A41C-01 INVITED 08:00h
Using measurements of OH, HO$_2$, and OH reactivity to study tropospheric oxidation processes
Much of tropospheric chemistry revolves around the hydroxyl radical, OH, and its reactions with a wide range of atmospheric constituents. Recognition of OH's importance more than three decades ago sparked an explosion in efforts to detect tropospheric OH. These efforts have in the last two decades culminated in a handful of different techniques that are now routinely being deployed around the globe in instrument complements designed to study tropospheric oxidation chemistry. Like other research groups, we have deployed our instruments to measure OH, HO$_2$, and OH reactivity in typically two intensive field campaigns a year for the last 5 years. What are the results of these recent studies? Modeled and measured OH and HO$_2$ generally agree to within a factor of two -- sometimes much better and sometimes much worse. However, two difficult-to-explain observations -- nighttime OH and the less-than-expected HO$_2$ decrease at high NO -- persist in most tropospheric environments. In this presentation, we discuss what our analyses of measurements have to say about tropospheric photochemistry and what we might want to do next.
A41C-02 08:30h
Interpretation of Recent Tropospheric Odd Hydrogen Radical Observations
There have been a number of airborne and ground-based measurement campaigns recently (e.g. TOPSE, TRACE-P, INTEX) that included measurements of peroxy radical and hydroxyl radical concentrations along with measurements of controlling species and parameters. These radicals are important indicators of our understanding of the fast photochemical processes operative in the troposphere. This paper will examine our understanding based on these measurements, and will suggest future observational strategies that could provide needed data to improve the representation of odd hydrogen radical chemistry in numerical models.
A41C-03 08:45h
Airborne Formaldehyde Measurements During INTEX-A: a Preliminary Look at Mixing Ratio Distributions Over North America
Formaldehyde (HCHO) is a key reactive trace gas present throughout the atmosphere and is involved in a number of important atmospheric processes, including hydrocarbon oxidation, ozone production, reactive hydrogen radical formation, and generation of carbon monoxide. Because of this importance and its integral role in helping to test photochemical reaction pathways, extensive measurements of HCHO and comparisons with box models have been carried out over wide geographic regions of the globe with varying levels of agreement. Despite this importance, very little is known about the vertical transport of HCHO from source regions in the boundary layer to the upper troposphere from convective outflow. This talk will present a preliminary look at HCHO mixing ratios, acquired by two independent instruments operated onboard NASA's DC-8 aircraft during the 2004 INTEX-A study, over wide geographic source regions of North America. This presentation will specifically highlight elevated HCHO mixing ratios in the upper troposphere from convective outflow
A41C-04 09:00h
Preliminary observations of methylhydroperoxide (CH$_{3}$OOH) and formaldehyde (CH$_{2}$O) in fresh convective outflow during INTEX-NA, summer 2004.
Studies have suggested that CH$_{3}$OOH and CH$_{2}$O are significant sources of HO$_{x}$, particularly in the upper atmosphere where the reactions of O$^{1}$D and H$_{2}$O do not dominate HO$_{x}$ production. Convection provides a transport mechanism for these HO$_{x}$ precursors to reach the upper atmosphere. In this study, we use the ratio of hydrogen peroxide (H$_{2}$O$_{2}$) to CH$_{3}$OOH, which can serve as an indication of fresh convective outflow given the difference in solubility of the two species, to identify convectively influenced air masses. Observations of H$_{2}$O$_{2}$, CH$_{3}$OOH, and CH$_{2}$O were made by two independent instruments on board the NASA DC-8 during the INTEX-NA science campaign (summer 2004). Preliminary results from INTEX-NA indicate the ratio of H$_{2}$O$_{2}$/CH$_{3}$OOH was 2-4 in the continental boundary layer while the ratio measured in fresh continental convective outflow was $\sim$1. Using this and other species ratios, we will examine fresh continental convective outflow over North America and ask the question, Do elevated CH$_{3}$OOH and CH$_{2}$O reflect simply the direct injection of these compounds or their precursors? Observations from the INTEX-NA campaign will be compared to results from the TOPSE program (winter/spring 2000), which showed CH$_{3}$OOH was not a significant source of HO$_{x}$, while CH$_{2}$O was a dominant source of HO$_{x}$ in the upper troposphere at higher latitudes.
A41C-05 09:15h
Measurements of Nitric Acid and Fine Aerosol Sulfate Made Aboard the NASA DC-8 During INTEX-A
We present measurements of HNO$_{3}$ and fine aerosol SO$_{4}$$^{=}$ made at 1.75 minute resolution from the NASA DC-8 during INTEX-A in summer 2004. INTEX-A addressed multiple objectives, including: satellite validation, testing chemical transport models, documenting the distribution of pollutants over North America (to better constrain emissions inventories), and sampling fresh North American outflow over the western North Atlantic as part of the multinational ICARTT experiment. Our preliminary analysis contributes to the last two objectives. In all regions over North America HNO$_{3}$ and aerosol SO$_{4}$$^{=}$ mixing ratios were highest in the boundary layer. Sulfate generally decreased less rapidly with altitude than HNO$_{3}$ did, reflecting production of HNO$_{3}$ in convective outflow and occasional stratospheric influence on the mid and upper troposphere. Regionally, boundary layer (below 1 kilometer) distributions reflected known large source regions with SO$_{4}$$^{=}$ mixing ratios highest in the south eastern U.S., while maximum HNO$_{3}$ (about 5 ppbv) was observed over CA and in the south east during regional pollution events. Average boundary layer HNO$_{3}$ mixing ratios in these regions were about 1 ppbv, twice the average of the north east boundary layer. Mixing ratios of HNO$_{3}$ and SO$_{4}$$^{=}$ in the outflow over the North Atlantic were lower than we observed immediately downwind of Asia during TRACE-P. This presumably reflects precipitation scavenging of these soluble compounds, since all major outflow events sampled during INTEX-A were associated with frontal systems and extensive clouds.
A41C-06 09:30h
Radiative Effect of Clouds on Tropospheric Chemistry in a Global Three-Dimensional Chemical Transport Model
Clouds exert an important influence on tropospheric photochemistry through modification of solar radiation which determines photolysis rates (J-values). We assess the radiative effect of clouds on photolysis rates and key oxidants in the troposphere with a global three-dimensional (3-D) chemical transport model (GEOS-CHEM) driven by assimilated meteorological observations from the Goddard Earth Observing System data assimilation system (GEOS DAS) at the NASA Global Modeling and Assimilation Office (GMAO). We focus on the year of 2001 with the GEOS-3 meteorological observations. Photolysis rates are calculated using the Fast-J algorithm of Wild et al. [2000], which uses a seven-wavelength quadrature scheme and accounts accurately for Mie scattering by clouds. The GEOS-3 global cloud optical depth and cloud fraction are evaluated with the satellite retrieval products from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the International Satellite Cloud Climatology Project (ISCCP). The impact of clouds on radiative transfer is assessed using three different methods: 1) the uniform cloud distribution method (most commonly used), 2) the random overlap scheme, and 3) the maximum-random overlap scheme. Results using the uniform cloud distribution method, which assumes linear scaling of cloud optical depth with cloud fraction in a grid-box, show global monthly mean OH concentrations generally increase by less than 5% due to the radiative effect of clouds. The OH distribution shows much larger changes, reflecting the opposite effects of enhanced (weakened) photochemistry above (below) clouds. Nevertheless, the change in the calculated methylchloroform lifetime is insignificant. The global monthly mean photolysis rates for J[O$_{3}$] and J[NO$_{2}$] in the troposphere are enhanced by less than 10% due to clouds; global mean O$_{3}$ concentrations in the troposphere are increased by less than 5%. Even though the uniform cloud distribution method presumably overestimates the backscattering of solar radiation above the clouds, our results emphasize that dominant effect of clouds is to influence the vertical redistribution of the intensity of photochemical activity while the global average effect remains modest, contrasting with previous studies. Using the random overlap scheme or the maximum-random overlap scheme reduces the impact of clouds on photochemistry, but does not change our results qualitatively. The results from sensitivity experiments where the model cloud optical depth is adjusted progressively will also be discussed.
A41C-07 09:45h
The Sensitivity of Ozone and Hydroxyl Radical Concentrations to Measured and Modeled Actinic Flux and Three Chemical Mechanisms
The field measurement program of the Central California Ozone Study (CCOS) was conducted during the summer of 2000 with an overall goal of improving the understanding of ozone formation over central and northern California. Measurements of actinic flux were made as part of the study using spectroradiometers located at University of California at Davis, Sunol, California and the Desert Research Institute at Reno, Nevada. The measured actinic flux was used, along with standard quantum yields and absorption cross-section data to calculate the photolysis rate parameters for nitrogen dioxide, ozone and formaldehyde, and a radiative transfer model was used to simulate these same photolysis rate parameters. An atmospheric chemistry box-model, OZIPR using the RADM2 mechanism, was used to estimate the significance of differences between rates based on models versus measurements for air quality assessment. The code for the OZIPR model was also modified to accommodate the Regional Atmospheric Chemistry Mechanism, (RACM), and the new Regional Atmospheric Chemistry Mechanism, Version 2 (RACM2). The modified model was used to compare the sensitivity of each mechanism to the photolytic rate parameters with respect to ozone formation and hydroxyl radical (HO) formation.