J. R. Olson

Assessment of upper tropospheric HOx sources over the tropical Pacific based on NASA GTE/PEM data: Net effect on HOx and other photochemical parameters

Data for the tropical upper troposphere (8–12 km, 20°N-20°S) collected during NASAs Pacific Exploratory Missions have been used to carry out a detailed examination of the photochemical processes controlling HOx (OH+HO2). Of particular significance is the availability of measurements of nonmethane hydrocarbons, oxygenated hydrocarbons (i.e., acetone, methanol, and ethanol) and peroxides (i.e., H2O2 and CH3OOH). These observations have provided constraints on model calculations permitting an assessment of the potential impact of these species on the levels of HOx, CH3O2, CH2O, as well as ozone budget parameters. Sensitivity calculations using a time-dependent photochemical box model show that when constrained by measured values of the above oxygenated species, model estimated HOx levels are elevated relative to unconstrained calculations. The impact of constraining these species was found to increase with altitude, reflecting the systematic roll-off in water vapor mixing ratios with altitude. At 11–12 km, overall increases in HOx approached a factor of 2 with somewhat larger increases being found for gross and net photochemical production of ozone. While significant, the impact on HOx due to peroxides appears to be less than previously estimated. In particular, observations of elevated H2O2 levels may be more influenced by local photochemistry than by convective transport. Issues related to the uncertainty in high-altitude water vapor levels and the possibility of other contributing sources of HOx are discussed. Finally, it is noted that the uncertainties in gas kinetic rate coefficients at the low temperatures of the upper troposphere and as well as OH sensor calibrations should be areas of continued investigation.

HOx chemistry during INTEX‐A 2004: Observation, model calculation, and comparison with previous studies

OH and HO_2 were measured with the Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) as part of a large measurement suite from the NASA DC-8 aircraft during the Intercontinental Chemical Transport Experiment-A (INTEX-A). This mission, which was conducted mainly over North America and the western Atlantic Ocean in summer 2004, was an excellent test of atmospheric oxidation chemistry. The HOx results from INTEX-A are compared to those from previous campaigns and to results for other related measurements from INTEX-A. Throughout the troposphere, observed OH was generally 0.95 of modeled OH; below 8 km, observed HO_2 was generally 1.20 of modeled HO_2. This observed-to-modeled comparison is similar to that for TRACE-P, another midlatitude study for which the median observed-to-modeled ratio was 1.08 for OH and 1.34 for HO_2, and to that for PEM-TB, a tropical study for which the median observed-to-modeled ratio was 1.17 for OH and 0.97 for HO_2. HO_2 behavior above 8 km was markedly different. The observed-to-modeled HO_2 ratio increased from ∼1.2 at 8 km to ∼3 at 11 km with the observed-to-modeled ratio correlating with NO. Above 8 km, the observed-to-modeled HO_2 and observed NO were both considerably greater than observations from previous campaigns. In addition, the observed-to-modeled HO_2/OH, which is sensitive to cycling reactions between OH and HO_2, increased from ∼1.5 at 8 km to almost 3.5 at 11 km. These discrepancies suggest a large unknown HO_x source and additional reactants that cycle HO_x from OH to HO_2. In the continental planetary boundary layer, the observed-to-modeled OH ratio increased from 1 when isoprene was less than 0.1 ppbv to over 4 when isoprene was greater than 2 ppbv, suggesting that forests throughout the United States are emitting unknown HO_x sources. Progress in resolving these discrepancies requires a focused research activity devoted to further examination of possible unknown OH sinks and HO_x sources.

Seasonal differences in the photochemistry of the South Pacific: A comparison of observations and model results from PEM-Tropics A and B

A time-dependent photochemical box model is used to examine the photochemistry of the equatorial and southern subtropical Pacific troposphere with aircraft data obtained during two distinct seasons: the Pacific Exploratory Mission-Tropics A (PEM-Tropics A) field campaign in September and October of 1996 and the Pacific Exploratory Mission-Tropics B (PEM-Tropics B) campaign in March and April of 1999. Model-predicted values were compared to observations for selected species (e.g., NO2, OH, HO2) with generally good agreement. Predicted values of HO2 were larger than those observed in the upper troposphere, in contrast to previous studies which show a general underprediction Of HO2 at upper altitudes. Some characteristics of the budgets Of HOx, NOx, and peroxides are discussed. The integrated net tendency for O3 is negative over the remote Pacific during both seasons, with gross formation equal to no more than half of the gross destruction. This suggests that a continual supply of O3 into the Pacific region throughout the year must exist in order to maintain O3 levels. Integrated net tendencies for equatorial O3 showed a seasonality, with a net loss of 1.06×1011 molecules cm−2 s−1 during PEM-Tropics B (March) increasing by 50% to 1.60×1011 molecules cm−2 s−1 during PEM-Tropics A (September). The seasonality over the southern subtropical Pacific was somewhat lower, with losses of 1.21×1011 molecules cm−2 s−1 during PEM-Tropics B (March) increasing by 25% to 1.51×1011 molecules cm−2 s−1 during PEM-Tropics A (September). While the larger net losses during PEM-Tropics A were primarily driven by higher concentrations of O3, the ability of the subtropical atmosphere to destroy O3 was ∼30% less effective during the PEM-Tropics A (September) campaign due to a drier atmosphere and higher overhead O3 column amounts.

OH and HO2 in the tropical Pacific: Results from PEM-Tropics B

OH and HO2 data collected on NASAs Pacific Exploratory Mission - Tropics B (PEM-Tropics B) are presented here and compared to results from a photochemical box model. PEM-Tropics B took place in the tropical Pacific in March and April of 1999 and examined photochemistry and sulfur chemistry in the remote tropical atmosphere. Altitude-resolved HOx budgets are presented. The model showed good overall agreement with the data, with a mean model to observed ratio of 0.86 for OH and 1.03 for HO2. The model tends to underpredict OH at higher altitudes and overpredict at low altitudes. The model agrees well with the HO2 observations at middle altitudes but tends to overpredict slightly at high and low altitudes.

Analysis of the distribution of ozone over the southern Atlantic region

Tropospheric ozone data measured by ozonesondes during the Transport and Atmospheric Chemistry Near the Equator-Atlantic (TRACE A) field mission and the multiyear pre-TRACE A program are analyzed jointly with tropospheric ozone amounts derived from remote satellite data (“residuals”). We present here the first detailed analysis of the entire Ascension Island pre-TRACE A data set. Data from the three pre-TRACE A ozonesonde sites are used to establish a coherent spatial and temporal climatology of ozone in the southern tropical Atlantic region. This analysis shows a significant ozone seasonality over the Atlantic region, with a period of maximum values that extends from the austral winter through at least October at Natal, Brazil, and Ascension Island. Concentrations begin to decline somewhat earlier at Brazzaville, Congo, especially at lower altitudes. Although Natal exhibits a significantly lower annual average than Ascension Island or Brazzaville by about 4 Dobson Units (DU), the magnitude of the seasonal amplitude at Natal is the largest of the three stations. Additionally, more of the seasonal amplitude at Natal is due to a contribution from ozone in the middle and upper troposphere than at either Ascension Island or Brazzaville. Amplitudes as large as 15 DU are measured at individual sites, and the residuals show an average amplitude over the southern tropical Atlantic region of 10–12 DU. Statistical comparison of the residuals to the ozonesonde climatology show that while the residuals tend to underpredict both the means and the seasonal amplitudes compared to the in situ data, they provide a good representation of the variance of ozone in this region and predict the local annual and seasonal means to within better than 10% and seasonal amplitudes to within 15%.

Radiative forcing of the Earth's climate system due to tropical tropospheric ozone production

The radiative forcing of the Earths climate system due to tropical tropospheric ozone is estimated using ozonesonde profiles and maps of the tropospheric ozone column derived from satellite data. The forcing is estimated using several different techniques in order to place bounds on its likely magnitude and to elucidate the role of biomass burning in producing the observed forcing. The results suggest that a widespread radiative forcing of at least 0.5 to 1 W m−2 exists over large areas in the tropics for much of the year. This radiative forcing is comparable in magnitude, but opposite in sign, to estimates of the aerosol forcing from tropical biomass burning. However, the burning contribution to ozone forcing is present over a larger geographic area than the aerosol forcing. The majority of the burning and thus these radiative forcings have likely been present only in the past century. These enhancements in tropical ozone are also estimated to be responsible for a radiative forcing of between 0.1 and 0.4 W m−2 when globally averaged. Comparison of these regional and global forcings due to tropical ozone changes to the estimates of forcing due to carbon dioxide and other trace gas increases since preindustrial times (about 2.45 W m−2) suggests that the effects of tropical ozone changes could be significant for evaluation of both regional and global anthropogenic forcing of climate.

Summertime distribution and relations of reactive odd nitrogen species and NOy in the troposphere over Canada

We report here large-scale features of the distribution of NOx, HNO3, PAN, particle NO3−, and NOy in the troposphere from 0.15 to 6 km altitude over central Canada. These measurements were conducted in July–August 1990 from the NASA Wallops Electra aircraft as part of the joint United States-Canadian Arctic Boundary Layer Expedition (ABLE) 3B-Northern Wetlands Study. Our findings show that this region is generally NOx limited, with NOx mixing ratios typically 20–30 parts per trillion by volume (pptv). We found little direct evidence for anthropogenic enhancement of mixing ratios of reactive odd nitrogen species and NOy above those in“background”air. Instead, it appears that enhancements in the mixing ratios of these species were primarily due to emissions from several day old or CO-rich-NOx-poor smoldering local biomass-burning fires. NOx mixing ratios in biomass-burning impacted air masses were usually <50 pptv, but those of HNO3 and PAN were typically 100–300 pptv representing a twofold-threefold enhancement over “background” air. During our study period, inputs of what appeared to be aged tropical air were a major factor influencing the distribution of reactive odd nitrogen in the midtroposphere over northeastern North America. These air masses were quite depleted in NOy (generally <150 pptv), and a frequent summertime occurrence of such air masses over this region would imply a significant influence on the reactive odd nitrogen budget. Our findings show that the chemical composition of aged air masses over subarctic Canada and those documented in the Arctic during ABLE 3A have strikingly similar chemistries, suggesting large-scale connection between the air masses influencing these regions.

Frequency and distribution of forest, savanna, and crop fires over tropical regions during PEM-Tropics A

Advanced very high resolution radiometer 1.1 km resolution satellite radiance data were used to locate active fires throughout much of the tropical region during NASAs Global Tropospheric Experiment (GTE) Pacific Exploratory Mission-Tropics (PEM-Tropics A) aircraft campaign, held in September and October 1996. The spatial and temporal distributions of the fires in Australia, southern Africa, and South America are presented here. The number of fires over northern Australia, central Africa, and South America appeared to decrease toward the end of the mission period. Fire over eastern Australia was widespread, and temporal patterns showed a somewhat consistent amount of burning with periodic episodes of enhanced fire counts observed. At least one episode of enhanced fire counts corresponded to the passage of a frontal system which brought conditions conducive to fire to the region, with strong westerlies originating over the hot, dry interior continent. Regions that were affected by lower than normal rainfall during the previous wet season (e.g., northern Australia and southwestern Africa) showed relatively few fires during this period. This is consistent with a drought-induced decrease in vegetation and therefore a decreased availability of fuel for burning. Alternatively, a heavier than normal previous wet season along the southeastern coast of South Africa may have contributed to high fuel loading and an associated relatively heavy amount of burning compared to data from previous years.

An assessment of cloud effects on photolysis rate coefficients: Comparison of experimental and theoretical values

An assessment ofthe effects of clouds on photolysis rate coefficients was carried out using three different experimental methods involving two different aircraft platforms. This evaluation was based on data recorded during NASAs Pacific Exploratory Mission (PEM)-Tropics A program in August-October 1996. On the NASA DC-8, upward and downward looking J(NO 2 ) filter radiometers and spectroradiometers were employed. For the NASA P-3B, the instrumentation consisted of Eppley radiometers. Although each aircraft typically sampled the same geographic region, coincident measurements occurred for only one brief period in the marine boundary layer near Christmas Island (2°N, 157°W). All three methods were compared for this flight period; however, only the J(NO 2 ) filter radiometers and spectroradiometers could be compared for the entire campaign. For the Christmas Island sampling period, all three radiometric measurements disagreed in magnitude but exhibited trends consistent with model-calculated photolysis rate coefficients. Overall, the results showed that the J(NO 2 ) filter radiometers and spectroradiometers exhibited a consistent disagreement of 30%, the J(NO 2 ) filter radiometers being higher. Eppley-derived values of J(NO 2 ) fell between those of the J(NO 2 ) filter radiometers and spectroradiometers. An examination of the variation in J(O 1 D) and J(NO 2 ) based on the output from the spectroradiometers and J(NO 2 ) filter radiometers suggests that differences between each photolysis rate coefficients response to cloud effects tend to be smaller than model uncertainties. Thus J(O 1 D) and other photolysis rate coefficients can be corrected for cloud effects based on the response of J(NO 2 ).

Role of convection in redistributing formaldehyde to the upper troposphere over North America and the North Atlantic during the summer 2004 INTEX campaign

Measurements of formaldehyde (CH2O) from a tunable diode laser absorption spectrometer (TDLAS) were acquired onboard the NASA DC-8 aircraft during the summer 2004 INTEX-NA campaign to test our understanding of convection and CH2O production mechanisms in the upper troposphere (UT, 6–12 km) over continental North America and the North Atlantic Ocean. The present study utilizes these TDLAS measurements and results from a box model to (1) establish sets of conditions by which to distinguish “background” UT CH2O levels from those perturbed by convection and other causes; (2) quantify the CH2O precursor budgets for both air mass types; (3) quantify the fraction of time that the UT CH2O measurements over North America and North Atlantic are perturbed during the summer of 2004; (4) provide estimates for the fraction of time that such perturbed CH2O levels are caused by direct convection of boundary layer CH2O and/or convection of CH2O precursors; (5) assess the ability of box models to reproduce the CH2O measurements; and (6) examine CH2O and HO2 relationships in the presence of enhanced NO. Multiple tracers were used to arrive at a set of UT CH2O background and perturbed air mass periods, and 46% of the TDLAS measurements fell within the latter category. In general, production of CH2O from CH4 was found to be the dominant source term, even in perturbed air masses. This was followed by production from methyl hydroperoxide, methanol, PAN-type compounds, and ketones, in descending order of their contribution. At least 70% to 73% of the elevated UT observations were caused by enhanced production from CH2O precursors rather than direct transport of CH2O from the boundary layer. In the presence of elevated NO, there was a definite trend in the CH2O measurement–model discrepancy, and this was highly correlated with HO2 measurement–model discrepancies in the UT.