Nitrogen dioxide, peroxynitrates, and the chemistry of tropospheric ozone production: New insights from in situ measurements

Nitrogen dioxide and its higher oxides (HNO₄, HNO₃) play a central role in tropospheric chemistry, regulating the production rates of ozone and controlling the chain length of the OH/HO₂ catalytic cycle. Conversion of NO₂ to long lived reservoirs such as peroxy(acyl)nitrates (RC(O)OONO₂) allows for the transport of NO_x (NO _x ≡ NO + NO₂) from source regions to the global atmosphere. This dissertation describes the design, development, and deployment of a time-gated laser-induced fluorescence (LIF) instrument capable of accurate (5%), precise (<5%), high time resolution (<1 Hz) measurements of ambient NO₂ at parts per trillion mixing ratios from the ground and from aircraft based platforms. The deployment of a two-channel version of this NO₂ LIF instrument fitted with heated inlets (TD-LIF) to measure ambient NO₂ together with the sum total concentration of peroxynitrates is also described. Instrument performance is demonstrated with in-situ measurements from three field campaigns: (1) Blodgett Forest Reasearch Station (August–October 1998), (2) 1999 Southern Oxidants Study (June 15–July 15, 1999), (3) Tropospheric Ozone Production about the Spring Equinox (January–May, 2000). Results from comparisons of the LIF NO₂ and total peroxynitrate measurements with independent techniques during these campaigns are presented, and suggest that the LIF and TD-LIF instruments provide substantial improvements to the accuracy, precision, and time resolution over existing nitrogen oxide measurement techniques.; These measurements are used together with models to investigate the chemical and physical processes controlling O₃ concentrations in the urban, rural, and remote troposphere. In urban and rural areas, the observations illustrate, for the first time, the transition of instantaneous O₃ production rates in an urban environment from NO_x-limited to NO _x-saturated regimes, and the dependence of O₃ production rates on both NO_x abundances and HO_x (HO_x ≡ OH + HO₂ + RO₂) radical production rates in a high NO _x and high hydrocarbon regime. An analysis along chemical coordinates suggests that observations and models of O₃ production in urban and rural environments with significant oxidation of biogenic hydrocarbons can be forced into agreement if the reaction rate or product yield for the reaction RO₂ + HO₂ → ROOH (R = organic moiety) is reduced by a factor of 10, implying that O₃ production in such environments is NO_x-saturated more often than current models would predict, and therefore, that current urban-regional O₃ control strategies may require more stringent reductions in NO_x emissions to be effective. In the remote atmosphere, measurements of total peroxynitrates provide evidence for the importance of HNO₄ in the nitrogen oxide budget and constrain the photochemistry of this species. Correlated increases in total peroxynitrate and in O₃ concentrations observed across the winter-spring transition, but not reproduced by global 3-D chemistry-transport models, suggests that the combination of mid-latitude springtime photochemistry and the transport of nitrogen oxides and O₃ from mid-latitudes to the poles is not accurately represented in current models.