J. Bradshaw

Ozone, hydroperoxides, oxides of nitrogen, and hydrocarbon budgets in the marine boundary layer over the South Atlantic

Author(s): Heikes, B; Lee, M; Jacob, D; Talbot, R; Bradshaw, J; Singh, H; Blake, D; Anderson, B; Fuelberg, H; Thompson, AM | Abstract: The NASA GTE TRACE A mission sampled air over the South Atlantic and western Indian Oceans. Thirteen flight legs were flown within the marine boundary layer (MBL). The MBL was typically the cleanest air sampled (e.g., CH4 l 1680 ppb, CO l 70 ppb, C2H6 l 400 ppt, C3H8 l 40 ppt, NOx l 15 ppt, and midday NO l 5 ppt) but was overlain by polluted air. The photochemistry of the MBL was influenced by oceanic emissions, surface deposition, and entrainment of pollutants from aloft. Chemical budgets were constructed for several species in the MBL in order to investigate these effects and are presented for ethane, ethylene, propane, propylene, n-butane, formic acid (HFo), methylhydroperoxide (CH3OOH), oxides of nitrogen (i.e., NO, NO2, PAN, HNO3), hydrogen peroxide (H2O2), and ozone (O3). A photochemical point model was used to evaluate local chemical production and loss. An entrainment model was used to assess material exchange between the lower free troposphere (FT) and the MBL and a resistance deposition model was used to evaluate material exchange across the air-sea interface. The results suggested the ocean to be the source of measured alkenes in the MBL and to be the most likely source of the shorter-lived alkanes: propane and n-butane. Ethane was the only hydrocarbon for which input from aloft may have exceeded its photochemical destruction. The estimated hydrocarbon sources from the ocean were in agreement with prior analyses. Transport from the lower FT together with surface loss could not account for measured concentrations of CH2O, HFo, and HNO3. The transport of peroxyacetylnitrate (PAN) from the FT to the MBL exceeded the rate of HNO3 production and was more than sufficient to maintain observed NOx levels without having to invoke an oceanic source for NO. The flux of NOx, PAN, and HNO3 was in balance with the surface deposition flux of HNO3. However, the predicted rates of HNO3 formation from the oxidation of NO2 and HNO3 entrainment from aloft were inadequate to maintain observed levels of HNO3 unless HNO3 was partitioned between the gas phase and a more slowly depositing aerosol phase. The estimated dry deposition flux of HNO3 to the South Atlantic during TRACE A, 2-4 × 109 molecules cm-2 s-1, was about 10 times the annual average estimate for this region. The destruction of O3 within the MBL was found to be exceeded by transport into the MBL from aloft, 6 ±2 × 1010 compared to 11 ± 10 × 1010 molecules cm-2 s-1. The principal O3 destruction process was mediated by the formation and surface deposition of H2O2 and CH3OOH, 4 ± 4 × 1010 and 1.1 ± 0.5 × 1010 molecules cm-2 s-1. The direct loss of O3 to the sea surface was estimated to be 1.7 ± 0.2 × 1010 molecules cm-2 s-1. CH3OOH was lost to the sea and transported into the FT from the MBL. Its first-order loss rate was estimated to be 7 × 10-6 s-1 for a mean MBL height of 700 m. H2O2 and CH2O losses from the MBL were estimated at rates of 1.3 × 10-5 s-1 for both species. The inclusion of surface deposition improved the agreement between predicted and measured concentrations of HNO3, CH3OOH, H2O2, and CH2O. However, model CH2O remained significantly greater than that measured in the MBL.

Hydrogen peroxide and methylhydroperoxide distributions related to ozone and odd hydrogen over the North Pacific in the fall of 1991

Hydrogen peroxide and methylhydroperoxide were measured in the troposphere over the western North Pacific as part of the airborne portion of NASAs Global Tropospheric Experiment/Pacific Exploratory Mission-West A field mission. The flights circled the North Pacific, focusing on the western Pacific, and extended from 300 to 13,000 m altitude. The hydroperoxides were uniquely separated and quantified using a high-pressure liquid chromatography system in conjunction with a continuous enzyme fluorometric instrument. Results show a latitudinal gradient in both peroxides at all altitudes; for example, between 3 and 5 km, H2O2 median values decrease from 1700 to 500 parts per trillion by volume (pptv) in going from 0°–15°N to 45°–60°N, and the corresponding decrease in CH3OOH was 1100 to 200 pptv. Concentration maxima are observed in both species at altitudes of 2 to 3 km with H2O2 concentrations below 1 km lower by 30%, 10% for CH3OOH, and even lower, by a factor of 10, for both above 9 km. The H2O2 to CH3OOH ratio increased with altitude and latitude with ratios 2 at midlatitude high altitude. Highest peroxide concentrations were encountered over the Celebes Sea in air which was impacted by aged biomass fire and urban pollutants. CH3OOH was below the level of detection in stratospheric air. H2O2 exceeded SO2 95% of the time, with the exceptions generally above 9 km. Above 3 km, O3 increases with decreasing H2O2 and CH3OOH. Below 3 km the O3-CH3OOH trend is the same but O3 increases with increasing H2O2. The measurements are compared with predictions based upon a photochemical steady state zero-dimensional model and a three-dimensional mesoscale time-dependent model. These models capture observed trends in H2O2 and CH3OOH, with the possible exception of H2O2 below 2 km where surface removal is important. A surface removal lifetime of 3.5 days brings the observed and zero-dimensional model-estimated H2O2 into agreement. The steady state model suggests a strong correlation between the ratios of NO/CO or HO2/HO and the ratio of H2O2/CH3OOH. The observed hydroperoxide ratios bracket the modeled relationship with occasionally much lower H2O2 than expected.

Trace chemical measurements from the northern midlatitude lowermost stratosphere in early spring: Distributions, correlations, and fate

In situ measurements of a large number of trace chemicals from the midlatitude (37–57°N) lower stratosphere were performed with the NASA DC-8 aircraft during March 1994. Deepest penetrations into the stratosphere (550 ppb O3, 279 ppb N2O, and 350 K potential temperature) corresponded to a region that has been defined as the “lowermost stratosphere” (LS) by Holton et al. [1995]. Analysis of data shows that the mixing ratios of long-lived tracer species (e. g. CH4, HNO3, NOy, CFCs) are linearly correlated with those of O3 and N2O. A ΔNOy/ΔO3 of 0.0054 ppb/ppb and ΔNOy/ΔN2O of −0.081 ppb/ppb is in good agreement with other reported measurements from the DC-8. These slopes are however, somewhat steeper than those reported from the ER-2 airborne studies. We find that the reactive nitrogen budget in the LS is largely balanced with HNO3 accounting for 80% of NOy, and PAN and NOx together accounting for 5%. A number of oxygenated species (e. g. acetone, H2O2) were present and may provide an important in situ source of HOx in the LS. SO2 mixing ratios were found to increase in the stratosphere at a rate that was comparable to the decline in OCS levels. No evidence of particle formation could be observed. Ethane, propane, and acetylene mixing ratios declined rapidly in the LS with Cl atoms likely playing a key role in this process. A number of reactive hydrocarbons/halocarbons (e. g. C6H6, CH3I) were present at low but measurable concentrations.

Photofragmentation two‐photon laser‐induced fluorescence detection of NO2 and NO: Comparison of measurements with model results based on airborne observations during PEM‐Tropics A

Measurements of NO and NO2 are reported using a highly modified photofragmentation two-photon laser-induced fluorescence (PF-TP-LIF) instrument. Available evidence suggests that the changes made substantially reduced wall decomposition of labile NOy species. In sharp contrast to the results reported from NASAs 1991 PEM-West A program where the median value for the ratio (NO2)meas/(NO2)calc was 3.36, the PEM-Tropics A observations produced a value for this ratio of 0.93. This represents the first time that remote upper tropospheric NO-NO2 data have shown a high degree of correspondence with current photochemical mechanisms.

Tropospheric reactive odd nitrogen over the South Pacific in austral springtime

The distribution of reactive nitrogen species over the South Pacific during austral springtime appears to be dominated by biomass burning emissions and possibly lightning and stratospheric inputs. The absence of robust correlations of reactive nitrogen species with source-specific tracers (e.g., C2H2 [combustion], CH3Cl [biomass burning], C2Cl4 [industrial], 210Pb [continental], and 7Be [stratospheric]) suggests significant aging and processing of the sampled air parcels due to losses by surface deposition, OH attack, and dilution processes. Classification of the air parcels based on CO enhancements indicates that the greatest influence was found in plumes at 3–8 km altitude in the distributions of HNO3 and peroxyacetyl nitrate (PAN). Here mixing ratios of these species reached 600 parts per trillion by volume (pptv), values surprisingly large for a location several thousand kilometers removed from the nearest continental areas. The mixing ratio of total reactive nitrogen (the NOy sum), operationally defined in this paper as measured (NO + HNO3 + PAN + CH3ONO2 + C2H5ONO2) + modeled (NO2), had a median value of 285 pptv within these plumes compared with 120 pptv in nonplume air parcels. Particle NO−3 was not included in this analysis of the NOy sum due to its 10- to 15-min sampling time resolution, but, in general, it was <10% of the NOy sum. Comparison of the two air parcel classifications for NOy and alkyl nitrate distributions showed no perceivable plume influence, but recycling of reactive nitrogen may have masked this direct effect. In the marine boundary layer, the NOy sum averaged 50 pptv in both air parcel classifications, being somewhat isolated from the polluted conditions above it by the trade wind inversion. In this region, however, alkyl nitrates appear to have an important marine source where they comprise 20–80% of the NOy sum in equatorial and high-latitude regions over the South Pacific.

Single photon laser-induced fluorescence detection of NO and SO 2 for atmospheric conditions of composition and pressure

Reported here are laboratory results from a laser-induced fluorescence (LIF) study of the molecules NO and SO(2) in which both the selectivity and sensitivity of the LIF method are examined. The laser excitation of these molecules occurred at 226 and 222 nm, respectively. The laser system employed consisted of a Nd: YAG-driven Quanta-Ray PDL dye laser, the fundamental of which was frequency doubled, and this output, in turn, was then frequency mixed with the Nd:YAG fundamental at 1064 nm. Two different dyes were required for generating the 226- and 222-nm wavelengths. To make these results as relevant as possible to the ultimate development of an atmospheric airborne field sampling system all experiments were carried out in atmospheric conditions of pressure and composition. In addition to the experimental data provided there has also been presented a theoretical assessment of the signal strength for both the NO and SO(2) LIF systems, and these results have been compared with the experimentally measured values. Current state-of-the-art technology would suggest that both NO and SO(2) can be measured by the LIF technique in atmospheric conditions at concentration levels of a few pptv.

Atmospheric ammonia measurement using a VUV/photo-fragmentation laser-induced fluorescence technique

Vacuum ultraviolet/photofragmentation laser-induced fluorescence has been demonstrated to be a highly specific and sensitive method for the quantitative measurement of atmospheric ammonia (NH(3)). The fluorescence detected in this approach results from the two 193-nm photon photofragmentation step NH(3)?NH(2)? NH(b(1)Sigma(+)) followed by the excitation of the NH(b(1)Sigma(+)) NH(c(1)Pi) transition via a 450-nm photon with final emission being observed from the NH(c(1) Pi) NH(a(1)Delta) transition at 325 nm. Limits of detection for the instrumentpresented here are < 10 pptv and < 4 pptv for 1- and 5-min integration periods, respectively, in ambient sampling conditions. The technique is free from interferences and system performance does not significantly degrade in adverse sampling conditions (i.e., rain, fog, clouds, haze, etc.). Spectroscopic selectivity in the NH(b(1)Sigma(+))?NH(c(1)Pi) transition is sufficient to resolve (15)NH(3) and (14)NH(3) contributions for use in atmospheric tracer studies. Average ammonia measurements at Stone Mountain, GA, ranged from approximately 110 pptv for air temperatures <5 degrees C to approximately 240 pptv for air temperatures >/=<5 degrees C over the period from Dec. 1987 to the end of Apr. 1988.

Sequential two-photon laser-induced fluorescence: a new technique for detecting hydroxyl radicals

A new approach to the detection of the chemically important transient species, the hydroxyl radical (OH), is reported. This new approach, labeled two-photon laser-induced fluorescence (TP-LIF), has now been tested in the laboratory under atmospheric conditions. The method involves combining IR laser pumping (e.g., 1.4 or 2.8 μm) with UV laser excitation (e.g., 345 or 351 nm). Theoretical predicted signal levels were found to agree very favorably with laboratory measured signal levels. Using laboratory data reported in this study, in conjunction with projected IR energy improvements, it appears that the TP-LIF OH method could be of great value both as a laboratory instrument for studying fundamental chemical processes and as an OH field detection system.

An Assessment of HOx Chemistry in the Tropical Pacific Boundary Layer: Comparison of Model Simulations with Observations Recorded during PEM Tropics A

Reported are the results from a comparison of OH,H2O2CH3OOH, and O3 observationswithmodel predictions based on current HOx–CH4reaction mechanisms. The field observations are thoserecorded during the NASA GTE field program, PEM-Tropics A. The major focus ofthis paper is on thosedata generated on the NASA P-3B aircraft during a mission flown in the marineboundary layer (MBL) nearChristmas Island, a site located in the central equatorial Pacific (i.e.,2° N, 157° W). Taking advantage of thestability of the southeastern trade-winds, an air parcel was sampled in aLagrangian mode over a significantfraction of a solar day. Analyses of these data revealed excellent agreementbetween model simulated andobserved OH. In addition, the model simulations reproduced the major featuresin the observed diurnalprofiles of H2O2 and CH3OOH. In the case ofO3, the model captured the key observational feature whichinvolved an early morning maximum. An examination of the MBL HOxbudget indicated that the O(1D) + H2Oreaction is the major source of HOx while the major sinks involveboth physical and chemical processes involving the peroxide species,H2O2 and CH3OOH. Overall, the generally goodagreement between modeland observations suggests that our current understanding ofHOx–CH4 chemistry in the tropical MBL isquite good; however, there remains a need to critically examine this chemistrywhen both CH2O and HO2are added to the species measured.

VUV/photofragmentation laser-induced fluorescence sensor for the measurement of atmospheric ammonia

Vacuum-ultraviolet/photofragmentation-laser-induced fluorescence (VUV/PF-LIF) has been demonstrated to be a highly specific and sensitive method for the quantitative measurement of atmospheric ammonia (NH3). The fluorescence detected in this approach results from the two photon (193 nm) photofragmentation of NH3 followed by the LIF excitation of the NH(b1(summation)+) yields NH(c1II) (at 452 nm) and the monitoring of fluorescence from the NH(c1II) yields NH(a1(Delta) ) transition at 325 nm. Limits of detection for the instrument presented here are < 10 pptv and < 4 pptv for one and five minute integration periods, respectively, under ambient sampling conditions. The technique is free from interferences and system performance does not significantly degrade under adverse sampling conditions (i.e. rain, fog, clouds, haze, etc.). Spectroscopic selectivity in the NH(b1(summation)+) yields NH(c1II) transition is sufficient to resolve 15NH3 and 14NH3 contributions for use in atmospheric tracer studies. Average ammonia measurements at Stone Mtn., GA, range from approximately equals 110 pptv for air temperatures < 5 degree(s)C to approximately equals 240 pptv for air temperatures >= 5 degree(s)C over the period from December 1987 to the end of April 1988. Ammonia levels measured at Green Mountain Mesa, Boulder CO, ranged from 10 pptv to 10 ppbv for measurements made during March 1989. Ammonia levels were seen to vary from about 300 pptv to greater than 5 ppbv over time scales of < 10 minutes in this latter data set. These results and future instrument improvements are discussed.