D. D. Montzka

Measurements and model simulations of the photostationary state during the Mauna Loa Observatory Photochemistry Experiment: Implications for radical concentrations and ozone production and loss rates

Simultaneous measurements of [NO2], [NO], [O3], and the NO2 photo-dissociation rate coefficient, J2, were made during a one-month field study in the spring of 1988 at Mauna Loa, Hawaii, and were used to evaluate the photostationary state ratio, ϕ = J2[NO2]/k1[NO][O3]. Over 5600 measurements were made for clear sky conditions, allowing a detailed comparison with photochemical theory. Values of ϕ determined from the observations were consistently higher than unity, approaching 2.0 for high sun, and indicated peroxy radical mixing ratios near 60 pptv. High sun values of ϕ were independent of NOx (NO + NO2), but correlated well with ozone and water vapor through the expression ϕ−1 = (0.11 ± 0.21) + (1.59 ± 0.64) × 10−3 × ([H2O]/[O3])½. A photochemical box model is shown to give good agreement with the values of ϕ, the peroxy radical concentrations, and the correlations with physical and chemical environmental variables determined from the observations. The rate of photochemical production of ozone was estimated from measurements of ϕ, and the rate of photochemical ozone destruction was estimated from the box model. For free tropospheric air samples characteristic of altitudes near 3.4 km, the 24-hour average net ozone production rate is shown to be −0.5 ppbv/d (net ozone destruction), and is determined primarily by photolytic destruction.

A study of the photochemistry and ozone budget during the Mauna Loa Observatory Photochemistry Experiment

Extensive measurements of trace species and parameters that are important to the photochemical production and loss of ozone have been made at Mauna Loa during the Mauna Loa Observatory Photochemistry Experiment experiment. These measurements are used as inputs as well as constraints in a model study of the photochemical budgets of ozone and five other trace species (CH2O, CH3OOH, H2O2, NO, and NOx) that are closely coupled to the photochemical production and loss of ozone. The study shows that there are significant discrepancies in the photochemical budgets of these trace species in this region and suggests that some important uncertainties exist in our understanding of the odd hydrogen photochemical processes.

Partitioning and budget of NO y species during the Mauna Loa Observatory Photochemistry Experiment

During the Mauna Loa Observatory Photochemistry Experiment (MLOPEX), measurements were made of total odd nitrogen (NOy) and the known individual daytime odd-nitrogen species. The individual species measured were NO, NO2, HNO3, paniculate NO3−, peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), methyl nitrate, and >C3 alkyl nitrates. The most abundant component of NOy was nitric acid; its median contribution to NOy in free tropospheric samples was 43%. The large fraction of HNO3 is consistent with the long transport times and photochemical processing of air masses reaching the mid-Pacific site as well as possible stratospheric input of NOy. The median contribution of NOx to NOy in the free troposphere near 3.4 km was ≈14%. PAN and other measured organic nitrates contributed < 7% to NOy. The median sum of the individually measured species was 102% of NOy in upslope periods which consist of a mixture of island-modified marine boundary layer and free tropospheric air. This total was 75% of NOy during downslope periods representative of the free troposphere. This shortfall in the odd-nitrogen budget in the free troposphere corresponds to 72 pptv of reactive nitrogen, which is over 2 times median NOx. The NOy shortfall and the composition of NOy appeared to have a regular variation in the free troposphere during the experiment which was related to air mass origin, recycling of odd nitrogen, and loss processes during transport. The presence of an odd-nitrogen deficit in the remote free troposphere suggests that our understanding of the NOy system is incomplete. Unidentified odd-nitrogen species, such as organic nitrates, may be present, but sampling limitations and analytical uncertainties in NOy and individual (NOy)i measurements still restrict our ability to accurately define an NOy budget, especially in remote regions.

Total reactive oxidized nitrogen (NOy) in the remote Pacific troposphere and its correlation with O3 and CO: Mauna Loa Observatory Photochemistry Experiment 1988

As part of the Mauna Loa Observatory Photochemistry Experiment (MLOPEX) total reactive oxidized nitrogen (NOy) was measured during May and early June of 1988 at the Mauna Loa Observatory, the NOAA-Geophysical Monitoring for Climatic Change Baseline Monitoring Station, located at 3.4-km elevation on the island of Hawaii. Gold catalytic surface conversion of individual reactive oxidized nitrogen species to NO and subsequent quantification of the NO by NO/O3 chemiluminescence was used to measure the NOy mixing ratio. The NOy abundance at the site was governed by the local downslope/upslope wind systems as well as synoptic-scale transport. With some exceptions, downslope wind brought air representative of the free troposphere, while upslope winds transported air from below the trade wind inversion to the site. The upslope air masses could be a mix of marine boundary layer air and free tropospheric air modified by anthropogenic and natural emissions from island sources. It was possible to identify free tropospheric air in the downslope flow through meteorological and chemical tracers. Reflecting the remote location, low NOy mixing ratios with median values of 262 and 239 pptv were found in free tropospheric and upslope air masses, respectively. The median NOy levels in free tropospheric air are consistent with airborne NOy measurements made during NASAs Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation (CITE 2) program over the northeastern Pacific Ocean at corresponding altitudes. The median NOy values in upslope flow are significantly higher than those measured in the remote marine boundary layer during CITE 2, reflecting probably the influence of island source and/or mixing of free tropospheric air with boundary layer air. The low correlation found between NOy and tracers of anthropogenic sources, such as carbon monoxide, tetrachloroethylene, and n-propyl nitrate, in free tropospheric air samples is consistent with a stratospheric or upper tropospheric source for NOy. Simultaneous particulate nitrate (NO3−) measurements suggest that at times not all aerosol NO3− was quantitatively converted to NO by the Au-surf ace converter technique. These episodes were usually found during upslope flow and were characterized by high sodium concentrations, suggesting that possibly the sodium nitrate contained in these aerosols was not converted efficiently by the Au converter.

Nitric Acid Uptake on Subtropical Cirrus Cloud Particles

The redistribution of HNO 3 via uptake and sedimentation by cirrus cloud particles is considered an important term in the upper tropospheric budget of reactive nitrogen. Numerous cirrus cloud encounters by the NASA WB-57F high-altitude research aircraft during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE) were accompanied by the observation of condensed-phase HNO 3 with the NOAA chemical ionization mass spectrometer. The instrument measures HNO 3 with two independent channels of detection connected to separate forward and downward facing inlets that allow a determination of the amount of HNO 3 condensed on ice particles. Subtropical cirrus clouds, as indicated by the presence of ice particles, were observed coincident with condensed-phase HNO 3 at temperatures of 197-224 K and pressures of 122-224 hPa. Maximum levels of condensed-phase HNO 3 approached the gas-phase equivalent of 0.8 ppbv. Ice particle surface coverages as high as 1.4 x 10 14 molecules cm -2 were observed. A dissociative Langmuir adsorption model, when using an empirically derived HNO 3 adsorption enthalpy of -11.0 kcal mol -1 , electively describes the observed molecular coverages to within a factor of 5. The percentage of total HNO 3 in the condensed phase ranged from near zero to 100% in the observed cirrus clouds. With volume-weighted mean particle diameters up to 700 μm and particle fall velocities up to 10 m s -1 , some observed clouds have significant potential to redistribute HNO 3 in the upper troposphere.

Measurements of nitric oxide and nitrogen dioxide during the Mauna Loa Observatory Photochemistry Experiment

NO and NO2 were simultaneously measured by photolytic conversion / chemiluminescence techniques during the Mauna Loa Observatory Photochemistry Experiment (MLOPEX). The field site, located at an elevation of 3.4 km on the north side of the Mauna Loa Volcano, was subject to two airflow regimes which typically corresponded to upslope (marine boundary layer plus island sources) conditions during the day and downslope (middle free tropospheric) conditions at night to mid-morning. Median values of NOx (NOx = NO + NO2) were 37 and 31 pptv during upslope and downslope conditions, respectively, with the downslope measurements consistent with previous measurements made from aircraft in the middle free troposphere over the North Pacific. Although the difference in median NOx mixing ratios in the upslope and downslope regimes is small, the influence of island sources of NOx is apparent. Indeed, the median upslope values were approximately 2.5 times greater than measurements made previously in the remote marine boundary layer. The data have been examined according to downslope / free tropospheric and upslope air flow regimes for relationships between NOx and the various species that were measured simultaneously (e.g., peroxyacetyl nitrate (PAN), HNO3, NO3, NOy, O3, CO, and hydrocarbons). While positive correlations between NOx and O3 and PAN were typically observed in free tropospheric air, these correlations were considerably weaker than those observed during previous campaigns. This is likely primarily due to the lower sampling altitude during the MLOPEX study. NOx and dew point temperature were weakly anticorrelated in free tropospheric air masses. Linear correlations between NOx and the peroxides, formaldehyde, alkyl nitrates, and hydrocarbons were also weak in the free tropospheric air masses at the MLO. NOx/NOy was typically on the order of 0.1–0.2 in free tropospheric flow. Considerably higher values of NOx/NOy, were occasionally observed under upslope conditions. The NOx/NOy and HNO3/NOx values obtained under downslope conditions were similar to those previously obtained during aircraft measurements in the middle free troposphere over the northeast Pacific. On the whole, the downslope air masses sampled appear to be characteristic of well-aged, marine free tropospheric air, and this conclusion is supported by 10-day trajectory analyses.

Evidence of the effect of summertime midlatitude convection on the subtropical lower stratosphere from CRYSTAL‐FACE tracer measurements

[1] Trace gas and particle measurements taken during the CRYSTAL-FACE mission are used to examine mixing in the summer subtropical lower stratosphere. Vigorous convection in the central and eastern United States injected a significant amount of tropospheric air into the lower stratosphere, which was subsequently advected over the region sampled during the CRYSTAL-FACE mission. Aerosols produced by biomass burning were observed over Florida during a time period with a large number of forest fires in the western United States and eastern Canada, providing evidence of convective injection of tropospheric air into the lower stratosphere. The circumstances of the large-scale flow pattern in the upper troposphere and lower stratosphere, vigorous summertime convection, abundant forest fires, and the downstream sampling allow a unique view of mixing in the lower stratosphere. We calculate the fractions of midlatitude tropospheric air in the sampled lower stratosphere and mixing rates on the basis of consistency between a number of tracer-tracer correlations. The tropospheric endpoints to the mixing estimates give an indication of midlatitude continental convective input into the lower stratosphere. We also discuss the possible impact of summertime midlatitude convection on the composition of the stratosphere as a whole.

Aircraft measurements made during the spring maximum of ozone over Hawaii: Peroxides, CO, O3, NOy, condensation nuclei, selected hydrocarbons, halocarbons, and alkyl nitrates between 0.5 and 9 km altitude

Between April 22 and May 11, 1992, ten flights of the University of Wyoming King Air were made during the maximum in tropospheric ozone over the central North Pacific Ocean in conjunction with the spring intensive of the second Mauna Loa Observatory Photochemistry Experiment. During the first week of flights, an episode of remarkably large total reactive nitrogen, NO y (∼2 ppbv) persisted in the 5-9 km altitude region for 3-4 days. Backtrajectory calculations combined with the trace gas and aerosol measurements confirm that its source was due primarily to export from northern latitude continental surface regions. The total amount of odd nitrogen transported over Hawaii during this event was estimated to be 1-2% of the annual emissions from subsonic aircraft or from stratospheric input. Throughout the measurement program layers of elevated O 3 , NO y , condensation nuclei (CN), and other species were frequently found between the onset of the marine boundary layer temperature inversion and 4-5 km altitude. Structure and strong gradients within these layers contribute to the daily variations seen at the 3.4 km elevation of the Mauna Loa Observatory during the nighttime downslope flow. The dryness of these low-altitude layers and the calculated air mass trajectories indicate that export from northern latitudes occurred mainly with subsidence to the Hawaii region rather than from transit just above the boundary layer inversion. There was no evidence of recent stratospheric input to the altitude region sampled below 9 km. However, the observations cannot distinguish whether O 3 input from the stratosphere occurred earlier in the air mass histories at higher latitudes. Fine vertical scale anticorrelations between CN and O 3 or NO y were also often observed particularly in the last week of the program when NO y mixing ratios were more typical of the remote troposphere. These features are attributed to new particle formation near the tops of cloud convection episodes and they illustrate the importance of such processes in contributing to the detailed layering and dilution of some chemical species in the free troposphere during this time of year. Mean and median profiles for many of the title species are given for high and low-to-moderate NO y categories.

Oxidation of mercury by bromine in the subtropical Pacific free troposphere

Mercury is a global toxin that can be introduced to ecosystems through atmospheric deposition. Mercury oxidation is thought to occur in the free troposphere by bromine radicals, but direct observational evidence for this process is currently unavailable. During the 2013 Nitrogen, Oxidants, Mercury and Aerosol Distributions, Sources and Sinks campaign, we measured enhanced oxidized mercury and bromine monoxide in a free tropospheric air mass over Texas. We use trace gas measurements, air mass back trajectories, and a chemical box model to confirm the origin and chemical history of the sampled air mass. We find the presence of elevated oxidized mercury to be consistent with oxidation of elemental mercury by bromine atoms in this subsiding upper tropospheric air mass within the subtropical Pacific High, where dry atmospheric conditions are conducive to oxidized mercury accumulation. Our results support the role of bromine as the dominant oxidant of mercury in the upper troposphere.

Measurements of NO x and PAN and estimates of O3 production over the seasons during Mauna Loa Observatory Photochemistry Experiment 2

Measurements of peroxyacetyl nitrate (PAN) and NOx and a variety of other constituents were made over approximately 1-month-long intensives in the autumn of 1991 and the winter, spring, and summer of 1992 during the second Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2). PAN and NOx in the free troposphere had maximum abundances in spring in concert with the well-known maximum in O3. The ratio of the spring to summer averages was a factor of 4.1 for PAN, a factor of 1.6 for O3, and only a factor of 1.4 for NOx. During most intensives, variations over periods of a few days to a week were often larger than the average seasonal amplitude. In free tropospheric air masses local to Hawaii, average PAN/NOx ratios were a maximum in winter through spring but in the range of 0.25–0.86 in all intensives. PAN decomposition is unlikely to be the major net source of NOx in local air masses in summer and fall. The low HNO3/NOx ratios determined during MLOPEX 1 were confirmed during MLOPEX 2. Intensive average ratios of 1.6–3.8 over the year are lower than some model predictions. Both the low ratio and the magnitude of NOx imply a shortcoming in our understanding of the transformations and sources of NOy constituents in the central Pacific, The 3- to 4-km altitude region near Hawaii was a net importer of O3, on average, over the year. The average net rate of production of O3 in free tropospheric air was near zero in winter, −0.4 to −0.8 ppbv/d in spring, −1.4 ppbv/d in summer, and −0.6 ppbv/d in autumn. Thus the spring maximum in O3 is not due to local photochemistry. We believe, as has been concluded from the long-term measurements of long-lived constituents by the Climate Monitoring and Diagnostics Laboratory, that the variation of ozone precursors over the year and on shorter timescales of a few days to a week is controlled predominantly by changes in long-range transport: more frequent sampling of higher-latitude and higher-altitude air masses in winter and spring versus more frequent sampling of well-aged air from lower altitudes and latitudes in summer and autumn.