An intercomparison has been conducted among five instruments which measure gas phase ammonia at low concentration in the atmosphere: (1) a photofragmentation/ laser-induced fluorescence (PF/LIF) instrument; (2) a molybdenum oxide annular denuder sampling/ chemiluminescence detection (MOADS)technique; (3) a tungsten oxide denuder sampling/chemiluminescence detection (DARE) system; (4) a citric acid coated denuder sampling/ion chromatographic analysis (CAD/IC) method; and (5)an oxalic acid coated filter pack sampling/colorimetric analysis (FP/COL) method. Mixtures of ammonia in air at flow rates of 1800 (STP) L/min and concentrations from 0.1 to 14 parts per billion by volume (ppbv) with the addition of possible interferants (CH3NH2, CH3CN, NO, NO2, 03, and H20) were provided for simultaneous tests. In addition, the five instruments made simultaneous ambient air measurements both from a common manifold and from their separate inlets located at a common height above the ground. Several conclusions were reached: (1) No artifacts or interferences were conclusively established for any of the techniques, although CH3NH2 may interfere in the MOADS system. (2) Measurements from the PF/LIF and the CAD/IC methods agreed well with the prepared mixtures over the full range of ammonia concentrations. The high specificity and t/me resolution (1 min) of the PF/LIF results allowed data from this technique to be used as a basis set for comparisons. (3) For fog-free conditions, ambient measurements from all of the instruments generally agreed to within a factor of 2 for ammonia levels above 0.5 ppbv. The CAD/IC and PF/L1F instruments agreed to within 15% on average for all ambient data. (4) The slope from linear regression analysis of separate inlet ambient air measurements indicated that the DARE data agreed with those from the PF/LIF system to within 7%. The linear regression line intercept was 216 parts per trillion by volume (pptv), which may indicate a positive interference in the DARE data, but the DARE data were closer to the PF/LIF data (50 -100 pptv higher) at the lowest ambient ammonia levels. (5) The FP/COL method measured about 66% of the ammonia as determined by the PF/LIF technique and measured even lower fractional levels in the prepared samples. These low values indicate a loss of ammonia, possibly on the Teflon prefilter, under the conditions of this study (cold temperatures Paper number 92JD00721. 0148-0227/92/92JD-00721 $05.00 and generally low relative humidity). (6) The MOADS ambient air data were about 64% of the ammonia that was observed by the PF/LIF method for levels greater than about 1 ppbv, and below this level it overestimated ammonia. Problems with the MOADS calibrations and inlets may have been responsible. (7) Ambient air data from the one period of fog formation indicated that ambient ammonia was primarily partitioned into the condensed phase, leaving the interstitial air greatly depleted. Volatilization of absorbed ammonia from water droplets entrained in the sampled air appeared to influence the M...

Measurements of biomass burn‐produced trace gases are presented that were obtained using a helicopter at low altitudes above burning Florida wetlands on November 9, 1987, and from both helicopter and light‐aircraft samplings on November 7, 1988. Carbon dioxide (CO2) normalized emission ratios (ΔX/ΔCO2; V/V; where X is trace gas) for carbon monoxide (CO), hydrogen (H2), methane (CH4), total nonmethane hydrocarbons (TNMHC), and nitrous oxide (N2O) were obtained over burning graminoid wetlands consisting primarily of Spartina bakeri and Juncus roemerianus. Some interspersed scrub oak (Quercus spp) and saw palmetto (Screnoa repens) were also burned. No significant differences were observed in the emission ratios determined for these gases from samples collected over flaming, mixed, and smoldering phases of combustion during the 1987 fire. Combustion‐categorized differences in emission ratios were small for the 1988 fire. Combustion efficiency was relatively good (low emission ratios for reduced gases) for both fires. We believe that the consistently low emission ratios were a unique result of graminoid wetlands fires, in which the grasses and rushes (both small‐size fuels) burned rapidly down to standing water and were quickly extinguished. Consequently, the efficiency of the combustion was good and the amount and duration of smoldering combustion was greatly diminished.

Aircraft measurements of peroxyacetyl nitrate (PAN) and other important reactive nitrogen species (NO, NO2, HNO3, and NOy) were performed over the continental United States and the eastern Pacific during August–September 1986 at all altitudes between O and 6 km as part of CITE 2. PAN measurements were conducted by two independent groups, allowing both intercomparisons and greater confidence in its observed atmospheric structure. PAN was found to be a dominant reactive nitrogen species in the troposphere with 98% of the mixing ratios falling in a range of 5–400 ppt. Typically, the highest mixing ratios (100–300 ppt) were observed aloft (4–6 km) with extremely low values (5–20 ppt) in the marine boundary layer. In the lower troposphere, continental air contained significantly more PAN than marine air. The vertical structure of PAN was largely dictated by its thermal destruction rate and equilibrium with available NO2. PAN mixing ratios showed a high degree of variability in both continental and marine atmospheres. Westerly marine air trajectories did not guarantee well‐mixed air of uniform composition. Mixing ratios of O3, NOy, NOx, HNO3, C2H6, CO, and CFCl3 were strongly correlated with those of PAN, indicating the important role played by transport processes. High PAN to NOx ratios in the mid‐troposphere further support the importance of long‐range transport from continental sources. Frequently, descending air masses from the upper troposphere suggested that PAN mixing ratios probably continued to increase above the 6‐km ceiling altitude. Air masses with O3 <20 ppb, CO <60 ppb, and C2H6 <500 ppt contained only miniscule amounts of PAN and are expected to be of tropical origin. Reasons for the observed PAN variability are discussed.

The first simultaneous measurements of ammonia and nitric acid in the troposphere have been made from an aircraft using a tungsten oxide denuder system. Vertical profiles of NH3 and HNO3 taken over coastal Virginia and Maryland in March and September, 1983, at altitudes from 150 m to 3000 m, show mixing ratios that decrease with altitude. Ammonia profiles show substantial seasonal variation, while nitric acid profiles do not. Using the measured profiles, we calculate, with a one‐dimensional photochemical model, life‐times due to heterogeneous loss of 1 day for HNO3 and 10 days for NH3. In contrast, NH3 profiles up to 5300 m over the North Atlantic ocean during August, 1982 show mixing ratios that increase slightly with altitude. These data represent the first ammonia profiles measured over the ocean. We suggest that the increase in NH3 with altitude is a result of an ammonia‐rich continental air mass advected over the ocean followed by the dissolution of NH3 in the marine boundary layer on water‐covered sea salt particles.

Nitric acid concentrations, measured by both teflon/nylon filter packs (FP) and the tungstic oxide denuder (DEN), are compared with the average NOx concentrations from laserinduced fluorescent and chemiluminescent methods. The HNO3/NOx ratio based on filter packs ranged from 0.8 to 10.4, with a mean of 3.4. The DEN nitric acid concentrations produced ratios ranging from <0.3 to 9.8, with a mean of 2.6. Average marine ratios were larger than those from continental regions, in part due to continental anthropogenic sources of NOx. Although we collected very few boundary layer samples, their average ratios were smaller than those in the free troposphere, apparently because of the effect of dry surface removal of nitric acid. The nitric acid to NOx ratio was greatest when the NOx/NOy ratio was smallest, such that the nitrogen photochemistry was nearing completion.