S. C. Liu

Reactive nitrogen and its correlation with ozone in the lower stratosphere and upper troposphere

Reactive nitrogen (NOy) and O3 were measured simultaneously from a NASA ER-2 aircraft during 1987 through 1989. These high resolution measurements cover a broad range of latitudes in the upper troposphere and lower stratosphere. The data show a striking positive correlation between NOy and O3 in the lower stratosphere at all scales sampled. The ratio NOy/O3 is nearly independent of altitude from the tropopause to above 20 km, with ratios in the stratosphere of 0.0015–0.002 in the tropics and 0.0025–0.004 elsewhere. The ratio is much more constant than either individual species, thus providing an excellent reference point for comparing limited data sets with models. Two-dimensional models reproduce general features of the vertical profile of NOy/O3 but not the gradient in the lower stratosphere between tropics and mid-latitudes. NOy and O3 are better correlated in the lower stratosphere than in the upper troposphere. The magnitude and variability of both NOy mixing ratios and NOy/O3 ratios indicate a source of NOy in the upper troposphere. The most plausible source in the tropics is lightning production of NOx. Condensation of NOy onto aerosol particles is often possible in the tropical upper troposphere and may play a role in determining the vertical distribution of NOy. In the mid-latitude upper troposphere the data suggest long-range transport of NOy. NOy mixing ratios in the tropical upper troposphere were usually between 150 and 600 pptv, enough so that upward transport can affect the NOy abundance in the tropical lower stratosphere.

Hydrocarbon ratios and photochemical history of air masses

An effective and commonly used technique for studying the sources, photochemistry, and even the “photochemical age” of trace species is to examine ratios of hydrocarbons by assuming the ratio is independent of transport processes. We present results from mesoscale model calculations that suggest a significant effect by atmospheric mixing on the ratio. We also show that the photochemical age of an air mass derived from the ratio of hydrocarbons is a function of both photochemistry and atmospheric transport. Without additional information, it is not possible to derive a unique value for the age of an air mass from hydrocarbon ratios alone.

The photochemistry of acetone in the upper troposphere: A source of odd-hydrogen radicals

This paper summarizes measured photodissociation quantum yields for acetone in the 290-320 nm wavelength region for pressures and temperatures characteristic of the upper troposphere. Calculations combine this laboratory data with trace gas concentrations obtained during the NASA and NOAA sponsored Stratospheric Tracers of Atmospheric Transport (STRAT) field campaign, in which measurements of OH, HO_(2), odd-nitrogen, and other compounds were collected over Hawaii, and west of California during fall and winter of 1995/1996. OH and HO_(2) concentrations within 2 to 5 km layers just below the tropopause are ∼50% larger than expected from O_(3), CH_(4), and H_(2)O chemistry alone. Although not measured during STRAT, acetone is inferred from CO measurements and acetone-CO correlations from a previous field study. These inferred acetone levels are a significant source of odd-hydrogen radicals that can explain a large part of the discrepancy in the upper troposphere. For lower altitudes, the inferred acetone makes a negligible contribution to HO_(x) (HO+HO_(2)), but influences NO_(y) partitioning. A major fraction of HO_(x) production by acetone is through CH_(2)O formation, and the HO_(x) discrepancy can also be explained by CH_(2)O levels in the 20 to 50 pptv range, regardless of the source.

Measurements of aromatic hydrocarbon ratios and NOx concentrations in the rural troposphere: Observation of air mass photochemical aging and NOx removal

Abstract Measurements of the aromatic hydrocarbons (benzene, toluene, ethylbenzene and ortho(o)-xylene) at Niwot Ridge, Colorado have shown distinct correlations between the ratios of the concentrations of these compounds and the degree of direct urban influence. The major atmospheric removal mechanism of aromatic hydrocarbons is reaction with the hydroxyl radical, OH. This allows the decrease in the ratios of aromatic hydrocarbon concentrations to be related to the transport time and average OH number density within an air mass, if assumptions are made concerning background sources of aromatic hydrocarbons. Measured ratios of aromatic compounds at this site, along with ratios reported for several cities in the western United States, and estimates of transport times from these cities were used to calculate temporally and spatially averaged OH number densities. Hydroxyl radical number density estimates using toluene-, ethylbenzene-, and o-xylene-to-benzene ratios, were 1.2 ± 0.6 × 106, 1.0 ±0.8 × 106 and 0.48 ± 0.8 × 106 molecules cm−3, respectively. Considering the uncertainties in the assumptions used in the above estimates, we obtain a diurnal-average upper limit of 2.4 × 106 molecules cm−3. The correlations between measured ratios are found to yield slopes consistent with those predicted by experimental OH rate constants for benzene, toluene and ethylbenzene, and approximately a factor of two different in the case of benzene, toluene and o-xylene. The ratio of NOx: benzene was found to yield no correlation with toluene: benzene ratio for periods of westerly flow, but was well correlated with toluene: benzene ratio during periods of direct urban impact on the site (upslope easterly winds). The correlation of these ratios in urban plume air masses was consistent with NO2 + OH + M being the major daytime removal mechanism of NOx in the summertime.

Reactive nitrogen and ozone over the western Pacific: Distribution, partitioning, and sources

Measurements of important reactive nitrogen species (NO, NO2, HNO3, PAN, PPN, NO3−, NOy), C1 to C6 hydrocarbons, O3, chemical tracers (C2Cl4, CO), and meteorological parameters were made in the troposphere (0 to 12 km) over the western Pacific (0°–50°N) during the Pacific Exploratory Mission-West A campaign (September–October 1991). Under clean conditions, mixing ratios of NO, NO2, NOy, and O3 increased with altitude and showed a distinct latitudinal gradient. PAN showed a midtropospheric maximum, while nitric acid mixing ratios were generally highest near the surface. Measured NOy concentrations were significantly greater than the sum of individually measured nitrogen species (mainly NOx, PAN, and HNO3), suggesting that a large fraction of reactive nitrogen present in the atmosphere is made up of hitherto unknown species. This shortfall was larger in the tropics (≈65%) compared to midlatitudes (≈40%) and was minimal in air masses with high HNO3 mixing ratios (>100 ppt). A global three-dimensional photochemical model has been used to compare observations with predictions and to assess the significance of major sources. It is possible that the tropical lightning source is much greater than commonly assumed, and both lightning source and its distribution remain a major area of uncertainty in the budgets of NOy and NOx. A large disagreement between measurement and theory exists in the atmospheric distribution of HNO3. It appears that surface-based anthropogenic emissions provide nearly 65% of the global atmospheric NOy reservoir. Relatively constant NOx/NOy ratios imply that NOy and NOx are in chemical equilibrium and the NOy reservoir may be an important in situ source of atmospheric NOx. Data are interpreted to suggest that only about 20% of the upper tropospheric (7–12 km) NOx is directly attributable to its surface NOx source, and free tropospheric sources are dominant. In situ release of NOx from the NOy reservoir, lightning, direct transport of surface NOx, aircraft emissions, and small stratospheric input collectively maintain the NOx balance in the atmosphere. It is shown that atmospheric ratios of reactive nitrogen and sulfur species, along with trajectory analysis, can be used to pinpoint the source of Asian continental outflow. Compared to rural atmospheres over North America, air masses over the Pacific are highly efficient in net O3 production. Sources of tropospheric NOx cannot yet be accurately defined due to shortcomings in measurements and theory.

Temperature dependence of global precipitation extremes

[1] Data from the Global Precipitation Climatology Project (GPCP) covering the period 1979-2007 are examined for changes of precipitation extremes as a function of global mean temperature by using a new method which focuses on interannual differences rather than time series. We find that the top 10% bin of precipitation intensity increases by about 95% for each degree Kelvin (K) increase in global mean temperature, while 30%-60% bins decrease by about 20% K -1 . The global average precipitation intensity increases by about 23% K -1 , substantially greater than the increase of about 7% K -1 in atmospheric water-holding capacity estimated by the Clausius-Clapeyron equation. The large increase of precipitation intensity is qualitatively consistent with the hypothesis that the precipitation intensity should increase by more than 7% K -1 because of the additional latent heat released from the increased moisture. Our results also provide an independent evidence in support for significant increases in the number and/or size of strong global tropical cyclones. However an ensemble of 17 latest generation climate models estimates an increase of only about 2% K -1 in precipitation intensity, about one order of magnitude smaller than our value, suggesting that the risk of extreme precipitation events due to global warming is substantially greater than that estimated by the climate models.

Is ozone pollution affecting crop yields in China

Newly available data from non-urban locations in China along with regional model simulations suggest that ground-level ozone may be sufficiently high to affect Chinas winter wheat production. As non-urban ozone increases with industrialization, its effects on crops could hinder efforts to meet increasing food demands in the coming decades, in China.

Hydrocarbon ratios during PEM‐WEST A: A model perspective

A useful application of the hydrocarbon measurements collected during the Pacific Exploratory Mission (PEM-West A) is as markers or indices of atmospheric processing. Traditionally, ratios of particular hydrocarbons have been interpreted as photochemical indices, since much of the effect due to atmospheric transport is assumed to cancel by using ratios. However, an ever increasing body of observatonial and theoretical evidence suggests that turbulent mixing associated with atmospheric transport influences certain hydrocarbon ratios significantly. In this study a three-dimensional mesoscale photochemical model is used to study the interaction of photochemistry and atmospheric mixing on select hydrocarbons. In terms of correlations and functional relationships between various alkanes, the model results and PEM-West A hydrocarbon observations share many similar characteristics as well as explainable differences. When the three-dimensional model is applied to inert tracers, hydrocarbon ratios andother relationships exactly follow those expected by simple dilution with model-imposed “background air,” and the three-dimensional results for reactive hydrocarbons are quite consistent with a combined influence of photochemistry and simple dilution. Analogous to these model results, relationships between various hydrocarbons collected during the PEM-West A experiment appear to be consistent with this simplified picture of photochemistry and dilution affecting individual air masses. When hydrocarbons are chosen that have negligible contributions to clean background air, unambiguous determinations of the relative contributions to photochemistry and dilution can be estimated from the hydrocarbon samples. Both the three-dimensional model results and the observations imply an average characteristic lifetime for dilution with background air roughly equivalent to the photochemical lifetime of butane for the western Pacific lower troposphere. Moreover, the dominance of OH as the primary photochemical oxidant downwind of anthropogenic source regions can be inferred from correlations between the highly reactive alkane ratios. By incorporating back-trajectory information within the three-dimensional model analysis, a correspondence between time and a particular hydrocarbon or hydrocarbon ratio can be determined, and the influence of atmospheric mixing or photochemistry can be quantified. Results of the three-dimensional model study are compared and applied to the PEM-West A hydrocarbon dataset, yielding a practical methodology for determining average OH concentrations and atmospheric mixing rates from the hydrocarbon measurements. Aircraft data taken below 2 km during wall flights east of Japan imply a diurnal average OH concentration of ∼3 × 106 cm−3. The characteristic time for dilution with background air is estimated to be ∼2.5 days for the two study areas examined in this work.

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.

A regional model study of the ozone budget in the eastern United States

In order to better understand the photochemical and meteorological processes controlling regional scale air quality problems such as ozone formation, we have developed a three-dimensional Eulerian model and applied this model to a high-pressure period (July 4 to July 7, 1986) over the eastern United States. Meteorological and physical variables from a three-dimensional primitive-equation model are used to drive the transport parameters over a grid with 60×60 km2 horizontal resolution, and 15 unequally spaced vertical layers extending from the ground to roughly 15 km. The treatment and incorporation of the dynamic model, the transport model, surface deposition, emission of anthropogenic and natural O3 precursors, the chemical mechanism for 35 individual species, solar radiation and the numerical methods are discussed in detail. Model performance is tested by comparing model predicted O3 concentrations with observations from the U.S. Environmental Protection Agency ozone-monitoring network. Although a significant correlation between model and observed O3 is found, systematic discrepancies also exist and are discussed in relation to the basic model formulation, and variability in the observed O3. Additionally, a comparison of time-averaged NOx and anthropogenic nonmethane hydrocarbon (NMHC) concentrations to relatively long term observations provides a qualitative assessment of the models ability to simulate certain aspects of these O3 precursors from the few available observations. The model is used as a diagnostic tool to analyze various aspects of regional scale O3 formation and the budgets of the primary O3 precursors. Ozone formation over much of the continental model domain is shown to be NOx. limited. On the other hand, for midday NOx levels greater than about 4 or 5 ppbv, O3 formation is generally suppressed because of the low NMHC to NOx ratios (1 to 7) that are characteristic of the emissions inventory. Regional-scale budget analyses show that very Hide NOx or NMHC is transported to the free troposphere for the high-pressure conditions of this study and in the absence of a significant subgrid-scale vertical mixing process (i.e., efficient cumulus transport). We calculate a net turnover time of about 1.5 days for continental O3 below 1800 m with in situ photochemical formation being balanced by photochemical loss and transport off the American continent. The results of this work are intended to serve as a baseline for further model development.