B. A. Ridley

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.

Sources and chemistry of NOx in the upper troposphere over the United States

The origin of NOx in the upper troposphere over the central United States is examined using aircraft observations obtained during the SUCCESS campaign in April–May of 1996. Correlations between NOy (sum of NOx and its oxidation products) and CO at 8–12 km altitude indicate that NOx originates primarily from convective transport of polluted boundary layer air. Lightning and aircraft emissions appear to be only minor sources of NOx. Chemical steady state model calculations constrained by local observations of NO underestimate the measured NOx/NOy concentration ratio at 8–12 km altitude by a factor of two on average. The magnitude of the underestimate is correlated with concentrations of condensation nuclei, which we take as a proxy for the age of air in the upper troposphere. We conclude that the NOx/NOy ratio is maintained above chemical steady state by frequent convective injections of fresh NOx from the polluted boundary layer and by the long lifetime of NOx in the upper troposphere (5–10 days). In contrast to previous studies, we find no evidence for fast heterogeneous recycling from HNO3 to NOx in the upper troposphere.

Distributions of NO, NOx, NOy, and O3 to 12 km altitude during the summer monsoon season over New Mexico

During late July and August 1989, 12 flights of the National Center for Atmospheric Research Sabreliner jet aircraft were made over New Mexico when the region was dominated by either synoptic high pressure or moist “monsoon” flow. In the latter case, sampling was made within and about deep convective clouds which were sometimes electrically active. A summary of the measurements of the species listed in the title and their ratios are given. These distributions include signatures from deep convection, lightning production of odd nitrogen, aircraft exhaust emissions, and possible stratospheric input. The averages and range of these distributions are considered to be more representative of typical summer conditions over the region compared to flights that are often restricted more to fair weather situations. Coherence between the O3 and the NOy observations is compared to results from other ground-based and aircraft programs and possible contributing factors are discussed. Because the measurements were made with then newly developed instrumentation, its capabilities and shortcomings are summarized.

Regional ozone and urban plumes in the southeastern United States: Birmingham, a case-study

Aircraft measurements of ozone and the oxides of nitrogen have characterized the horizontal and vertical extent of the urban plume downwind of Birmingham, Alabama. Derived NOx emission rate estimates of 0.6×1025 molecules s−1, with an uncertainty of a factor of 2, for this metropolitan area are in reasonable accord with the 1985 National Acid Precipitation Assessment Program inventory, which gives 1.2×1025 molecules s−1 for daytime emissions. These estimates are from two flights in 1992 when the urban plume was well separated from the plumes from two power plants northwest of the city. During three flights in 1990 the plumes of the Birmingham metropolitan area and the two power plants were combined; good agreement was found between the estimated fluxes (2.0 to 5.5×1025 molecules s−1) and the emission inventory (3.7×1025 molecules s−1) for the combined sources. The enhancement of O3 in the urban plume indicates photochemical formation and shows that during the summertime, approximately seven O3 molecules can be formed per NOx molecule added by the urban and power plant emissions. This production efficiency applies both to the isolated urban plume and to the combined urban-power plant plumes and is similar to that derived for rural areas from surface studies. Comparison of the results from several flights indicates the contribution of the regional ozone levels to the O3 concentrations observed within the urban plumes. The aircraft measurements, in combination with surface measurements of ozone, show that the interaction of ozone concentrations entering the urban area, the photochemical formation of ozone during the oxidation of the urban emissions, and the wind speed and direction determine the location and the magnitude of the peak ozone concentrations in the vicinity of this metropolitan area.

Airborne in-situ OH and HO2 observations in the cloud-free troposphere and lower stratosphere during SUCCESS

The hydroxyl (OH) and hydroperoxyl (HO2) radicals were measured for the first time throughout the troposphere and in the lower stratosphere with a new instrument aboard the NASA DC-8 aircraft during the 1996 SUCCESS mission. Typically midday OH was 0.1-0.5 pptv and HO2 was 3-15 pptv. Comparisons with a steady-state model yield the following conclusions. First, even in the lower stratosphere OH was sensitive to the albedo of low clouds and distant high clouds. Second, although sometimes in agreement with models, observed OH and HO2 were more than 4 times larger at other times. Evidence suggests that for the California upper troposphere on 10 May this discrepancy was due to unmeasured HOx sources from Asia. Third, observed HO2/OH had the expected inverse dependence with NO, but was inexplicably higher than modeled HO2/OH by an average of 30%. Finally, small-scale, midday OH and HO2 features were strongly linked to NO variations.

HNO3/NOx ratio in the remote troposphere During MLOPEX 2: Evidence for nitric acid reduction on carbonaceous aerosols?

The [HNO 3 ]/[NO x ] ratio is generally overestimated by a factor of 5-10 in photochemical models in comparison to tropospheric measurements. In this study, the heterogeneous reduction of HNO 3 into NO on carbonaceous aerosols [Lary et al., 1996] has been introduced in a photochemical box-model on the basis of black carbon mass densities measured during MLOPEX 2. This recycling to NO x decreases the [HNO 3 ]/[NO x ] ratio close to observed values. The concomitant increase in modeled NO x concentration is also in better agreement with the observations, and has substantial implications for the ozone budget in the remote atmosphere. Large uncertainties in the estimate of black carbon surface area and of accomodation coefficients preclude definitive conclusions until more detailed measurements are carried out.

Sources of HOx and production of ozone in the upper troposphere over the United States

The sources of HO x (OH+peroxy radicals) and the associated production of ozone at 8-12 km over the United States are examined by modeling observations of OH, HO 2 , NO, and other species during the SUCCESS aircraft campaign in April-May 1996. The HO x concentrations measured in SUCCESS are up to a factor of 3 higher than can be calculated from oxidation of water vapor and photolysis of acetone. The highest discrepancy was seen in the outflow of a convective storm. We show that convective injection of peroxides (CH 3 OOH and H 2 O 2 ) and formaldehyde (CH 2 O) from the boundary layer to the upper troposphere could resolve this discrepancy. More generally, the data collected over the central United States during SUCCESS suggest that local convection was a major source of HO x and NO x to the upper troposphere. The OH and HO 2 observations together with the observations of NO allow us to directly calculate the ozone production in the upper troposphere and its dependence on NO x . We find an average net ozone production of 2 ppbv day -1 between 8 and 12 km over the continental United States in the spring. Ozone production was NO x -limited under essentially all the conditions encountered in SUCCESS. The high levels of HO x present in the upper troposphere stimulate ozone production and increase the sensitivity of ozone to NO x emissions from aircraft and other sources.

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.

A cloud‐scale model study of lightning‐generated NO x in an individual thunderstorm during STERAO‐A

Understanding lightning NOx (NO 1 NO2) production on the cloud scale is key for developing better parameterizations of lightning NOx for use in regional and global chemical transport models. This paper attempts to further the understanding of lightning NOx production on the cloud scale using a cloud model simulation of an observed thunderstorm. Objectives are (1) to infer from the model simulations and in situ measurements the relative production rates of NOx by cloud-to-ground (CG) and intracloud (IC) lightning for the storm; (2) to assess the relative contributions in the storm anvil of convective transport of NOx from the boundary layer and NOx production by lightning; and (3) to simulate the effects of the lightning-generated NOx on subsequent photochemical ozone production. We use a two-dimensional cloud model that includes a parameterized source of lightning-generated NOx to study the production and advection of NOx associated with a developing northeast Colorado thunderstorm observed on July 12, 1996, during the Stratosphere-Troposphere Experiment—Radiation, Aerosols, Ozone (STERAO-A) field campaign. Model results are compared with the sum of NO measurements taken by aircraft and photostationary state estimates of NO2 in and around the anvil of the thunderstorm. The results show that IC lightning was the dominant source of NOx in this thunderstorm. We estimate from our simulations that the NOx production per CG flash (PCG) was of the order of 200 to 500 mol flash 21 .N O x production per IC flash (PIC) appeared to be half or more of that for a CG flash, a higher ratio of P IC/PCG than is commonly assumed. The results also indicate that the majority of NOx (greater than 80%) in the anvil region of this storm resulted from lightning as opposed to transport from the boundary layer. The effect of the lightning NOx on subsequent photochemical ozone production was assessed using a column chemical model initialized with values of NOx ,O 3, and hydrocarbons taken from a horizontally averaged vertical profile through the anvil of the simulated storm. The lightning NOx increased simulated ozone production rates by a maximum of over 7 ppbv d 21 in the upper troposphere downwind of this storm.