Mary Anne Carroll

MOZART, a global chemical transport model for ozone and related chemical tracers: 2. Model results and evaluation

In this second of two companion papers, we present results from a new global three-dimensional chemical transport model, called MOZART (model for ozone and related chemical tracers). MOZART is developed in the framework of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM) and includes a detailed representation of tropospheric chemistry. The model provides the distribution of 56 chemical species at a spatial resolution of 2.8° in both latitude and longitude, with 25 levels in the vertical (from the surface to level of 3 mbar) and a time step of 20 min. The meteorological information is supplied from a 2-year run of the NCAR Community Climate Model. The simulated distributions of ozone (O 3 ) and its precursors are evaluated by comparison with observational data. The distribution of methane, nonmethane hydrocarbons (NMHCs), and CO are generally well simulated by the model. The model evaluation in the tropics stresses the need for a better representation of biomass burning emissions in order to evaluate the budget of carbon monoxide, nitrogen species, and ozone with more accuracy in these regions. MOZART reproduces the NO observations in most parts of the troposphere. Nitric acid, however, is overestimated over the Pacific by up to a factor of 10 and over continental regions by a factor of 2-3. Discrepancies are also found in the simulation of PAN in the upper troposphere and in biomass burning regions. These results highlight shortcomings in our understanding of the nitrogen budget in the troposphere. The seasonal cycle of ozone in the troposphere is generally well reproduced by the model in comparison with ozone soundings. MOZART tends, however, to underestimate O 3 at higher latitudes, and specifically above 300 mbar. The global photochemical production and destruction of ozone in the troposphere are 3018 Tg/yr and 2511 Tg/yr, respectively (net ozone production of 507 Tg/yr). The stratospheric influx of O 3 is estimated to be 391 Tg/yr and the surface dry deposition 898 Tg/yr. The calculated global lifetime of methane is 9.9 years in the annual average.

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.

Data composites of airborne observations of tropospheric ozone and its precursors

Tropospheric data from a number of aircraft campaigns have been gridded onto global maps, forming “data composites” of chemical species important in ozone photochemistry. Although these are not climatologies in the sense of a long temporal average, these data summaries are useful for providing a picture of the global distributions of these species and are a start to creating observations-based climatologies. Using aircraft measurements from a number of campaigns, we have averaged observations of O3, CO, NO, NOx, HNO3, PAN, H2O2, CH3OOH, HCHO, CH3COCH3, C2H6, and C3H8 onto a 5° latitude by 5° longitude horizontal grid with a 1-km vertical resolution. These maps provide information about the distributions at various altitudes, but also clearly show that direct observations of the global troposphere are still very limited. A set of regions with 10°–20° horizontal extent has also been chosen wherein there is sufficient data to study vertical profiles. These profiles are particularly valuable for comparison with model results, especially when a full suite of chemical species can be compared simultaneously. The O3 and NO climatologies generated from measurements obtained during commercial aircraft flights associated writh the MOZAIC and NOXAR programs are incorporated with the data composites at 10–11 km. As an example of the utility of these data composites, observations are compared to results from two global chemical transport models, MOZART and IMAGES, to help identify incorrect emission sources, incorrect strength of convection, and missing chemistry in the models. These comparisons suggest that in MOZART the NOx biomass burning emissions may be too low and convection too weak and that the transport of ozone from the stratosphere in IMAGES is too great. The ozone profiles from the data composites are compared with ozonesonde climatologies and show that in some cases the aircraft data agree with the long-term averages, but in others, such as in the western Pacific during PEM-Tropics-A, agreement is lacking. Finally, the data composites provide temporal and spatial information, which can help identify the locations and seasons where new measurements would be most valuable. All of the data composites presented here are available via the Internet (http://aoss.engin.umich.edu/SASSarchive/).

HO x budgets in a deciduous forest: Results from the PROPHET summer 1998 campaign

Results from a tightly constrained photochemical point model for OH and HO2 are compared to OH and HO2 data collected during the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) summer 1998 intensive campaign held in northern Michigan. The PROPHET campaign was located in a deciduous forest marked by relatively low NOx levels and high isoprene emissions. Detailed HOx budgets are presented. The model is generally unable to match the measured OH, with the observations 2.7 times greater than the model on average. The model HO2, however, is in good agreement with the measured HO2. Even with an additional postulated OH source from the ozonolysis of unmeasured terpenes, the measured OH is 1.5 times greater than the model; the model HO2 with this added source is 15% to 30% higher than the measured HO2. Moreover, the HO2/OH ratios as modeled are 2.5 to 4 times higher than the measured ratios, indicating that the cycling between OH and HO2 is poorly described by the model. We discuss possible reasons for the discrepancies.

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.

Climatologies of NOx and NOy: A comparison of data and models

Abstract Climatologies of tropospheric NOx (NO + NO2) and NOy (total reactive nitrogen: NOx + N03 + 2 × N2O5 + HNO2 + HNO3 + HNO4 + ClONO2 + PAN (peroxyacetylnitrate) + other organic ni trates) have been compiled from data previously published and, in most cases, publicly archived. Emphasis has been on non-urban measurements, including rural and remote ground sites, as well as aircraft data. Although the distribution of data is sparse, a compilation in this manner can begin to provide an understanding of the spatial and temporal distributions of these reactive nitrogen species. The cleanest measurements in the boundary layer are in Alaska, northern Canada and the eastern Pacific, with median NO mixing ratios below 10 pptv, NOx below 50 pptv, and NOy below 300 pptv. The highest NO values (greater than 1 ppbv) were found in eastern North America and Europe, with correspondingly high NOy (∼ 5 ppbv). A significantly narrower range of concentrations is seen in the free troposphere, particularly at 3–6 km, with NO typically about 10 pptv in the boreal summer. NO increases with altitude to ∼ 100 pptv at 9–12 km, whereas NOy does not show a trend with altitude, but varies between 100 and 1000 pptv. Decreasing mixing ratios eastward of the Asian and North American continents are seen in all three species at all altitudes. Model-generated climatologies of NOx and NOy from six chemical transport models are also presented and are compared with observations in the boundary layer and the middle troposphere for summer and winter. These comparisons test our understanding of the chemical and transport processes responsible for these species distributions. Although the model results show differences between them, and disagreement with observations, none are systematically different for all seasons and altitudes. Some of the differences between the observations and model results may likely be attributed to the specific meteorological conditions at the time that measurements were made differing from the model meteorology, which is either climatological flow from GCMs or actual meteorology for an arbitrary year. Differences in emission inventories, and convection and washout schemes in the models will also affect the calculated NOα and NOy distributions.

Nighttime observations of anomalously high levels of hydroxyl radicals above a deciduous forest canopy

Diurnal measurements of hydroxyl and hydroperoxy radicals (OH and HO2) made during the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) summer intensive of 1998 indicate that these key components of gas phase atmospheric oxidation are sustained in significant amounts throughout the night in this northern forested region. Typical overnight levels of OH observed were 0.04 parts per trillion (pptv) (1.1 × 106 molecules/cm3), while HO2 concentrations ranged from 1 to 4 pptv. Results of diagnostic testing performed before, after, and during the deployment suggest little possibility of interferences in the measurements. Collocated measurements of the reactive biogenic hydrocarbon isoprene corroborate the observed levels of OH by exhibiting significant decays overnight above the forest canopy. The observed isoprene lifetimes ranged from 1.5 to 12 hours in the dark, and they correlate well to those expected from chemical oxidation by the measured OH abundances. Possible dark reactions that could generate such elevated levels of OH include the ozonolysis of extremely reactive biogenic terpenoids. However, in steady state models, which include this hypothetical production mechanism, HO2 radicals are generated in greater quantities than were measured. Nonetheless, if the measurements are representative of the nocturnal boundary layer in midlatitude temperate forests, this observed nocturnal phenomenon might considerably alter our understanding of the diurnal pattern of atmospheric oxidation in such pristine, low-NOx environments.

Nucleation in the equatorial free troposphere: Favorable environments during PEM-Tropics

A combination of aerosol and gas phase instrumentation was employed aboard the NASA-P3B as part of the Pacific Exploratory Mission-Tropics (PEM-T) in the eastern equatorial Pacific during August-October 1996. Recent particle production was found in cloud-processed air over extended regions aloft (6–8 km). These were clearly associated with clean marine air lofted by deep convection and scavenged of most aerosol mass in the Intertropical Convergence Zone (ITCZ) and in more aged cloud-scavenged air influenced by a distant continental combustion near the South Pacific Convergence Zone (SPCZ). Recent particle production was evident in regions where sulfuric acid concentrations were about 0.5 to 1 × l07 molecules cm−3, when surface areas were near or below 5 µm2 cm−3, and when relative humidity (RH) was elevated over adjacent regions. In regions of recent particle production, the calculated critical sulfuric acid concentrations, based upon classical binary nucleation theory and corrected for in situ conditions near cloud, were generally consistent with nearby observed sulfuric acid concentrations. This indicates that classical binary nucleation theory and natural sources of sulfuric acid can account for nucleation in the near-cloud environment. Data from six equatorial flights between 20°N and 20°S demonstrate that this process populates extensive regions of the equatorial free troposphere with new particles. Vertical profiles suggest that nucleation, subsidence, and mixing into the MBL can supply the MBL with new aerosol.

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.

Meteorological Research Needs for Improved Air Quality Forecasting Report of the 11th Prospectus Development Team of the U.S. Weather Research Program

Abstract The U.S. Weather Research Program convenes expert working groups on a one-time basis to identify critical research needs in various problem areas. The most recent expert working group was charged to “identify and delineate critical meteorological research issues related to the prediction of air quality.” In this context, “prediction” is denoted as “forecasting” and includes the depiction and communication of the present chemical state of the atmosphere, extrapolation or nowcasting, and numerical prediction and chemical evolution on time scales up to several days. Emphasis is on the meteorological aspects of air quality. The problem of air quality forecasting is different in many ways from the problem of weather forecasting. The latter typically is focused on prediction of severe, adverse weather conditions, while the meteorology of adverse air quality conditions frequently is associated with benign weather. Boundary layer structure and wind direction are perhaps the two most poorly determined met...