UC Berkeley

Nitrous oxide (N₂O) and molecular nitrogen (N₂) isotopic compositions and aerosol optical properties were investigated through experiments and observations to elucidate their roles in atmospheric radiative transfer and chemistry. In Earth's atmosphere, the isotopic composition of N₂O, a potent greenhouse gas, is a useful tool for investigating its sources and sinks. N₂ is the main component of the atmospheres of Earth and Titan, and isotope effects in its photoionization may lead to isotopic fractionation. The optical properties of aerosols, a component of most planetary atmospheres, determine how they affect radiative transfer. A polarimeter was developed to measure aerosol optical properties in situ in an existing apparatus.

Three sets of measurements of N₂O isotopic composition provide new insight into its budget. First, a time-series from 1940 to 2005 from firn and archived air samples is consistent with the observed N₂O increase being largely due to isotopically light N₂O emissions from agriculture and reveals seasonal cycles due to the seasonally-varying influences of multiple N₂O sources and stratosphere-troposphere exchange. Second, measurements from the tropical free troposphere show coherent vertical variations in N₂O isotopic compositions consistent with the persistent influence of a regional surface source, most likely the ocean. Third, samples from the marine boundary layer with anomalously high N₂O mixing ratios and perturbed isotopic compositions were used to deduce a source isotopic composition that is perhaps representative of N₂O emitted from the South Atlantic Ocean.

Isotope effects in the non-dissociative photoionization of N₂ -- investigated by measuring the photoionization efficiency spectrum for its three isotopologues -- clarify peak identities and show that these previously ignored isotope effects may be important in planetary atmospheres. The shifts in peak energy due to isotopic substitution show that the controversial peak at 15.68 eV for ¹⁴N₂ is most likely due to a Rydberg state converging to the v'=2 level of the A²π_u N₂⁺ state. A model of Titan's atmosphere shows that isotopic self-shielding in ¹⁴N₂ photoionization may cause isotopic fractionation between N₂ and other N-bearing molecules distinct from that caused by N₂ photodissociation, providing a possible mechanism for determining the relative importance of ion versus neutral photochemistry.