[1] The distribution and atmospheric budgets for molecular hydrogen and its deuterium component dD are simulated with the GEOS-Chem global chemical transport model and constrained by observations of H 2 from the NOAA Climate Monitoring and Diagnostics Laboratory network and dD observations from ship and ground stations. Our simulation includes a primary H 2 source of 38.8 Tg a À1 (22.7 Tg a À1 from fossil and biofuels, 10.1 Tg a À1 from biomass burning, 6.0 Tg a À1 from the ocean) (where a is years) and a secondary photochemical source from photolysis of formaldehyde of 34.3 Tg a À1 . The simulated global tropospheric mean H 2 is 525 ppbv, with a tropospheric burden of 141 Tg and tropospheric lifetime of 1.9 a. Uptake by enzymes in soils accounts for 75% of the H 2 sink, with the remainder due to reaction with OH. The model captures the observed latitudinal, vertical, and seasonal variations of H 2 . For dD we find that a photochemical source signature from methane and biogenic volatile organic compound oxidation of 162% yields a global mean atmospheric dD of 130%, consistent with atmospheric observations. The model captures the observed latitudinal gradient in dD, simulating a 21% greater enrichment in the Southern Hemisphere because of the predominance of isotopically depleted fossil fuel emissions in the Northern Hemisphere. We find that stratospheric-tropospheric exchange results in 37% enrichment of tropospheric dD. Our simulation provides new simultaneous constraints on the H 2 soil sink (55 ± 8 Tg a À1 ), the ocean source (6 ± 3 Tg a À1 ), and the isotopic signature for photochemical production (162 ± 57%).Citation: Price, H., L. Jaeglé, A. Rice, P. Quay, P. C. Novelli, and R. Gammon (2007), Global budget of molecular hydrogen and its deuterium content: Constraints from ground station, cruise, and aircraft observations,

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Using a 15‐year record of O3 from Lassen Volcanic National Park, a rural elevated site in northern California, data from two aircraft campaigns conducted in 1984 and 2002 over the eastern North Pacific, and observations spanning 18 years from five U.S. west coast, marine boundary layer sites, we show that O3 in air arriving from the Eastern Pacific in spring has increased by approximately 10 ppbv, i.e. 30% from the mid 1980s to the present. This positive trend in O3 correlates with the increasing trend in global nitrogen oxide emissions, which is especially pronounced in Asia. As spring is the season of strongest transport of Asian emissions to the Pacific, we conclude that the emission trend is the most likely cause of the O3 trend.

[1] Measurements during the Intercontinental Transport and Chemical Transformation 2002 (ITCT 2K2) field study characterized the springtime, eastern Pacific ozone distribution at two ground sites, from the National Oceanic and Atmospheric Administration WP-3D aircraft, and from a light aircraft operated by the University of Washington. D. Jaffe and colleagues compared the 2002 ozone distribution with measurements made in the region over the two previous decades and show that average ozone levels over the eastern midlatitude Pacific have systematically increased by $10 ppbv in the last two decades. Here we provide substantial evidence that a marked change in the photochemical environment in the springtime troposphere of the North Pacific is responsible for this increased O 3 . This change is evidenced in the eastern North Pacific ITCT 2K2 study region by (1) larger increases in the minimum observed ozone levels compared to more modest increases in the maximum levels, (2) increased peroxyacetyl nitrate (PAN) levels that parallel trends in NO x emissions, and (3) decreased efficiency of photochemical O 3 destruction, i.e., less negative O 3 photochemical tendency (or net rate of O 3 photochemical production; P(O 3 )). This changed photochemical environment is hypothesized to be due to anthropogenic emissions from Asia, which are believed to have substantially increased over the two decades preceding the study. We propose that their influence has changed the springtime Pacific tropospheric photochemistry from predominately ozone destroying to more nearly ozone producing. However, chemical transport model calculations indicate the possible influence of a confounding factor; unusual transport of tropical air to the western North Pacific during one early field study may have played a role in this apparent change in the photochemistry.

During the spring of 2002, vertical profiles of O3, CO, nonmethane volatile organic compounds (VOCs), and total aerosol scattering were collected over the northwestern coast of Washington State as part of the University of Washington's Photochemical Ozone Budget of the Eastern North Pacific Atmosphere (PHOBEA) research campaign. These observations coincided with NOAA's Intercontinental Transport and Chemical Transformation 2002 (NOAA‐ITCT 2K2) project. Thirteen research flights were conducted from 29 March through 23 May and several well‐defined polluted layers of varying thickness (∼0.2 to >3 km) were observed at altitudes between 0 and 6 km. These layers were characterized by correlated enhancements of O3, CO, VOCs, and particles. We observed rapid transpacific transport of polluted air masses on 15 April and 14, 17, and 23 May 2002, with ΔO3 and ΔCO (where Δ refers to the enhancement over background) exceeding 30 and 60 ppbv, respectively, and total aerosol scattering of green light (“σsp (550 nm)”) exceeding 65 Mm−1. These episodes were efficient in transporting O3 to the northeast (NE) Pacific troposphere, with ΔO3/ΔCO ratios in the pollution layers varying from 0.22 to 0.42 mol mol−1. In contrast, the average Δσsp (550 nm)/ΔCO ratio of the mid‐May events (0.66 ± 0.21 (1σ)) was more than twice that of the 15 April event (0.32 ± 0.05). The correlation between O3, CO, aerosols, and VOCs coupled with back‐trajectory analyses, satellite data, and the GEOS‐CHEM global chemical transport model indicate that the primary source of pollution observed on 15 April originated from a mixture of Asian anthropogenic and biomass‐burning emissions. For the May events, our analyses indicate that the early onset of the 2002 Siberian fire season was a significant source of the pollution episodes observed in May.

[1] On the basis of observations from the 1997-2002 Photochemical Ozone Budget of the Northeast Pacific (PHOBEA) experiments, we have identified 11 transpacific long-range transport (LRT) episodes, which contain significantly elevated levels of CO, O 3 , and aerosol scattering. The LRT episodes were determined from aircraft and ground-based observations of CO, O 3 , aerosol scattering coefficient, and 281 whole air samples analyzed for nonmethane hydrocarbons (NMHC). The ratio of excess O 3 to excess CO (DO 3 /DCO) for the 11 LRT episodes ranged from À0.06 to 1.52. Lower DO 3 /DCO ratios (<0.10) are characteristic of LRT episodes transported in the boundary layer or in the presence of substantial mineral dust. These events lack O 3 enhancements, even though O 3 precursors (CO, NMHCs) are elevated. Ratios of DO 3 /DCO of 0.2-0.5 are characteristic of LRT episodes of industrial and/or biomass burning where excess CO is coincident with expected excesses in O 3 . High DO 3 /DCO ratios (>0.50) are found in LRT episodes transported higher in the free troposphere and are probably due to a mixing of LRT pollution plumes with ozone-rich upper tropospheric air. Using PHOBEA observations, backward trajectories, and data from other experiments in the North Pacific (TRACE-P, ACE-Asia, PEM-West B) we calculate OH concentrations using two different methods. For the LRT episodes we obtain mean OH concentrations between 1.9 Â 10 5 and 1.3 Â 10 6 molecules cm À3 . We also present a method using dispersion models and observations to calculate the rate of dilution, k dil , with background air during LRT. A low k dil indicates less mixing with background air during transport, while a high value represents more entrainment with background air. For the April 2001 LRT episode we calculate a mean k dil of 0.010 ± 0.004 hr À1 and an OH radical concentration of 2 Â 10 5 molecules cm À3 . On the basis of these calculations we find that the large mineral dust transport episode, which took place in April 2001, was associated with the lowest OH concentration of the 11 episodes considered, implicating a strong role for heterogeneous chemistry during LRT.

Airborne observations of NMHCs, O3, CO, and aerosol scatter were made near the coast of Washington State from 29 March to 6 May 2001 as part of the Photochemical Ozone Budget of the Eastern North Pacific‐II (PHOBEA‐II) experiment. These observations overlapped the time period of the TRACE‐P (24 February to 10 April 2001) and ACE‐ASIA (27 March to 30 April 2001) experiments operating in the Western Pacific. Measurements were made during 12 flights at 48.31 ± 0.03°N latitude, 124.63 ± 0.08°W longitude at altitudes from 0 to 6 km. On several flights, significant enhancements in all species were observed and are attributed to transport from the Eurasian continent, including a long‐range transport event observed on 14 April 2001. This event contained substantial CO, NMHC, and aerosol loadings and was identified by the Total Ozone Mapping Spectrometer (TOMS) aboard the Earth Probe Satellite and airborne and surface measurements throughout North America. This airmass was unique in that it contained the highest levels of aerosol scatter, CO, and various NMHCs observed in 2001, was the only flight with a low Ångstrom coefficient (0.7) indicating dominance of super micron aerosols, and had a negative relationship between ozone and aerosol scatter (r = −0.30). Within this mineral dust and pollution layer, aerosol scatter, propane, and CO were enhanced by 1054%, 85%, and 36%, respectively, over the observed spring 2001 median values between 3.5 and 6 km. A comparison of our previous aircraft campaign in 1999 with 2001 observations shows that ozone, aerosol scatter, and most NMHCs were significantly lower in the spring of 2001. The exact cause is still under investigation, but the combination of elevated ozone, aerosol scatter, and NMHCs suggests a combustion source that was enhanced and/or transported more efficiently during the spring of 1999.

The atmospheric molecular hydrogen concentration and its deuterium abundance were measured in remote air samples collected onboard six Pacific Ocean ship transects between 37°N and 77°S during years 2001 through 2005. The data reveal a year‐round interhemispheric gradient in H2 concentration and isotopic composition with the extratropical Northern Hemisphere lower in H2 concentration by 17 ± 11 ppb and δD of H2 by 16 ± 12‰ than the Southern Hemisphere (95% confidence). On the basis of these snapshots, the interhemispheric gradient in δD was observed to be smallest in September through November, a time that experiences the largest gradient in concentration, and the largest in April, a time that has a small gradient in concentration. A simple hemispheric box model of the atmosphere indicates that, while the hemispheric asymmetry in soil sink of H2 is primarily responsible for the observed interhemispheric gradient in H2 concentration, the hemispheric difference in the δD of the H2 sources and sinks are equally responsible for the observed interhemispheric gradient in δD. Both the inverse correlation between interhemispheric H2 and δD gradients and their seasonal changes point to the importance of the H2 produced by photochemical sources. Comparisons with a three‐dimensional chemical transport model shows reasonable agreement with mean behavior in both variables and provides an accounting for H2 sources and sinks within ±15% without a dramatic change in the H2 budget. Anomalous H2 concentrations and δD in tropics and low‐latitude regions observed during the November–December 2001 meridional H2 and δD snapshot is thought to be a result of H2 emissions from biomass burning, possibly from continental Africa.