Publications

Abstract. Glyoxal (CHOCHO), the simplest dicarbonyl in the troposphere, is a potential precursor for secondary organic aerosol (SOA) and brown carbon (BrC) affecting air quality and climate. The airborne measurement of CHOCHO concentrations during the KORUS-AQ (KORea–US Air Quality study) campaign in 2016 enables detailed quantification of loss mechanisms pertaining to SOA formation in the real atmosphere. The production of this molecule was mainly from oxidation of aromatics (59 %) initiated by hydroxyl radical (OH). CHOCHO loss to aerosol was found to be the most important removal path (69 %) and contributed to roughly ∼ 20 % (3.7 µg sm−3 ppmv−1 h−1, normalized with excess CO) of SOA growth in the first 6 h in Seoul Metropolitan Area. A reactive uptake coefficient (γ) of ∼ 0.008 best represents the loss of CHOCHO by surface uptake during the campaign. To our knowledge, we show the first field observation of aerosol surface-area-dependent (Asurf) CHOCHO uptake, which diverges from the simple surface uptake assumption as Asurf increases in ambient condition. Specifically, under the low (high) aerosol loading, the CHOCHO effective uptake rate coefficient, keff,uptake, linearly increases (levels off) with Asurf; thus, the irreversible surface uptake is a reasonable (unreasonable) approximation for simulating CHOCHO loss to aerosol. Dependence on photochemical impact and changes in the chemical and physical aerosol properties “free water”, as well as aerosol viscosity, are discussed as other possible factors influencing CHOCHO uptake rate. Our inferred Henry's law coefficient of CHOCHO, 7.0×108 M atm−1, is ∼ 2 orders of magnitude higher than those estimated from salting-in effects constrained by inorganic salts only consistent with laboratory findings that show similar high partitioning into water-soluble organics, which urges more understanding on CHOCHO solubility under real atmospheric conditions.

We use a 3‐D chemical transport model (the GEOS‐Chem CTM) to evaluate a global emission inventory for ethane (C 2 H 6 ), with a best estimate for the global source of 13 Tg yr −1 , 8.0 Tg yr −1 from fossil fuel production, 2.6 Tg yr −1 from biofuel, and 2.4 Tg yr −1 from biomass burning. About 80% of the source is emitted in the Northern Hemisphere. The model generally provides a reasonable and unbiased simulation of surface air observations, column measurements, and aircraft profiles worldwide, including patterns of geographical and seasonal variability. The main bias is a 20%–30% overestimate at European surface sites. Propagation of the C 2 H 6 seasonal signal from northern midlatitudes to the equatorial western Pacific and the southern tropics demonstrates the dominance of northern midlatitudes as a source of C 2 H 6 worldwide. Interhemispheric transport provides the largest C 2 H 6 source to the Southern Hemisphere (1.7 Tg yr −1 ), and southern biomass burning provides the other major source (1.0 Tg yr −1 ). The C 2 H 6 emission inventory for the United States from the Environmental Protection Agency (0.6 Tg yr −1 ) is considerably lower than our estimate constrained by extensive aircraft observations in the continental boundary layer (2.4 Tg yr −1 ). This appears to reflect a factor 7 underestimate in the fossil fuel source over the south‐central United States. Our estimate of C 2 H 6 emissions, together with observed ratios of CH 4 :C 2 H 6 , suggests that CH 4 emissions from energy production in the U.S. may be underestimated by as much as 50%–100%.

During the Pacific Exploration of Asian Continental Emission (PEACE) phase A mission in January 2002, we launched ozonesondes in subtropical southeast China at Hong Kong (114.17°E, 22.31°N), middle latitude northeast China at Beijing (116.47°E, 39.81°N), and northwest China at Xining (101.45°E, 36.43°N) in order to study long‐range ozone (O 3 ) transport from Eurasia, tropospheric O 3 sources in China, and O 3 outflow to the Pacific. Tropospheric O 3 showed a complex vertical distribution with average tropospheric O 3 columns of 39 ± 4, 23 ± 3, and 30 ± 6 DU in Hong Kong, Beijing, and Xining, respectively, which accounted for 17 ± 2%, 7 ± 1%, and 10 ± 1% of the total O 3 column. The lower tropospheric and boundary layer (BL) O 3 over Xining and especially Beijing exhibited low values, suggesting negligible O 3 formation in middle latitudes of China during the winter season. The results also revealed frequent propagation of enhanced O 3 layers from the lower stratosphere to the upper troposphere over Xining and especially Beijing, suggesting that stratospheric O 3 is an important source of O 3 in the upper troposphere of northern China. This “natural” O 3 is transported downwind by the prevailing westerly wind and acts as a source of O 3 to the east Asian coast and northwestern Pacific. We observed elevated O 3 , with a maximum mixing ratio up to 111 ppbv, at 1.5 km in the upper BL over Hong Kong. The elevated O 3 was resulted from transport of pollutants from northwest‐central or southwest China and regional O 3 formation and accumulation in south China including the Pearl River Delta and Hong Kong. We also observed enhanced O 3 (>95 ppbv) in the middle and upper troposphere over Hong Kong in air masses transported along the subtropical jet from tropical and subtropical East Africa, south Asia, and Southeast Asia. The O 3 enhancements were most likely due to intrusion of stratospheric O 3 into the troposphere in the Indo‐Burmese region of tropical Southeast Asia, where substantial downward motion had been observed.

Abstract. The sulfur cycle plays a key role in atmospheric air quality, climate, and ecosystems. In this study, we compare the spatial and temporal distribution of four sulfur-containing species, dimethyl sulfide (DMS), sulfur dioxide (SO2), particulate methanesulfonate (MSA), and particulate sulfate (SO4), that were measured during the airborne NASA Atmospheric Tomography (ATom) mission and simulated by five AeroCom-III models to analyze the budget of sulfur cycle from the models. This study focuses on remote regions over the Pacific, Atlantic, and Southern Oceans from near the ocean surface to ~12-km altitude range, and covers all four seasons. These regions provide us with highly heterogeneous natural and anthropogenic source environments, which is not usually the case for traditional continental studies. We examine the vertical and seasonal variations of these sulfur species over tropical, mid-, and high-latitude regions in both hemispheres. We identify their origins from land versus ocean and from anthropogenic versus natural sources with sensitivity studies by applying tagged tracers linking to emission types and regions. In general, the differences among model results can be greater than one-order of magnitude. Comparing with observations, simulated SO2 is generally low while SO4 is high, and the model-observation agreement is much better in ATom-4 (April–May, 2018). There are much larger DMS concentrations simulated close to the sea surface than observed, indicating that the DMS emissions may be too high from all models. Anthropogenic emissions are the dominant source (40–60 % of the total amount) for atmospheric sulfate simulated at locations and times along the ATom flight tracks at almost every altitude, followed by volcanic emissions (18–32 %) and oceanic sources (16–32 %). Similar source contributions can also be derived at broad ocean basin and monthly scales, indicating that any reductions of anthropogenic sulfur emissions would have global impacts in modern times.

Bulk aerosol sampling of soluble ionic compounds from the NASA Wallops Island P‐3 aircraft and a tower on Christmas Island during PEM‐Tropics B provides an opportunity to assess the magnitude of particle losses in the University of New Hampshire airborne bulk aerosol sampling system. We find that most aerosol‐associated ions decrease strongly with height above the sea surface, making direct comparisons between mixing ratios at 30 m on the tower and the lowest flight level of the P‐3 (150 m) open to interpretation. Theoretical considerations suggest that vertical gradients of sea‐salt aerosol particles should show exponential decreases with height. Observed gradients of Na + and Mg 2+ , combining the tower observations with P‐3 samples collected below 1 km, are well described by exponential decreases ( r values of 0.88 and 0.87, respectively), though the curve fit underestimates average mixing ratios at the surface by 25%. Cascade impactor samples collected on the tower show that >99% of the Na + and Mg 2+ mass is on supermicron particles, 65% is in the 1–6 micron range, and just 20% resides on particles with diameters larger than 9 microns. These results indicate that our airborne aerosol sampling probes must be passing particles up to at least 6 microns with high efficiency. We also observed that nss SO 4 2− and NH 4 + , which are dominantly on accumulation mode particles, tended to decrease between 150 and 1000 m, but they were often considerably higher at the lowest P‐3 sampling altitudes than at the tower. This finding is presently not well understood.

▪ Abstract This paper reviews the available data and models on energy and material flows through the world's 25 largest cities. Throughput is categorized as stored, transformed, or passive for the major flow modes. The aggregate, fuel, food, water, and air cycles are all examined. Emphasis is placed on atmospheric pathways because the data are abundant. Relevant models of urban energy and material flows, demography, and atmospheric chemistry are discussed. Earth system–level loops from cities to neighboring ecosystems are identified. Megacities are somewhat independent of their immediate environment for food, fuel, and aggregate inputs, but all are constrained by their regional environment for supplying water and absorbing wastes. We elaborate on analogies with biological metabolism and ecosystem succession as useful conceptual frameworks for addressing urban ecological problems. We conclude that whereas data are numerous for some individual cities, cross-cutting compilations are lacking in biogeochemical analysis and modeling. Synthesis of the existing information will be a crucial first step. Cross-cutting field research and integrated, multidisciplinary simulations will be necessary.

Gas phase elemental mercury (Hg°) was measured aboard the NASA DC-8 research aircraft during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign conducted in spring 2008 primarily over the North American Arctic. We examined the vertical distributions of Hg° and ozone (O3) together with tetrachloroethylene (C2Cl4), ethyne (C2H2), and alkanes when Hg°- and O3-depleted air masses were sampled near the surface (<1 km). This study suggests that Hg° and O3 depletions commonly occur over linear distances of ~20–200 km. Horizontally there was a sharp decreasing gradient of ~100 ppqv in Hg° over <10 km in going from the bay near Ellesmere Island to the frozen open ocean. There was a distinct land-ocean difference in the vertical thickness of the Hg°-depleted layer – being variable but around a few 100 meters over the ocean whereas occurring only very near the surface over land. Data support that atmospheric mercury depletion events are driven by Hg° reactions with halogen atoms. Derived from data collected aboard the DC-8, we present mathematical expressions giving the rates of Hg° and O3 depletions as a function of the radical concentrations. These relationships can be a useful metric to evaluate models that attempt to reproduce springtime Arctic Hg° and/or O3 depletion events, and they can also be employed to obtain order-of-magnitude estimates of radical concentrations and the ratio [Br]/[Cl].

Phase A of the NASA Pacific Exploratory Mission in the western North Pacific (PEM‐West A) region was conducted during September–October 1991. The background meteorology of eastern Asia and the western North Pacific region during the PEM‐West A study is described. Mean large‐scale flow patterns are discussed along with transient synoptic scale features (e.g., midlatitude cyclones, anticyclones, and frontal systems) responsible for long‐range transport of trace species over the study region. Synoptic summaries are given for each of the 18 data flights, together with selected examples of meteorological processes that gave rise to some of the changes observed in the measured trace gases. Examples of large‐scale ozone features observed above and below the DC‐8 flight altitude by an onboard lidar system are also related to meteorological processes such as stratospheric‐tropospheric exchange and upward transport of air from the boundary layer.

The atmospheric CO 2 mixing ratio was measured using a continuous measurement system onboard a Gulfstream‐II aircraft between the northern midlatitudes and the southern subtropics during the Biomass Burning and Lightning Experiment Phase A (BIBLE A) campaign in September–October 1998. The vertical distribution of CO 2 over tropical regions was almost constant from the surface to an altitude of 13 km. CO 2 enhancements from biomass burning and oceanic release were observed in the tropical boundary layer. Measurements in the upper troposphere indicate interhemispheric exchange was effectively suppressed between 2°N–7°N. Interhemispheric transport of air in the upper troposphere was suppressed effectively in this region. The CO 2 mixing ratios in the Northern and Southern Hemispheres were almost constant, with an average value of about 365 parts per million (ppm) and 366 ppm, respectively. The correlation between the CO 2 and NO y mixing ratios observed north of 7°N was apparently different from that obtained south of 2°N. This fact strongly supports the result that the north‐south boundary in the upper troposphere during BIBLE A was located around 2°N–7°N as the boundary is not necessary a permanent feature.