Publications

A series of large mesoscale convective systems that occurred during the Brazilian phase of GTE/TRACE A (Transport and Atmospheric Chemistry near the Equator‐Atlantic) provided an opportunity to observe deep convective transport of trace gases from biomass burning. This paper reports a detailed analysis of flight 6, on September 27, 1992, which sampled cloud‐ and biomass‐burning‐perturbed regions north of Brasilia. High‐frequency sampling of cloud outflow at 9–12 km from the NASA DC‐8 showed enhancement of CO mixing ratios typically a factor of 3 above background (200–300 parts per billion by volume (ppbv) versus 90 ppbv) and significant increases in NO x and hydrocarbons. Clear signals of lightning‐generated NO were detected; we estimate that at least 40% of NO x at the 9.5‐km level and 32% at 11.3 km originated from lightning. Four types of model studies have been performed to analyze the dynamical and photochemical characteristics of the series of convective events. (1) Regional simulations for the period have been performed with the NCAR/Penn State mesoscale model (MM5), including tracer transport of carbon monoxide, initialized with observations. Middle‐upper tropospheric enhancements of a factor of 3 above background are reproduced. (2) A cloud‐resolving model (the Goddard cumulus ensemble (GCE) model) has been run for one representative convective cell during the September 26–27 episode. (3) Photochemical calculations (the Goddard tropospheric chemical model), initialized with trace gas observations (e.g., CO, NO x , hydrocarbons, O 3 ) observed in cloud outflow, show appreciable O 3 formation postconvection, initially up to 7–8 ppbv O 3 /d. (4) Forward trajectories from cloud outflow levels (postconvective conditions) put the ozone‐producing air masses in eastern Brazil and the tropical Atlantic within 2–4 days and over the Atlantic, Africa, and the Indian Ocean in 6–8 days. Indeed, 3–4 days after the convective episode (September 30, 1992), upper tropospheric levels in the Natal ozone sounding show an average increase of ∼30 ppbv (3 Dobson units (DU) integrated) compared to the September 28 sounding. Our simulated net O 3 production rates in cloud outflow are a factor of 3 or more greater than those in air undisturbed by the storms. Integrated over the 8‐ to 16‐km cloud outflow layer, the postconvection net O 3 production (∼5–6 DU over 8 days) accounts for ∼25% of the excess O 3 (15–25 DU) over the South Atlantic. Comparison of TRACE A Brazilian ozonesondes and the frequency of deep convection with climatology [ Kirchhoff et al. , this issue] suggests that the late September 1992 conditions represented an unusually active period for both convection and upper tropospheric ozone formation.

Supplement to: BrCN in the global troposphere 1 Formation of BrCN from HOBr chemistryThe formation of XCN compounds from reactions of HCN/CN¯ in the condensed phase is well known, so deserves consideration in the context of the polar boundary layer.The major sources of HCN to the atmosphere are biomass burning and vehicle exhaust, two sources that favor the northern hemisphere.HCN is a very weak acid (pKa=9.3), is not very soluble in water (Yoo et al., 1986), and its gas phase loss processes are quite slow (several years against reaction with OH).Consequently, HCN is widely distributed throughout the troposphere as is apparent in the ATom data shown in the main text.Vertical profiles of HCN imply a surface sink and the effective lifetime of HCN has been estimated to be about 5 months (Li et al., 2003).The data from ATom-3&4, including the vertical profile and inter-hemisphere distribution, are for the large part in agreement with this.

Remote and in situ measurements of gases and aerosols were made with airborne instrumentation to investigate the sources and sinks of tropospheric gases and aerosols over the western Pacific during the NASA Global Tropospheric Experiment (GTE)/Pacific Exploratory Mission‐West A (PEM‐West A) conducted in September–October 1991. This paper discusses the general characteristics of the air masses encountered during this experiment using an airborne lidar system for measurements of the large‐scale variations in ozone (O 3 ) and aerosol distributions across the troposphere and airborne in situ instrumentation for comprehensive measurements of air mass composition. In low latitudes of the western Pacific the airflow was generally from the east, and under these conditions the air was observed to have low aerosol loading and low ozone levels throughout the troposphere. Ozone was found to be below 10 parts per billion volume (ppbv) near the surface to 40–50 ppbv in the middle to upper troposphere. In the middle and high latitudes the airflow was mostly westerly, and the background O 3 was generally less than 55 ppbv. On 60% of the PEM‐West A flights, O 3 was observed to exceed these levels in regions that were determined to be associated with stratospheric intrusions. In convective outflows from typhoons, near‐surface air with low ozone (<25 ppbv) was transported into the upper troposphere (>10 km). Several cases of continental plumes from Asia were observed over the Pacific during westerly flow conditions. These plumes were found in the lower troposphere with ozone levels in the 60–80 ppbv range and enhanced aerosol scattering. At low latitudes over the central Pacific the troposphere primarily contained air with background or low ozone levels; however, stratospherically influenced air with enhanced ozone (40–60 ppbv) was observed several times in the lower troposphere. The frequency of observation of the air masses and their average chemical composition are also discussed in this paper.

Observations (0–8 km) from the Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment are analyzed to examine air masses contributing to the observed variability of springtime O 3 and its seasonal increase at 40°–85°N over North America. Factor analysis using the positive matrix factorization and principal component analysis methods is applied to the data set with 14 chemical tracers (O 3 , NO y , PAN, CO, CH 4 , C 2 H 2 , C 3 H 8 , CH 3 Cl, CH 3 Br, C 2 Cl 4 , CFC‐11, HCFC‐141B, Halon‐1211, and 7 Be) and one dynamic tracer (potential temperature). Our analysis results are biased by the measurements at 5–8 km (70% of the data) due to the availability of 7 Be measurements. The identified tracer characteristics for seven factors are generally consistent with the geographical origins derived from their 10 day back trajectories. Stratospherically influenced air accounts for 14 ppbv (35–40%) of the observed O 3 variability for data with O 3 concentrations <100 ppbv at middle and high latitudes. It accounts for about 2.5 ppbv/month (40%) of the seasonal O 3 trend at midlatitudes but for only 0.8 ppbv/month (<20%) at high latitudes, likely reflecting more vigorous midlatitude dynamical systems in spring. At midlatitudes, reactive nitrogen‐rich air masses transported through Asia are much more significant (11 ppbv in variability and 3.5 ppbv/month in trend) than other tropospheric contributors. At high latitudes the O 3 variability is significantly influenced by air masses transported from lower latitudes (11 ppbv), which are poor in reactive nitrogen. The O 3 trend, in contrast, is largely defined by air masses rich in reactive nitrogen transported through Asia and Europe across the Pacific or the Arctic (3 ppbv/month). The influence from the stratospheric source is more apparent at 6–8 km, while the effect of O 3 production and transport within the troposphere is more apparent at lower altitudes. The overall effect of tropospheric photochemical production, through long‐range transport, on the observed O 3 variability and its seasonal trend is more important at high latitudes relative to more photochemically active midlatitudes.

We estimate methane emissions from North America with high spatial resolution by inversion of Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) satellite observations using the Goddard Earth Observing System Chemistry (GEOS-Chem) chemical transport model and its adjoint. The inversion focuses on summer 2004 when data from the Intercontinental Chemical Transport Experiment-North America (INTEX-A) aircraft campaign over the eastern U.S. are available to validate the SCIAMACHY retrievals and evaluate the inversion. From the INTEX-A data we identify and correct a water vapor-dependent bias in the SCIAMACHY data. We conduct an initial inversion of emissions on the horizontal grid of GEOS-Chem (1/2° × 2/3°) to identify correction tendencies relative to the Emission Database for Global Atmospheric Research (EDGAR) v4.2 emission inventory used as a priori. We then cluster these grid cells with a hierarchical algorithm to extract the maximum information from the SCIAMACHY observations. A 1000 cluster ensemble can be adequately constrained, providing ~100 km resolution across North America. Analysis of results indicates that the Hudson Bay Lowland wetlands source is 2.1Tga-1, lower than the a priori but consistent with other recent estimates. Anthropogenic U.S. emissions are 30.1 ± 1.3 Tg a-1, compared to 25.8Tga-1 and 28.3 Tg a-1 in the EDGAR v4.2 and Environmental Protection Agency (EPA) inventories, respectively. We find that U.S. livestock emissions are 40% greater than in these two inventories. No such discrepancy is apparent for overall U.S. oil and gas emissions, although this may reflect some compensation between overestimate of emissions from storage/distribution and underestimate from production. We find that U.S. livestock emissions are 70% greater than the oil and gas emissions, in contrast to the EDGAR v4.2 and EPA inventories where these two sources are of comparable magnitude.

In situ measurements of a large number of trace chemicals from the midlatitude (37–57°N) lower stratosphere were performed with the NASA DC‐8 aircraft during March 1994. Deepest penetrations into the stratosphere (550 ppb O 3 , 279 ppb N 2 O, and 350 K potential temperature) corresponded to a region that has been defined as the “lowermost stratosphere” (LS) by Holton et al. [1995]. Analysis of data shows that the mixing ratios of long‐lived tracer species (e. g. CH 4 , HNO 3 , NO y , CFCs) are linearly correlated with those of O 3 and N 2 O. A ΔNO y /ΔO 3 of 0.0054 ppb/ppb and ΔNO y /ΔN 2 O of −0.081 ppb/ppb is in good agreement with other reported measurements from the DC‐8. These slopes are however, somewhat steeper than those reported from the ER‐2 airborne studies. We find that the reactive nitrogen budget in the LS is largely balanced with HNO 3 accounting for 80% of NO y , and PAN and NO x together accounting for 5%. A number of oxygenated species (e. g. acetone, H 2 O 2 ) were present and may provide an important in situ source of HO x in the LS. SO 2 mixing ratios were found to increase in the stratosphere at a rate that was comparable to the decline in OCS levels. No evidence of particle formation could be observed. Ethane, propane, and acetylene mixing ratios declined rapidly in the LS with Cl atoms likely playing a key role in this process. A number of reactive hydrocarbons/halocarbons (e. g. C 6 H 6 , CH 3 I) were present at low but measurable concentrations.

We report here large‐scale features of the distribution of NO x , HNO 3 , PAN, particle NO 3 − , and NO y 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 NO x limited, with NO x 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 NO y 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‐NO x ‐poor smoldering local biomass‐burning fires. NO x mixing ratios in biomass‐burning impacted air masses were usually <50 pptv, but those of HNO 3 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 NO y (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.

We have identified terrestrial sources of methyl iodide (CH 3 I) and assessed their importance in its atmospheric budget using a synthesis of field observations. Measurements include those from NASA DC‐8 research flights over the United States and the North Atlantic, the AIRMAP long‐term ground‐observing network in New England, and a field campaign at Duke Forest, North Carolina. We found an average CH 3 I flux of ∼2,700 ng m −2 d −1 to the atmosphere from midlatitude vegetation and soils, a value similar in magnitude to previous estimates of the oceanic source strength. The large‐scale aircraft measurements of vertical profiles over the continental U.S. showed CH 3 I‐mixing ratios comparable to and greater than those observed over the North Atlantic. Overall, midlatitude terrestrial biomes appear to contribute 33 Gg yr −1 to the CH 3 I global budget.

Abstract Aircraft observations of the four chloromethanes: carbon tetrachloride (CCl 4 ), methyl chloride (CH 3 Cl), dichloromethane (CH 2 Cl 2 ), and chloroform (CHCl 3 ), collected over North America between 2000 and 2022, were used to evaluate their vertical distributions and temporal trends in the atmosphere. We examine the vertical profiles, from the surface to the lower stratosphere (LS), of these increasingly important contributors to ozone‐depleting chlorine in both altitude and potential temperature space. Airborne chloromethane trends were compared with those measured at long‐term, ground‐based monitoring stations. Below 20 km altitude, CCl 4 trends were decreasing at all levels studied in the North American atmosphere (−1.1 ppt yr −1 ). CHCl 3 and CH 2 Cl 2 airborne observations were comparable to ground network measurements: CHCl 3 increased between 2000 and 2018 and then decreased leading to a negligible trend over the 22 years studied and CH 2 Cl 2 has been increasing at all levels in the troposphere (+2.41 ppt yr −1 , 2000–2022, <20 km).

Airborne measurements of a large number of oxygenated volatile organic chemicals (OVOC) were carried out in the Pacific troposphere (0.1–12 km) in winter/spring of 2001 (24 February to 10 April). Specifically, these measurements included acetone (CH 3 COCH 3 ), methylethyl ketone (CH 3 COC 2 H 5 , MEK), methanol (CH 3 OH), ethanol (C 2 H 5 OH), acetaldehyde (CH 3 CHO), propionaldehyde (C 2 H 5 CHO), peroxyacylnitrates (PANs) (C n H 2 n +1 COO 2 NO 2 ), and organic nitrates (C n H 2 n +1 ONO 2 ). Complementary measurements of formaldehyde (HCHO), methyl hydroperoxide (CH 3 OOH), and selected tracers were also available. OVOC were abundant in the clean troposphere and were greatly enhanced in the outflow regions from Asia. Background mixing ratios were typically highest in the lower troposphere and declined toward the upper troposphere and the lowermost stratosphere. Their total abundance (ΣOVOC) was nearly twice that of nonmethane hydrocarbons (ΣC 2 ‐C 8 NMHC). Throughout the troposphere, the OH reactivity of OVOC is comparable to that of methane and far exceeds that of NMHC. A comparison of these data with western Pacific observations collected some 7 years earlier (February–March 1994) did not reveal significant differences. Mixing ratios of OVOC were strongly correlated with each other as well as with tracers of fossil and biomass/biofuel combustion. Analysis of the relative enhancement of selected OVOC with respect to CH 3 Cl and CO in 12 plumes originating from fires and sampled in the free troposphere (3–11 km) is used to assess their primary and secondary emissions from biomass combustion. The composition of these plumes also indicates a large shift of reactive nitrogen into the PAN reservoir thereby limiting ozone formation. A three‐dimensional global model that uses state of the art chemistry and source information is used to compare measured and simulated mixing ratios of selected OVOC. While there is reasonable agreement in many cases, measured aldehyde concentrations are significantly larger than predicted. At their observed levels, acetaldehyde mixing ratios are shown to be an important source of HCHO (and HO x ) and PAN in the troposphere. On the basis of presently known chemistry, measured mixing ratios of aldehydes and PANs are mutually incompatible. We provide rough estimates of the global sources of several OVOC and conclude that collectively these are extremely large (150–500 Tg C yr −1 ) but remain poorly quantified.