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Measurements of the 14C content of carbon dioxide in air collected by high‐altitude balloon flights in 2003–2005 reveal the contemporary radiocarbon distribution in the northern midlatitude stratosphere, four decades after the Limited Test Ban Treaty restricted atmospheric testing of nuclear weapons. Comparisons with results from a 3‐D chemical‐transport model show that the 14CO2distribution is now largely governed by the altitude/latitude dependence of the natural cosmogenic production rate, stratospheric transport, and propagation into the stratosphere of the decreasing radiocarbon trend in tropospheric CO2due to fossil fuel combustion. From the observed correlation of 14CO2with N2O mixing ratios, an annual global mean net flux of 14CO2to the troposphere of 1.6(±0.4) × 1017‰ mol CO2yr−1and a global production rate of 2.2(±0.6) × 1026atoms 14C yr−1are empirically derived. The results also indicate that contemporary 14CO2observations provide highly sensitive diagnostics for stratospheric transport and residence times in models. Stratospheric 14C is governed by cosmogenic production, transport, and stratosphere‐troposphere exchangeGlobal annual mean 14C production rate and net flux to the troposphere are determined empirically14C is a sensitive tracer of stratospheric transport and residence times

A recent airborne field campaign over the remote western Pacific obtained the first intensive in situ ozone sampling over the warm pool region from oceanic surface to 15 km altitude (near 360 K potential temperature level). The new data set quantifies ozone in the tropical tropopause layer under significant influence of convective outflow. The analysis further reveals a bimodal distribution of free tropospheric ozone mixing ratio. A primary mode, narrowly distributed around 20 ppbv, dominates the troposphere from the surface to 15 km. A secondary mode, broadly distributed with a 60 ppbv modal value, is prominent between 3 and 8 km (320 K to 340 K potential temperature levels). The latter mode occurs as persistent layers of ozone‐rich drier air and is characterized by relative humidity under 45%. Possible controlling mechanisms are discussed. These findings provide new insight into the physical interpretation of the “S”‐shaped mean ozone profiles in the tropics. New airborne in situ ozone data over tropical western Pacific from surface to 15 kmData show a dominant feature in ozone distribution near 20 ppbv throughout the troposphereA secondary mode near 60 ppbv is formed by O3‐enhanced and H2O‐reduced air masses

Environmental context. Air over the remote Southern Atlantic Ocean is amongst the cleanest anywhere on the planet. Yet in summer a large-scale natural phytoplankton bloom emits numerous natural reactive compounds into the overlying air. The productive waters also support a large squid fishing fleet, which emits significant amounts of NO and NO2. The combination of these natural and man-made emissions can efficiently produce ozone, an important atmospheric oxidant. Abstract. Ship-borne measurements have been made in air over the remote South Atlantic and Southern Oceans in January–March 2007. This cruise encountered a large-scale natural phytoplankton bloom emitting reactive hydrocarbons (e.g. isoprene); and a high seas squid fishing fleet emitting NOx(NO and NO2). Using an atmospheric chemistry box model constrained by in-situ measurements, it is shown that enhanced ozone production ensues from such juxtaposed marine biogenic and anthropogenic emissions. The relative impact of shipping and phytoplankton emissions on ozone was examined on a global scale using the EMAC model. Ozone in the marine boundary layer was found to be over ten times more sensitive to NOxemissions from ships, than to marine isoprene in the region south of 45°. Although marine isoprene emissions make little impact on the global ozone budget, co-located ship and phytoplankton emissions may explain the increasing ozone reported for the 40–60°S southern Atlantic region.

Elevated carbon monoxide (CO) mixing ratios in excess of those derived from emissions inventories have been observed in plumes from one gas- and coal-fired power plant and three of four lignite coal-fired electric utility power plants observed in east and central Texas. Observations of elevated CO on days characterized by differing wind directions show that CO emissions from the lignite plants were relatively constant over time and cannot be ascribed to separate sources adjacent to the power plants. These three plants were found to be emitting CO at rates 22 to 34 times those tabulated in State and Federal emissions inventories. Elevated CO emissions from the gas- and coal-fired plant were highly variable on time scales of hours to days, in one case changing by a factor of 8 within an hour. Three other fossil-fueled power plants, including one lignite-fired plant observed during this study, did not emit substantial amounts of CO, suggesting that a combination of plant operating conditions and the use of lignite coal may contribute to the enhanced emissions. Observed elevated CO emissions from the three lignite plants, if representative of average operating conditions, represent an additional 30% of the annual total CO emissions from point sources for the state of Texas.

Unprecedented growth of East Asian economies has led to increases of anthropogenic pollutants in the regional atmosphere. This pollutant burden is transported into the global atmosphere and is a significant source of intercontinental and transboundary anthropogenic pollution. This work analyzes pollution transport into the western Pacific associated with the dispersion of East Asian pollution during Northern Hemisphere winter. To examine transport characteristics, we use chemical and dynamical data sets obtained during the CONvective TRansport of Active Species in the Tropics (CONTRAST) field campaign, conducted from Guam during January–February 2014. We identify that the evolution of shear lines from decaying cold fronts and their southward advancement facilitates polluted air transport into low latitudes of the Western Pacific Ocean. Observations from two cases of shear line passage are analyzed. The result shows that this transport process significantly elevates anthropogenic trace gases in the marine boundary layer and lowermost free troposphere up to 3–4 km. Results of our analysis show that chemical influence of the shear line on the background tropical marine atmosphere varies as a function of pollution source, intensity, shear line strength, and the speed of advancement, as well as local background conditions. To quantify the contribution of shear‐line‐related transport, we introduce an index, the Anthropogenic Enhancement Factor (AEF), defined as a fractional change in mixing ratio of a gas brought about by the advancing front. This index shows that the most significant enhancements are for species with photochemical lifetimes comparable to their transport times from source regions. Elevated levels of anthropogenic trace gases transported in the lower atmosphere from East Asia were found behind shear lines in the deep tropical West Pacific during winterThe chemical impact of the pollutants associated with the shear line depends on source strength, shear line organization, speed of advancement, and background conditionsAnthropogenic Enhancement Factor is a metric developed to quantify the chemical impact of the air behind the shear line on the local background conditions

During the NEAQS‐ITCT2k4 campaign in New England, anthropogenic VOCs and CO were measured downwind from New York City and Boston. The emission ratios of VOCs relative to CO and acetylene were calculated using a method in which the ratio of a VOC with acetylene is plotted versus the photochemical age. The intercept at the photochemical age of zero gives the emission ratio. The so determined emission ratios were compared to other measurement sets, including data from the same location in 2002, canister samples collected inside New York City and Boston, aircraft measurements from Los Angeles in 2002, and the average urban composition of 39 U.S. cities. All the measurements generally agree within a factor of two. The measured emission ratios also agree for most compounds within a factor of two with vehicle exhaust data indicating that a major source of VOCs in urban areas is automobiles. A comparison with an anthropogenic emission database shows less agreement. Especially large discrepancies were found for the C2‐C4alkanes and most oxygenated species. As an example, the database overestimated toluene by almost a factor of three, which caused an air quality forecast model (WRF‐CHEM) using this database to overpredict the toluene mixing ratio by a factor of 2.5 as well. On the other hand, the overall reactivity of the measured species and the reactivity of the same compounds in the emission database were found to agree within 30%.

Bayesian inference has been used to determine rigorous estimates of hydroxyl radical concentrations () and air mass dilution rates (K) averaged following air masses between linked observations of nonmethane hydrocarbons (NMHCs) spanning the North Atlantic during the Intercontinental Transport and Chemical Transformation (ITCT)‐Lagrangian‐2K4 experiment. The Bayesian technique obtains a refined (posterior) distribution of a parameter given data related to the parameter through a model and prior beliefs about the parameter distribution. Here, the model describes hydrocarbon loss through OH reaction and mixing with a background concentration at rate K. The Lagrangian experiment provides direct observations of hydrocarbons at two time points, removing assumptions regarding composition or sources upstream of a single observation. The estimates are sharpened by using many hydrocarbons with different reactivities and accounting for their variability and measurement uncertainty. A novel technique is used to construct prior background distributions of many species, described by variation of a single parameter α. This exploits the high correlation of species, related by the first principal component of many NMHC samples. The Bayesian method obtains posterior estimates of , Kand αfollowing each air mass. Median values are typically between 0.5 and 2.0 × 106molecules cm−3, but are elevated to between 2.5 and 3.5 × 106molecules cm−3, in low‐level pollution. A comparison of estimates from absolute NMHC concentrations and NMHC ratios assuming zero background (the “photochemical clock” method) shows similar distributions but reveals systematic high bias in the estimates from ratios. Estimates of Kare ∼0.1 day−1but show more sensitivity to the prior distribution assumed.

Nonmethane hydrocarbon (NMHC) concentration ratios provide useful indicators of tropospheric oxidation and transport processes. However, the influences of both photochemical and mixing processes are inextricably linked in the evolution of these ratios. We present a model for investigating these influences by combining the transport treatment of the Lagrangian particle dispersion model FLEXPART with an ultrasimple (i.e., constant OH concentration) chemical treatment. Required model input includes NMHC emission ratios, but not ad hoc assumed background NMHC concentrations. The model results give NMHC relationships that can be directly compared, in a statistical manner, with measurements. The measured concentration ratios of the longest‐lived alkanes show strong deviations from purely kinetic behavior, which the model nicely reproduces. In contrast, some measured aromatic ratio relationships show even stronger deviations that are not well reproduced by the model for reasons that are not understood. The model‐measurement comparisons indicate that the interaction of mixing and photochemical processing prevent a simple interpretation of “photochemical age,” but that the average age of any particular NMHC can be well defined and can be approximated by a properly chosen and interpreted NMHC ratio. In summary, the relationships of NMHC concentration ratios not only yield useful measures of photochemical processing in the troposphere, but also provide useful test of the treatment of mixing and chemical processing in chemical transport models.

During the summer of 2004 large wildfires were burning in Alaska and Canada, and part of the emissions were transported toward the northeast United States, where they were measured during the NEAQS‐ITCT 2k4 (New England Air Quality Study–Intercontinental Transport and Chemical Transformation) study on board the NOAA WP‐3 aircraft and the NOAA research vessel Ronald H. Brown. Using acetonitrile and chloroform as tracers the biomass burning and the anthropogenic fraction of the carbon monoxide (CO) enhancement are determined. As much as 30% of the measured enhancement is attributed to the forest fires in Alaska and Canada transported into the region, and 70% is attributed to the urban emissions of mainly New York and Boston. On some days the forest fire emissions were mixed down to the surface and dominated the CO enhancement. The results compare well with the FLEXPART transport model, indicating that the total emissions during the measurement campaign for biomass burning might be about 22 Tg. The total U.S. anthropogenic CO sources used in FLEXPART are 25 Tg. FLEXPART model, using the U.S. EPA NEI‐99 data, overpredicts the CO mixing ratio around Boston and New York in 2004 by about 50%.