Mauna Loa Observatory Photochemistry Experiment Research Articles

Intercontinental transport, chemical transformations, and baroclinic systems

The adjoint sensitivity technique is used to diagnose the transport pathway and chemical transformations of a pollutant plume from Asia to Hawaii in a three‐dimensional chemical transport model (HANK) driven by winds from the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) Mesoscale Modeling System (MM5). The simulation takes place within the timeframe of the Mauna Loa Observatory (MLO) Photochemistry Experiment (MLOPEX) 2c (15 April to 15 May 1992). Unlike trajectories, the adjoint technique takes into account all model processes including diffusion and convection in accounting for the transport of the pollutant plume and clearly defines the footprint of the plume. The transport of the pollutant plume is primarily analyzed from an isentropic perspective. The plume is lofted upward out of the Asian boundary layer in a very narrow band immediately above the surface cold front in the warm conveyor belt of a developing synoptic storm. Once lofted into the free troposphere, the plume travels almost isentropically to Hawaii, with very little additional mixing. The chemical and physical transformations within the plume are consistent with measurements on Hawaii as part of MLOPEX 2c: in particular, the initial loss of water and HNO3, the conversion of NOy from PAN to NOx to HNO3, the slow mixing rate in the free troposphere and the small ratio of HNO3 to NOx are consistent with the measurements. The chemical signature of pollutant plumes at MLO is investigated. The plumes have much higher concentrations of hydrocarbons than average and low concentrations of HNO3. The concentrations of PAN, NOx, H2O, and H2O2 were not found to distinguish rapid transport from Asia.

Model and measurement analysis of springtime transport and chemistry of the Pacific basin

Extensive chemical measurements from the Mauna Loa Observatory (MLO) Photochemistry Experiment 2c (MLOPEX 2c) (April 15 to May 15, 1992) field campaign are analyzed in relation to the photochemistry, transport, and physical losses in the Pacific basin utilizing an episodic chemical transport model christened HANK. The budgets of NOy and O3 are examined in particular, as are model‐measurement discrepancies in the NOy budget. For the long‐lived species the bias of the simulated means (measured minus simulated divided by measured) of CO, CH4, C2H6, C3H8, O3, and NOx is less than 20%. The bias of ln ([C3H8]/[C2H6]) is less than 4%, and the modeled and measured means of C3H8 are not significantly different. H2O2, CH3OOH, and radon agree within a standard deviation. The simulated concentrations of NOy, HNO3, and peroxyacetyl nitrate (PAN) are high, while the short‐lived hydrocarbons (C2H6 and C2H4) are low. Frequency distributions and species' cross correlations are also used to diagnose and evaluate the simulation. Sensitivities in the model formulation are discussed. The results indicate that the subtropics demarcate a transition between a regime characterized by Hadley cell transport to the south and one characterized by rapid isentropic mixing to the north. Vigorous photochemical processing occurs in the remote subtropical Pacific as PAN is converted to NOx, which in turn is converted to HNO3. This rapid photochemistry is related to the slope of the isentropic surfaces in the subtropics. It maintains elevated levels of NOx in the remote Pacific, reduced levels of HNO3/NOx, and increases the net ozone production. The model and measurements show that as air masses are processed photochemically (as measured by AGE) the relative concentrations of PAN and NOx decrease, as does PAN/NOy; corresponding increases occur in HNO3, HNO3/NOx, and HNO3/NOy. In both the model and measurements, H2O2, CH3OOH, and HNO3/NOx are tracers of photochemically processed air, while high concentrations of C3H8, C2H6, NOx, and PAN are associated with younger air.

Episodic modeling of the chemical structure of the troposphere as revealed during the spring MLOPEX 2 intensive

An episodic regional chemical transport model (CTM) is described and applied to modeling the vertical chemical structure of the troposphere, as revealed from aircraft flights in the vicinity of Hawaii during the springtime Mauna Loa Observatory Photochemistry Experiment 2c intensive. The CTM simulates the chemistry and transport of 50 species within the North Pacific basin. It is driven by output fields from the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Modeling System. The modeled meteorology is highly correlated with European Centre for Medium‐Range Weather Forecasts analyses on the large scale but is not accurate enough to correctly simulate the long‐range transport of chemical plumes. The simulated total cloud cover is approximately correct when compared with International Satellite Cloud Climatology Project cloud cover. However, the coverage of low‐lying clouds is overpredicted, while the upper level cloudiness is underpredicted. Modeled and satellite‐derived deep convection are reasonably correlated. The measured concentrations of H2O2, CH3OOH, CO, NOy, and O3 are compared with simulated concentrations from the marine boundary layer to approximately 9 km. Although pronounced chemical plumes and vertical layering complicate the model‐measurement comparison, modeled and measured concentrations are generally within a factor of 2. with H2O2 particularly well simulated. A number of measured transient chemical events are examined. This study provides the necessary background for subsequent papers examining the chemistry and transport of the Pacific basin in more detail.

Photochemistry and budget of ozone during the Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2)

During the Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2), simultaneous measurements of a large number of photochemical species were measured during different seasons at Mauna Loa Observatory (MLO), Hawaii. In this study, these measurements are used to constrain a detailed photochemical box model and evaluate our understanding of the tropospheric photochemistry in this region of the Pacific. The simulations generally reproduce satisfactorily the NO/NO2 photostationary state, which controls the ozone production rate. However, the model fails in simulating the concentration of peroxy radicals (PO2) during all seasons and of hydroxyl radical (OH) during summer. Several hypotheses are considered to assess this discrepancy, including the removal of radicals by unidentified mechanisms and the potential impact of biogenic organic compounds. None of the tested hypotheses give satisfactorily results in terms of OH, PO2 and NO/NO2 simultaneously. Although experimental uncertainties are large for radicals, this issue constitutes a major inconsistency between measurements and model results during MLOPEX. Another disagreement arises from the simulation of peroxides for free tropospheric conditions. The model tends to overestimate H2O2 and CH3OOH by a factor of 1.5–2.5. On the other hand, a fair agreement is achieved in simulating formaldehyde when CH3OOH is constrained in the model. Finally, we find that the gross ozone production and destruction rates are nearly in balance in this region of the Pacific troposphere. The net production is slightly negative, ranging from nearly 0 in winter to about −1.4 ppbv/d during summer. In contrast, the NOx budget shows a severe imbalance. Our results indicate that an additional source of NOx ranging from 18 to 48 pptv/d (in winter and summer, respectively) would be required to sustain the 30 pptv of NOx measured on average at the site during free tropospheric conditions. Acetone has little effect on the budget of HOx at the altitude of MLO (3.4 km). However, including this species in the model induces an even larger imbalance in the NOx budget through the production of peroxyacetylnitrate.

Measurements of NOx 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.

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, NOy (∼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 O3, NOy, 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 O3 input from the stratosphere occurred earlier in the air mass histories at higher latitudes. Fine vertical scale anticorrelations between CN and O3 or NOy were also often observed particularly in the last week of the program when NOy 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 NOy categories.

Some considerations of the origin of nighttime peroxy radicals observed in MLOPEX 2

The “chemical amplifier” (CA) was used to measure the concentration of the peroxy radicals present in the nighttime atmosphere at the Mauna Loa Observatory (MLO) during the Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2) from April 15 to May 15, 1992. Simultaneous measurements were also made of peroxy radicals using a modified OH instrument during one period of 56‐hour duration. Both instruments observe experimentally significant nighttime signals, but the magnitude of the larger CA signal appears to result from organic peroxy radicals to which the modified OH instrument is insensitive. The data from about one half of the nights showed a decay of the radical signal from 2000 to 0400 HST the next morning. The decay rates followed roughly those expected for primary or secondary alkyl peroxy radicals. However, data from the other half of nights showed an increase in radical concentration with time, which reflected significant radical generation rates. Several alternatives are considered in explanation of the results. (1) The possible origin of the signals from the interference of HO2NO2, presumably transported from reservoirs at higher elevations, was considered. The data are inconclusive, and this hypothesis is tentatively rejected. (2) The nighttime chemistry of the O3 and NO3 was also considered. The observed trace gases provide in theory only a small fraction of the peroxy radical signal seen on many nights. However, the presence of 1–30 parts per trillion by volume (pptv) of CH3SCH3, a compound not measured but expected to be present at MLO, can react with NO3 to produce peroxy radical signals of the magnitude observed. It is concluded that this ubiquitous compound over the oceans offers the best current hypothesis to rationalize the nighttime generation of peroxy radicals at MLO.

Peroxy radicals measured during Mauna Loa Observatory Photochemistry Experiment 2: The data and first analysis

Peroxy radicals were measured using chemical amplification during the four intensive measurement periods of the Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2) campaign from September 1991 to August 1992. One‐minute average radical concentrations were obtained with overall uncertainties of 8 to 16 parts per trillion by volume (pptv), depending on the variability of ambient ozone. Thirty‐minute averaged concentrations have uncertainties of ±30%. The peroxy radicals exhibited diurnal behavior expected of an atmospheric species with photochemical origins and correlated with a simple expression that describes radical formation and loss. The application of a steady state analysis to the radical levels suggested that for some conditions, loss processes other than gas phase chemistry may be required to explain the observed concentrations.

Tunable diode laser absorption measurements of H2O2 and HCHO during the Mauna Loa Observatory Photochemistry Experiment

The tunable diode laser absorption spectrometric technique was employed to measure H2O2 and HCHO during the Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) fall and winter intensives. Comparisons were made between the measurements made with the Tunable Diode Laser Absorption Spectrometer (TDLAS) technique and several other methods. The half‐hourly average mixing ratios varied between 1300 parts per trillion by volume (pptv) and the instrument detection limit of 50 – 100 pptv for H2O2 and between 400 pptv and the instrument detection limit ∼25 pptv for HCHO. Daily average mixing ratios for H2O2 were 550 ± 79 pptv (fall) and 540 ± 80 pptv (winter), and for HCHO were 130 ± 21 pptv (fall) and 200 ± 35 (winter). HCHO showed strong diurnal behaviour with maxima in the 1200 – 1700 time period whereas H2O2 showed essentially no diurnal variation. Clean tropospheric air sampled during downslope conditions between 2000 and 0600 had HCHO mixing ratios averaging 80 pptv in the fall and 160 pptv in the winter. These values are considerably lower than model predictions.

Theoretical, actinometric, and radiometric determinations of the photolysis rate coefficient of NO2 during the Mauna Loa Observatory Photochemistry Experiment 2

Measurements of the photolysis rate coefficient of NO2 (jNO2) were made using a chemical actinometer during the fall (September 15 to October 23, 1991), winter (January 15 to February 15, 1992), spring (April 15 to May 15, 1992), and summer (July 15 to August 15, 1992) intensives during the second Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2), Hawaii. The measured clear sky jNO2 values of MLOPEX 2 for all four intensives have substantially increased over the measured jNO2 values of MLOPEX 1 (May, 1988); a 37% increase occurs in midday clear sky jNO2 from the spring intensive of MLOPEX 1 to the spring intensive of MLOPEX 2. The ratio of actinometer measurements to theoretical calculations of jNO2 are 1.44 ± 0.03, 1.45 ± 0.02, 1.43 ± 0.03, and 1.34 ± 0.02 for the fall, winter, spring, and summer intensives, respectively. Inclusion of stratospheric aerosols or aerosols of any reasonable optical parameters in a detailed discrete ordinate model cannot account for the discrepancy between measurements and model. The photolysis rate coefficient of NO2 is also derived from simultaneous measurements of the ultraviolet irradiance measured with an Eppley radiometer using a semiempirical relationship for each of the four intensives. A simplified cloud model is proposed to explain jNO2 values that exceed clear sky values during days that have partial cloud coverage.

Peroxy radical concentrations measured and calculated from trace gas measurements in the Mauna Loa Observatory Photochemistry Experiment 2

Measurements of peroxy radical concentrations ([HO2] + [RO2]) were made by the “chemical amplifier” technique during the four intensives of the Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) at the Mauna Loa Observatory in 1991–1992. In this study these data are compared with the theoretical values of the peroxy radical concentrations obtained from steady state analysis of the complete suite of trace gas measurements and other relevant parameters also measured during the experiment. The data from 33 days of the study contain time overlap of the concentration and physical data which allow a meaningful theoretical treatment. The experimental results for [HO2] + [RO2] agree well with theory for many of the days, but are significantly suppressed from the theoretical expectations on other days. Two hypotheses are presented and tested to explain the observed suppression. The first involves the reaction of the peroxy radicals at aerosol surfaces. The second proposes the loss of [OH] through its reaction with unknown and undetected species to develop peroxy radicals subsequently to which the “chemical amplifier” is insensitive. Evidence at hand does not allow a clear choice between these or possible alternative explanations. The data suggest that the net rate of O3 generation in the free troposphere is about −1.5 parts per billion by volume per day (ppbv d−1; 24‐hour average).

Comparison of hydroperoxide measurements made during the Mauna Loa Observatory Photochemistry Experiment 2

From the fall of 1991 through the summer of 1992 the Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) took place in Hawaii. The experiment consisted of four intensive research periods, each approximately 1 month in duration. We report a comparison of measurements of atmospheric hydroperoxide and methylhydroperoxide as measured during MLOPEX 2a (September 16 to October 23, 1991) and MLOPEX 2b (January 15 to February 15, 1992). The analytical techniques used for measuring hydroperoxides included tunable diode laser infrared absorption (TDLAS) and fluorescence from an enzymatically produced dimer. The TDLAS technique measured hydrogen peroxide (H2O2) only. Three different procedures were utilized to distinguish between H2O2 and organic hydroperoxides in the fluorescence analytical techniques. These were high performance liquid chromatographic (HPLC) separation, enzymatic discrimination, and differential aqueous solubility. The measured concentrations ranged from 0 to 2 parts per billion by volume (ppbv) for H2O2 and 0 to ppbv for methylhydroperoxide (CH3OOH). Depending on the time period of measurement, the agreement varied between 20% and a factor of 2 or more for the measurement of H2O2. The discrepancy for the CH3OOH data was larger.

Tropospheric formaldehyde concentration at the Mauna Loa Observatory during the Mauna Loa Observatory Photochemistry Experiment 2

The concentration of formaldehyde at Mauna Loa Observatory, Hawaii, was determined during four Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) measurement intensives between September 1991 and August 1992. The observed diurnal variations, 200–900 parts per trillion by volume (pptv) during daytime and 60–200 pptv during nighttime, resulted mainly from the local air circulation pattern whereby island modified marine boundary layer air prevailed during the day and free tropospheric air dominated during the night. A seasonal variation was also observed; the median/mean values of all data points are: 149/196, 129/149, 143/178, and 181/211 pptv for autumn, winter, spring, and summer intensives, respectively. During nighttime downslope flow periods which brought in free tropospheric air to the measurement site, the formaldehyde concentrations (median/mean) were 122/123, 110/112, 120/125, and 140/137 pptv for autumn, winter, spring, and summer, respectively. This seasonal dependence may be attributable to changes in solar insolation and NO concentrations. A simple box model calculation constrained by the experimentally determined concentrations of CH3OOH yielded a formaldehyde concentration (without/with heterogeneous removal) for free tropospheric air, at 7°C, of 155/140, 125/115, 210/195, and 220/205 pptv for autumn, winter, spring and summer, respectively. The calculated values are in good agreement with the measured concentrations for winter (within 27/15%, without/with heterogeneous removal) and fall (within 14/5%), but are significantly higher for spring (75/63%) and summer (57/46%).

Actinometric and radiometric measurement and modeling of the photolysis rate coefficient of ozone to O(1D) during Mauna Loa Observatory Photochemistry Experiment 2

The in situ photolysis rate coefficient of O3 to O(1D) has been measured at Mauna Loa Observatory using a new actinometric instrument based on the reaction of O(1D) with N2O and with a hemispherical radiometer. One minute averaged photolysis rate coefficients were determined with an overall uncertainty of approximately ±11% at the 1 σ level for the actinometer and ±15% at the 1 σ level for the radiometer. Over 120 days of data were collected with varying cloud cover, aerosol loadings, and overhead ozone representing the first set of long term measurements. Clear sky solar noon values vary between approximately 3.0 × 10−5 and 4.5 × 10−5 sec−1. Modeling of the photolysis rate coefficients was done using a discrete ordinate radiative transfer scheme and results were compared with the actinometric measurements. The quantum yields for O(1D) production are reevaluated from existing data and reported here. The comparisons were done using the quantum yields for the photolysis of ozone recommended by DeMore et al. [1994], the newer evaluation of Michelsen et al. [1994], and also with reevaluated values in this paper. An analysis of the measured photolysis rate coefficient of O3 to O(1D) and model simulations of the photolysis rate coefficient data from clear days during the study provides added insight into the choice of quantum yield data for use in photochemical models of the troposphere.

A comparison of aircraft and ground‐based measurements at Mauna Loa Observatory, Hawaii, during GTE PEM‐West and MLOPEX 2

During October 19–20, 1991, one flight of the NASA Global Tropospheric Experiment (GTE) Pacific Exploratory Mission (PEM‐West A) mission was conducted near Hawaii as an intercomparison with ground‐based measurements of the Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2) and the NOAA Climate Modeling and Diagnostics Laboratory (CMDL). Ozone, reactive nitrogen species, peroxides, hydrocarbons, and halogenated hydrocarbons were measured by investigators aboard the DC‐8 aircraft and at the ground site. Lidar cross sections of ozone revealed a complex air mass structure near the island of Hawaii which was evidenced by large variation in some trace gas mixing ratios. This variation limited the time and spatial scales for direct measurement intercomparisons. Where differences occurred between measurements in the same air masses, the intercomparison suggested that biases for some trace gases was due to different calibration scales or, in some cases, instrumental or sampling biases. Relatively large uncertainties were associated with those trace gases present in the low parts per trillion by volume range. Trace gas correlations were used to expand the scope of the intercomparison to identify consistent trends between the different data sets.

Seasonal measurements of nonmethane hydrocarbons and carbon monoxide at the Mauna Loa Observatory during the Mauna Loa Observatory Photochemistry Experiment 2

Measurements of carbon monoxide and nonmethane hydrocarbons were made in situ at the Mauna Loa Observatory from September 1991 through August 1992. A distinct seasonal cycle in concentrations was observed for CO and most NMHCs, with higher average concentrations from January through April and much lower average concentrations from mid‐May through October. The sources of CO and NMHCs appeared to vary with season and appeared more homogeneous in winter (urban/industrial) and spring (unidentified). The ocean around Hawaii did not appear to be a major source of CO and most NMHCs, but was the major source of ethane and propene. Ethene and propene were always present in free tropospheric air sampled, indicating daily exchange between the free troposphere and the marine boundary layer. Air arriving at MLO during free tropospheric flow periods had seasonally similar photochemical age (approximately 20 days), with no significant fresh source inputs of NMHCs (except ethene and propene) within at least 10 days. The influence of atmospheric dilution processes on atmospheric concentrations was weakest in the spring period, when the influence of chemical loss processes was greatest. Aircraft measurements made during the spring agreed in trends and absolute concentrations with ground‐based measurement made at MLO. The temporal trends in concentrations of NMHCs within altitude ranges were similar. These trends varied with altitude and characterized a layered atmosphere with differing chemical composition. Acetone and methyl ethyl ketone were detected in most samples. N‐alkyl aldehydes were also detected but may be artifacts formed from organic aerosols collected in the sampling process.

Trajectories and related variations in the chemical composition of air for the Mauna Loa Observatory during 1991 and 1992

Isentropic 10‐day back trajectories are computed each day for the fall 1991 and winter, spring, and summer 1992 intensives of the second Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2). The trajectories are computed on an isentropic surface at the approximate height of the Mauna Loa Observatory (MLO). Trajectory variations on the daily, as well as annual timescale, are documented and compared to trajectory climatologies for Hawaii. During MLOPEX 2 the Pacific anticyclone was consistently farther west than normal, with important consequences for the computed trajectories. The concentrations of ozone, total reactive oxidized nitrogen, water, methyl nitrate, carbon monoxide, acetylene, radon, and peroxyacetyl nitrate (PAN) measured at MLO are examined in relation to trajectory characteristics on the daily and annual timescales. During the fall intensive and the summer intensive, changes in trajectory characteristics are related, in a broad sense, to changes in the measured concentrations of the above species; during the winter and especially the spring intensives, the trajectories prove to be poor indicators of the measured concentrations. For most species the seasonal variation in the computed trajectories does not satisfactorily explain the annual cycle in their measured concentrations.

Formaldehyde methods comparison in the remote lower troposphere during the Mauna Loa Photochemistry Experiment 2

Five methods for the measurement of CH2O vapor were compared under remote tropospheric conditions. The techniques included an aqueous‐scrubber enzyme fluorescence method (URIcoil), TDLAS (UNI), 2,4‐dinitrophenylhydrazine (DNPH) impregnated‐cartridges (BNL/WSUcart), an aqueous‐scrubber DNPH method (BNL/WSUcoil), and an unpublished aqueous‐scrubber immobilized‐enzyme fluorescence method (NCARbound). This was part of the Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) which was performed in four ∼30‐day intensives. In MLOPEX 2a (fall 1991), the URIcoil technique indicated higher concentrations relative to the BNL/WSUcoil which were higher than UNI. The limited number of NCARbound measurements for MLOPEX 2a were greater than the other measurements and least reliable. During MLOPEX 2b (winter 1992), URIcoil, UNI, BNL/WSUcoil, and NCAR‐bound gave comparable measurements of CH2O with the latter again being consistently higher than the first three methods and its performance and reliability was improved over MLOPEX 2a. URIcoil and BNL/WSUcoil were comparable in MLOPEX 2c (spring 1992) and 2d (summer 1992), whereas BNL/WSUcart was consistently higher than both for these intensives. The major source of difference between techniques was ascribed to variations in the analytical‐procedural field blanks. Calibration differences were the most likely cause of URIcoil being higher than BNL/WSUcoil and BNL/WSUcoil being higher than UNI during MLOPEX 2a. Lower free troposphere median concentrations were between 100 and 150 ppt for all seasons and without a seasonal trend. These values were a factor of 2 to 3 lower than model estimates for this study site and were in keeping with earlier work.

Observed and model‐calculated photostationary state at Mauna Loa Observatory during MLOPEX 2

During the Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2), simultaneous clear sky measurements of [NO2], [NO], [O3], and NO2 photodissociation rate were collected during autumn 1991, winter, spring, and summer 1992 intensive measurement periods at Mauna Loa Observatory (MLO), Hawaii. These measurements were used to assess the O3‐NO‐NO2 photostationary state, and to infer peroxy radical concentration, ozone photochemical production and destruction rates, as well as the relationship between the deviation of the photostationary state parameter, ϕ = j2[NO2]/k1[O3][NO], from unity and environmental variables. In addition, a photochemical box model was used for the purpose of comparison with observations and interpretation of the results. A systematic deviation of ϕ from unity was determined from measurements, this parameter reached values of 1.5–2 for high‐sun conditions depending on the season. The inferred total peroxy radical concentrations were generally larger (25–70 pptv for high‐sun conditions depending on the season) than those directly measured (20–25 pptv during noontime). Model results seem generally to underestimate the value of ϕ and to overpredict the hydroxyl and total peroxy radical concentrations, suggesting loss processes not accounted for in the model for these species. Analysis of the contributions to the net ozone photochemical production indicates total production and destruction terms reaching 10–15 ppbv/day which tend to nearly cancel on a daily average basis. High‐sun values of ϕ − 1 were shown to be independent of NOx (= NO + NO2) for NOx concentrations above about 50 pptv. However, below this level, observations show that ϕ − 1 generally decreases with increasing nitrogen oxide concentrations. Furthermore, the measured ϕ − 1 correlation with suggests the existence of NO to NO2 oxidants not accounted for in a simplified HOx chemistry.

Aerosol measurements during the Mauna Loa Photochemistry Experiment 2

Aerosol measurements have been made continuously at Mauna Loa Observatory (MLO) from 1974 to the present. Condensation nucleus (CN) concentration has been measured using automatic CN counters, and aerosol scattering extinction (σsp) has been measured using a four‐wavelength nephelometer. The Mauna Loa Observatory Photochemistry Experiment (MLOPEX) was conducted in 1991–1992 to study intensively many important variables in the field of atmospheric chemistry. Because of a strong diurnal cycle in nearly everything measured at MLO, caused by an upslope‐downslope wind system, it is important to develop data‐editing criteria that can safely identify background conditions as opposed to other conditions when the site may be contaminated by local sources. Ordinarily, background conditions occur during nighttime downslope wind conditions, and contaminated conditions occur during daytime upslope wind conditions. However, occasionally unusual weather conditions or contamination caused by a local source such as the Mauna Loa caldera can confuse the issue. It is recommended that background aerosol data be chosen during 0000–0800 Hawaiian standard time (HST) to generally avoid upslope wind conditions, and that wind direction and speed, CN, and SO2 data be used if available to further eliminate local pollution episodes. In addition, all data should be examined by a human editor, if possible, in order to recognize certain episodes that may not fit automated criteria.