPL) MkIV Fourier transform infrared (FTIR) spectrometer. The derived column abundances of 03 declined by 35% over this period (20% in April and May, and 15% during the summer), whereas those of HF, a long-lived tracer, changed by less than 5%. High-latitude, summertime balloon observations reveal similar shapes for the volume mixing ratio profiles of O 3 and HF in the lower stratosphere, where most of their column abundance resides. Vertical transport should therefore have similar effects on the column abundances of O 3 and HF. Data from the Halogen Occultation Experiment (HALOE) show a poleward decrease in the O 3 /HF ratio at all stratospheric altitudes, so that any reductions in column O 3 due to horizontal meridional transport would have been accompanied by even larger reductions in column HF. Therefore the observed column O 3 decrease must be the result of chemical loss processes. Column measurements of other atmospheric gases show a summertime maximum in the NO x /NO y column ratio and little change in the chlorine partitioning. We conclude that most of the reduction in column O 3 over Fairbanks from March to September 1997 was likely driven by NO x chemistry. These conclusions are supported by the similar behavior of column abundances measured by another ground-based FTIR spectrometer based in Ny Alesund, Spitsbergen, (79°N, 12°E).