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bstractThe formation of nitrous acid (HONO) in the dark from initial concentrations of N0 2 of 0.1-20 ppm in air, and the concurrent disappearance of N0 2 , were monitored quantitatively by UV differential optical absorption spectroscopy in two different environmental chambers of ca.4300-and 5800-L volume (both with surface/volume ratios of 3.4 m' 1). In these environmental chambers the initial HONO formation rate was first order in the N0 2 concentration and increased with the water vapor concentration. However, the BONO formation rate was independent of the NO concentration and relatively insensitive to temperature. The initial pseudo-first-Qrder consumption rate of N0 2 was (2.8 :!: 1.2) x 10-4 min-1 in the 5800-L Tefion-coated evacuable chamber and (1.6 ± 0.5) x 10-4 min-1 in a 4300-L all·Teflon reaction chamber at ca.300 K and ca.50% RH. The initial HONO yields were ca.40-50% of the N0 2 reacted in the evacuable chamber and ca. 10-30% in the all-Tefton chamber. Nitric oxide formation was observed during the later stages of the reaction in the evacuable chamber, but ca.50% of the nitrogen could not be accounted for, and gas phase HN0 3 was not detected. The implications of these data concerning radical sources in environmental chamber irradiations of NO xorganic-air mixtures, and of HONO formation in polluted atmospheres. are discussed.

The nitrate radical, NO3, has been identified and measured for the first time in the polluted troposphere using long path (970 and 750 m) differential optical absorption spectroscopy at two sites in the Los Angeles basin. NO3 concentrations of up to 355 ppt were measured using the strong NO3 absorption bands at 623 and 662 nm. During pollution episodes from September 11 to September 19, 1979 concentrations increased sharply after sunset and peaked about one hour later at ∼ 20:00 (PDT). In many other cases peak concentrations were much lower and sometimes below the detection limit of several ppt. Possible sinks for the NO3 radical under polluted conditions are considered, including reaction with NO, reaction with organic species, and the hydrolysis of N2O5 for which a new upper limit rate constant is derived.

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Absolute rate constants determined by using the flash photolysis-resonance fluorescence technique are reported for the reactions of hydroxyl radicals with isoprene, a-and /3-pinene, methyl vinyl ketone, and methacrolein in the temperature range 297-424 K, and with methylglyoxal at 297 K. These results contribute to a more quantitative understanding of the tropospheric fate of gas-phase biomass-related organics and serve as input to models of the chemistry of the natural troposphere.

An experimental and modeling study of an ambient RO* (HO + H02 + RO + R02) detector Is presented. As described previously, the detector utilizes chemical amplification of the radical concentration through a chain reaction Involving NO and CO to produce N02. Modifications reported here overcome a PAN and PNA Interference to produce a detector that Is a factor of ~25 less sensitive to these Interferences than the conventional design, has Improved rejection of artifact signals, and can have variable-length Inlets without requiring a controlled nitrogen flow. A model of the chemical amplifier chemistry, which Includes chemical and wall loss of radicals, showed that simplified calculations greatly overestimate the chain length. The variation of the chain length with wall loss rates, radical concentration, reaction time, and radical type has been Investigated. The wide variation In reported chain lengths has been attributed to a chain length dependence on radical concentration and Inadequacies associated with one of the calibration techniques. Absolute radical calibration of the Instrument was performed by using the thermal decomposition of PAN as a source of known concentrations of peroxyacetyl radicals. Ambient measurements over a 4-day period show a diurnal variation of RO* radical concentrations and that daytime maximum concentrations of 3 pptv are readily discernible.

Abstract. Air samples for nonmethane hydrocarbon (NMHC) analysis were collected at two ground-based sites: Alert, Northwest Territories (82.5øN, 62.3øW) and Narwhal ice camp, an ice floe 140 km northwest of Alert, from Julian days 90 to 117, 1994, and on a 2-day aerial survey conducted on Julian days 89 and 90, 1994 over the Arctic archipelago. Several ozone depletion events and concurrent decreases in hydrocarbon concentrations relative to their background levels were observed at Alert and Narwhal ice camp. At Nap•hal, a long period (>-7 days) of ozone depletion was observed during which a clear decay of alkane concentration occurred. A kinetic analysis led to a calculated C1 atom concentration of 4.5 x 103 cm -3 during this period. Several low-ozone periods concurrent with NMHC concentration decreases were observed over a widespread region of the Arctic region (82ø-85øN, and 51ø-65øW). Hydrocarbon measurements during the aerial survey indicated that the low concentrations of these species occurred only in the boundary layer. In all ozone depletion periods, concentration changes of alkanes and toluene were consistent with C1 atom reactions. The changes in ethyne concentration from its background level were in excess of those expected from C1 atom kinetics alone and are attributed to additional Br atom reactions. A box modeling exercise suggested that the C1 and particularly Br atom concentrations required to explain the hydrocarbon behavior are also sufficient to destroy ozone. IntroductionThe depletion of O3 in the boundary layer at the onset of 24 hour daylight in the spring has been reported for several locations in the high Arctic such as Alert ( Hence the relative rates of removal of hydrocarbons can be used as an indicator for the dominance of particular atmospheric photochemical reactions such as those initiated by HO radicals or those started by C1 or Br atoms. 13,169