No abstract

Understanding the processes that control the terrestrial exchange of carbon is critical for assessing atmospheric CO2 budgets. Carbonyl sulfide (COS) is taken up by vegetation during photosynthesis following a pathway that mirrors CO2 but has a small or nonexistent emission component, providing a possible tracer for gross primary production. Field measurements of COS and CO2 mixing ratios were made in forest, senescent grassland, and riparian ecosystems using a laser absorption spectrometer installed in a mobile trailer. Measurements of leaf fluxes with a branch‐bag gas‐exchange system were made across species from 10 genera of trees, and soil fluxes were measured with a flow‐through chamber. These data show (1) the existence of a narrow normalized daytime uptake ratio of COS to CO2 across vascular plant species of 1.7, providing critical information for the application of COS to estimate photosynthetic CO2 fluxes and (2) a temperature‐dependent normalized uptake ratio of COS to CO2 from soils. Significant nighttime uptake of COS was observed in broad‐leafed species and revealed active stomatal opening prior to sunrise. Continuous high‐resolution joint measurements of COS and CO2 concentrations in the boundary layer are used here alongside the flux measurements to partition the influence that leaf and soil fluxes and entrainment of air from above have on the surface carbon budget. The results provide a number of critical constraints on the processes that control surface COS exchange, which can be used to diagnose the robustness of global models that are beginning to use COS to constrain terrestrial carbon exchange.

No abstract

Nitrate radical (NO(3)), an important nighttime tropospheric oxidant, was measured continuously for two years (July 2005 to September 2007) in Jerusalem, a semiarid urban site, by long-path differential optical absorption spectroscopy (LP-DOAS). From this period, 21 days with the highest concentrations of nitrate radical (above 220 pptv) were selected for analysis. Joint measurements with the University of Heidelberg's LP-DOAS showed good agreement (r = 0.94). For all daytime measurements, NO(3) remained below the detection limit (8.5 pptv). The highest value recorded was more than 800 pptv (July 27, 2007), twice the maximum level reported previously. For this subset of measurements, mean maximum values for the extreme events were 345 pptv (SD = 135 pptv). Concentrations rose above detection limits at sunset, peaked between midnight and early morning, and returned to zero at sunrise. These elevated concentrations of NO(3) were a consequence of several factors, including an increase in ozone concentrations parallel to a substantial decrease in relative humidity during the night; Mean nighttime NO(2) levels above 10 ppbv, which prevented a deficiency in NO(3) precursors; Negligible NO levels during the night; and a substantial decrease in the loss processes, which led to a lower degradation frequency and allowed NO(3) lifetimes to build up to a maximum mean of 25 min. The results indicate that the major sink pathway for NO(3) was direct homogeneous gas phase reactions with VOC, and a smaller indirect pathway via hydrolysis of N(2)O(5). The Jerusalem measurements were used to estimate the oxidation potential of extreme NO(3) levels at an urban location. The 24 h average potential of NO(3), OH, and O(3) to oxidize hydrocarbons was evaluated for 30 separate VOCs. NO(3) was found to be responsible for approximately 70% of the oxidation of total VOCs and nearly 75% of the olefinic VOCs; which was more than twice the VOC oxidation potential of the OH radical. These results establish the NO(3) radical as an important atmospheric oxidant in Jerusalem.