E. Kosciuch

Measurements of OH, H2SO4, and MSA at the South Pole during ISCAT

The first measurements of OH, H2SO4, and MSA performed at the South Pole as part of the Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) study are presented. OH concentrations were found to be quite elevated for such a dry environment, with average values of 2x106 molecule cm−3. Model simulations suggest that much of the observed OH is a result of unexpectedly high NO concentrations. Concentrations of H2SO4 and MSA were generally low with average values of 2.5x105 and 1x105 molecule cm−3, respectively. Major variations in the concentration levels of the above species were found to have a high correlation with changes in the polar mixing layer as estimated from the measured temperature difference from 22 to 2m above the snow surface. Chemical details are discussed.

Peroxy radical behavior during the Transport and Chemical Evolution over the Pacific (TRACE‐P) campaign as measured aboard the NASA P‐3B aircraft

[1] Peroxy radical concentrations were measured aboard the NASA P-3B aircraft during the Transport and Chemical Evolution over the Pacific (TRACE-P) campaign in the spring of 2001 and varied in ways that depended on radical production rates and reactive nitrogen concentrations. Measurements of HO2 ,H O2 +R O2, and OH during this study allowed calculation of radical ratios, examination of functional relationships of these ratios on controlling variables, and comparison with numerical model estimations. Radical production terms show changes in relative contributions at low, middle, and high total production rates that are understandable in terms of systematic variations in the controlling components (trace gas concentrations and photolysis rate coefficients). Ozone tendency calculations indicate net ozone production in the western Pacific basin because the concentrations of critical precursor trace gases (e.g., NOx, hydrocarbons) are highest there. The dependence of ozone tendency follows the concentration of NO systematically. Peroxy radical levels on the two aircraft (HO2 +R O2 on the P-3B and HO2 on the DC-8) during two relatively short prescribed intercomparison periods were in good agreement in one instance and poorer in another given reasonable assumptions about the apportioning of radicals between HO2 and RO2. Recommended changes to CH2O photolysis quantum yields, HO2 self reaction, and O( 1 D) quenching kinetics lead to small changes (<5%) in calculated peroxy radical levels for TRACE-P conditions. There is evidence from this campaign that peroxy radicals are lost by interaction with aerosols and cloud droplets. INDEX TERMS: 0317 Atmospheric Composition and Structure: Chemical kinetic and photochemical properties; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; KEYWORDS: photochemistry, peroxy radicals, ozone

Airborne observations of DMSO, DMS, and OH at marine tropical latitudes

This paper reports the first simultaneous fast-time resolution measurements of dimethylsulfide (DMS), dimethylsulfoxide (DMSO), and the hydroxyl radical (OH) at marine tropical latitudes. These observations were recorded during the PEM-Tropics B field program on NASAs P-3B aircraft. The observations of DMSO, using a Selected Ion Chemical Ionization Mass Spectrometry (SICIMS) technique, are of particular significance. They have revealed two unique findings: 1) average midday-tropical levels of DMSO are significantly higher than those predicted from current models when constrained by observed DMS and OH levels (e.g., 10 pptv versus 1 to 3 pptv); and 2) DMSO concentration profiles are significantly out-of-phase with model predictions, maximum values being seen under near dark conditions and minimum values being observed at midday. Although no simple explanation has yet been found for these unusual results, the fact that no evidence points to a problem in the measurements suggests that others may exist. Clearly, if the observations are correct, they indicate that at least for tropical upwelling regions the atmospheric sulfur budget may need to be adjusted to accommodate additional sources of DMSO.

Atmospheric chemistry results from the ANTCI 2005 Antarctic plateau airborne study

One of the major goals of the 2005 Antarctic Tropospheric Chemistry Investigation (ANTCI) was to bridge the information gap between current knowledge of South Pole (SP) chemistry and that of the plateau. The former has been extensively studied, but its geographical position on the edge of the plateau makes extrapolating these findings across the plateau problematic. The airborne observations reported here demonstrate that, as at SP, elevated levels of nitric oxide (NO) are a common summertime feature of the plateau. As in earlier studies, planetary boundary layer (PBL) variations were a contributing factor leading to NO fluctuations. Thus, extensive use was made of in situ measurements and models to characterize PBL depths along each flight path and over broader areas of the plateau. Consistent with earlier SP studies that revealed photolysis of nitrate in surface snow as the source of NO x , large vertical gradients in NO were observed over most plateau areas sampled. Similar gradients were also found for the nitrogen species HNO3 and HO2NO2 and for O3. Thus, a common meteorological-chemical feature found was shallow PBLs associated with nitrogen species concentrations that exceeded free tropospheric levels. Collectively, these new results greatly extend the geographical sampling footprint defined by earlier SP studies. In particular, they suggest that previous assessments of the plateau as simply a chemical depository need updating. Although the evidence supporting this position comes in many forms, the fact that net photochemical production of ozone occurs during summer months over extensive areas of the plateau is pivotal.

Nucleation in the equatorial Pacific during PEM-Tropics B: Enhanced boundary layer H2SO4 with no particle production

During the second phase of the NASA Pacific Exploratory Mission to the Pacific Tropics (PEM-Tropics B), regions of unusually high concentrations of sulfuric acid vapor ranging from 107 to 108 molecules cm−3 were detected near the ocean surface in equatorial regions between Hawaii and Tahiti. No 3–4 nm diameter nanoparticles were observed near the ocean surface where acid concentrations were highest; however, 3–4 nm particles were detected at higher elevations in regions near clouds. Calculations show that in some regions of high acid concentrations newly formed particles would be readily detected by our instruments and thus the lack of nanoparticles suggests that there was no nucleation. In contrast, in the previous PEM-Tropics A mission Clarke et al. [1998a] observed a large nucleation event in the equatorial marine boundary layer under similar temperatures, relative humidity, and sulfuric acid concentrations. Comparison between these two studies further demonstrates that some additional species or unknown process is necessary to significantly enhance nucleation in the remote marine troposphere and that this component is not always present at levels sufficient to sustain nucleation throughout the region, even at a low continuous rate. We speculate that if ammonia is one example of a critical nucleation precursor, tropospheric ternary nucleation (sulfuric acid/ammonia/water) under some conditions requires ammonia concentrations to be greater than typical background concentrations for particle production via this mechanism.

Relationship between OH measurements on two different NASA aircraft during PEM Tropics B

OH measurements were performed on both the P3-B and the DC-8 aircraft throughout Pacific Exploratory Mission (PEM) Tropics B, using two very different measurement techniques. While no direct comparison was possible because of the difference in flight paths of the two aircraft, two brief measurement periods in close proximity allow at least a glimpse of how the two measurements compare. The first comparison took place in the marine boundary layer and showed exceptionally good agreement between the two aircraft OH measurements. The second set of close proximity flights at 5.5-km altitude resulted in an average concentration difference approximately equal to the uncertainty limit associated with either individual instrument but much less than the combined uncertainties from all the contributing measurements. In both cases the comparisons were made by normalizing the data with a photostationary state model. In addition, a comparison of the OH concentrations measured by each aircraft from 25°N latitude to 25°S latitude over the entire longitude and altitude range of the mission resulted in agreement within ∼10% but with a trend of higher DC-8/P-3B OH ratios at higher altitudes. While a definitive comparison of these two OH instruments must await a far more rigorous future test, the present results show that there were no clear OH measurement discrepancies observed. The dramatic differences in these two measurement techniques also minimizes the chance of common interferences or calibration errors.