Publications

Emissions of C₂-C₅ alkanes from the U.S. oil and gas sector have changed rapidly over the last decade. We use a nested GEOS-Chem simulation driven by updated 2011NEI emissions with aircraft, surface and column observations to 1) examine spatial patterns in the emissions and observed atmospheric abundances of C₂-C₅ alkanes over the U.S., and 2) estimate the contribution of emissions from the U.S. oil and gas industry to these patterns. The oil and gas sector in the updated 2011NEI contributes over 80% of the total U.S. emissions of ethane (C₂H₆) and propane (C₃H₈), and emissions of these species are largest in the central U.S. Observed mixing ratios of C₂-C₅ alkanes show enhancements over the central U.S. below 2 km. A nested GEOS-Chem simulation underpredicts observed C₃H₈ mixing ratios in the boundary layer over several U.S. regions and the relative underprediction is not consistent, suggesting C₃H₈ emissions should receive more attention moving forward. Our decision to consider only C₄-C₅ alkane emissions as a single lumped species produces a geographic distribution similar to observations. Due to the increasing importance of oil and gas emissions in the U.S., we recommend continued support of existing long-term measurements of C₂-C₅ alkanes. We suggest additional monitoring of C₂-C₅ alkanes downwind of northeastern Colorado, Wyoming and western North Dakota to capture changes in these regions. The atmospheric chemistry modeling community should also evaluate whether chemical mechanisms that lump larger alkanes are sufficient to understand air quality issues in regions with large emissions of these species.

Abstract. Data obtained in Hong Kong during the Hong Kong and the Pearl River Delta (PRD) Pilot Air Monitoring Study in autumn 2002 are analyzed to unravel the relationship between ground-level ozone (O3), pollution precursors, and cross-border transport. Ten ozone episodes, during which the hourly O3 concentration exceeded 100 ppbv in 9 cases and 90 ppbv in one case, are subject to detailed analysis, including one case with hourly O3 of 203 ppbv, which is the highest concentration on record to date in Hong Kong. Combined with high-resolution back trajectories, dCO/dNOy (the ratio of enhancement of CO concentration above background to that of NOy) is used to define whether O3 is locally or regionally produced. Five out of the ten Hong Kong O3-episodes studied show a "pollution signature" that is indicative of impact from Guangdong Province. Examination of speciated volatile organic compounds (VOCs) shows that the reactivity of VOCs is dominated by anthropogenic VOCs, of which the reactive aromatics dominate, in particular xylenes and toluene. Calculations using a photochemical box model indicate that between 50–100% of the O3 increase observed in Hong Kong during the O3 episodes can be explained by photochemical generation within the Hong Kong area, provided that nitrous acid (HONO) is present at the concentrations derived from this study. An Observation-Based Model (OBM) is used to calculate the sensitivity of the O3 production to changes in the concentrations of the precursor compounds. Generally the production of O3 throughout much of the Hong Kong area is limited by VOCs, while high nitric oxide (NO) concentrations suppress O3 concentration.

Terpenoids play a significant role in the formation of tropospheric ozone (O3) and secondary organic aerosol (SOA). While terpenoids are largely attributed to biogenic sources, they are also widely used in consumer products that end up in the atmosphe

NASA's Pacific Exploratory Mission‐Tropics (PEM‐T) experiment investigated the atmospheric chemistry of a large portion of the tropical and subtropical Pacific Basin during August to October 1996. This paper summarizes meteorological conditions over the PEM‐T domain. Mean flow patterns during PEM‐T are described. Important circulation systems near the surface include subtropical anticyclones, the South Pacific Convergence Zone (SPCZ), the Intertropical Convergence Zone (ITCZ), and middle latitude transient cyclones. The SPCZ and ITCZ are areas of widespread ascent and deep convection; however, there is relatively little lightning in these oceanic regions. A large area of subsidence is associated with the subtropical anticyclone centered near Easter Island. PEM‐T occurred during a period of near normal sea surface temperatures. When compared to an 11 year climatology (1986–1996), relatively minor circulation anomalies are observed during PEM‐T. Some of these circulation anomalies are consistent with much stronger anomalies observed during previous La Nina events. In general, however, the 1996 PEM‐T period appears to be climatologically representative. Meteorological conditions for specific flights from each major operations area are summarized. The vertical distribution of ozone along selected DC‐8 flights is described using the DIAL remote sensing system. These ozone distributions are related to thermodynamic soundings obtained during aircraft maneuvers and to backward trajectories that arrived at locations along the flight tracks. Most locations in the deep tropics are found to have relatively small values of tropospheric ozone. Backward trajectories calculated from global gridded analyses show that much of this air originates from the east and has not passed over land within 10 days. The deep convection associated with the ITCZ and SPCZ also influences the atmospheric chemistry of these regions. Flights over portions of the subtropics and middle latitudes document layers of greatly enhanced tropospheric ozone, sometimes exceeding 80 ppbv. In situ carbon monoxide in these layers often exceeds 90 ppbv. These regions are located near, and especially south of Tahiti, Easter Island, and Fiji. The layers of enhanced ozone usually correspond to layers of dry air, associated with widespread subsiding air. The backward trajectories show that air parcels arriving in these regions originate from the west, passing over Australia and even extending back to southern Africa. These are regions of biomass burning. The in situ chemical measurements support the trajectory‐derived origins of these ozone plumes. Thus the enhanced tropospheric ozone over the central Pacific Basin may be due to biomass burning many thousands of kilometers away. Middle‐latitude portions of the PEM‐T area are influenced by transient cyclones, and the DC‐8 traversed tropopause folds during several flights. The flight area just west of Ecuador experiences outflow from South America. Thus the biomass burning that is prevalent over portions of Brazil influences this area.

While observations at global background sites in east Asia suggested unexpectedly increased emissions of trichlorofluoromethane (CFC-11) after 2012 in eastern China, particularly in Shandong province, there is a lack of local monitoring data to constrain the emission estimates. Here we report observations of ambient CFC-11 during 2012–2018 in Shandong province at five sites, including three rural sites and two background sites [one at Tuoji Island (TJI) in the Bohai Sea and the other at Mount Tai (MT), 1534 m above sea level]. The mixing ratios of CFC-11 at rural sites were 17–23% above the global background levels at Mauna Loa, Hawaii, and observations at MT and TJI revealed larger enhancements occurring in air masses traveling through the polyurethane foam industry region of the Shandong Peninsula. On the basis of the ratios of CFC-11 to tracers such as carbon monoxide (CO), chloroform (CHCl3), and carbon tetrachloride (CCl4) at the MT and TJI sites, the estimated emissions of CFC-11 in eastern China ranged from 12.0 ± 1.6 to 20.8 ± 3.9 Gg year–1 with an average of 14.7 ± 4.3 Gg year–1 in 2014 and 2017–2018. These tracer-based estimates may represent the upper limits due to relatively higher ratios of CFC-11 to tracers in the hot spot province.

Abstract. The National Aeronautics and Space Administration (NASA) conducted the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) mission during 2008 as a part of the International Polar Year (IPY). The purpose of ARCTAS was to study the factors responsible for changes in the Arctic's atmospheric composition and climate. A major emphasis was to investigate Arctic haze, which is most pronounced during winter and early spring. This study focuses on the spring phase of ARCTAS (ARCTAS-A) that was based in Alaska during April 2008. Although anthropogenic emissions historically have been associated with Arctic haze, biomass burning dominated the ARCTAS-A period and has been the focus of many ARCTAS related studies. This study determines the common pathways for anthropogenic emissions during ARCTAS-A. Trajectories (air parcels) are released each day from three historically significant regions of anthropogenic emissions (Asia, North America, and Europe). These fifteen day forward trajectories are calculated using data from the Weather Research and Forecasting (WRF) model at 45 km horizontal resolution. The trajectories then are examined to determine: origins of emissions that reach the Arctic (defined as north of 70° N) within fifteen days, pathways of the emissions reaching the Arctic, Arctic entry locations, and altitudes at which the trajectories enter the Arctic. These results serve as regional "climatologies" for the ARCTAS-A period. Three cases during the ARCTAS-A period (one for each of the regions above) are examined using backward trajectories and chemical fingerprinting based on in situ data sampled from the NASA DC-8. The fingerprinting utilizes volatile organic compounds that represent pure anthropogenic tracers, Asian anthropogenic pollution, incomplete combustion, and natural gas emissions. We determine flight legs containing anthropogenic emissions and the pathways travelled by these emissions. Results show that the DC-8 sampled anthropogenic emissions from Asia, North America, and Europe during the spring phase of ARCTAS. The pathways travelled by these emissions agree with our derived "climatologies" and previous studies of Arctic transport. Meteorological analysis and trajectory calculations indicate that middle latitude cyclones and their associated warm conveyor belts play an important role in lofting the surface based emissions to their sampling altitude in all three cases.

Airborne measurements of volatile organic compounds (VOCs) were performed during CalNex 2010 (California Research at the Nexus of Air Quality and Climate Change) in the Los Angeles (LA) basin in May–June 2010 and during ITCT2k2 (Intercontinental Transport and Chemical Transformation) in May 2002. While CO 2 enhancements in the basin were similar between the two years, the ΔCO/ΔCO 2 ratio had decreased by about a factor of two. The ΔVOC/ΔCO emission ratios stayed relatively constant between the two years. This indicates that, relative to CO 2 , VOCs in the LA basin also decreased by about a factor of two since 2002. These data are compared with the results from various previous field campaigns dating back as early as 1960 and from the extensive air quality monitoring system in the LA basin going back to 1980. The results show that the mixing ratios of VOCs and CO have decreased by almost two orders of magnitude during the past five decades at an average annual rate of about 7.5%. Exceptions to this trend are the small alkanes ethane and propane, which have decreased slower due to the use and production of natural gas. A comparison with trends in London, UK shows that, due to stricter regulations at the time, VOC mixing ratios in LA decreased earlier than in London, albeit at a slower rate, such that typical mixing ratios in both cities in 2008 were at about the same level.

A three‐dimensional mesoscale transport/photochemical model is used to study the transport and photochemical transformation of trace species over eastern Asia and western Pacific for the period from September 20 to October 6, 1991, of the Pacific Exploratory Mission‐West A experiment. The influence of emissions from the continental boundary layer that was evident in the observed trace species distributions in the lower troposphere over the ocean is well simulated by the model. In the upper troposphere, species such as O 3 , NO y (total reactive nitrogen species), and SO 2 which have a significant source in the stratosphere are also simulated well in the model, suggesting that the upper tropospheric abundances of these species are strongly influenced by stratospheric fluxes and upper tropospheric sources. In the case of SO 2 the stratospheric flux is identified to be mostly from the Mount Pinatubo eruption. Concentrations in the upper troposphere for species such as CO and hydrocarbons, which are emitted in the continental boundary layer and have a sink in the troposphere, are significantly underestimated by the model. Two factors have been identified to contribute significantly to the underestimate: one is emissions upwind of the model domain (eastern Asia and western Pacific); the other is that vertical transport is underestimated in the model. Model results are also grouped by back trajectories to study the contrast between compositions of marine and continental air masses. The model‐calculated altitude profiles of trace species in continental and marine air masses are found to be qualitatively consistent with observations. However, the difference in the median values of trace species between continental air and marine air is about twice as large for the observed values as for model results. This suggests that the model underestimates the outflow fluxes of trace species from the Asian continent and the Pacific rim countries to the ocean. Observed altitude profiles for species like CO and hydrocarbons show a negative gradient in continental air and a positive gradient in marine air. A mechanism which may be responsible for the altitude gradients is proposed.

The radiative impact of the mixing state of black carbon (BC) aerosol is investigated in Asian outflow. The mixing state and size distribution of BC aerosol were measured with a ground‐based single‐particle soot photometer at a remote island (Fukue) in Japan in spring 2007. The mass concentration of BC in Asian continental air masses reached 0.5 μ g m −3 , with a mass median diameter of 200–220 nm. The median value of the shell/core diameter ratio increased to ∼1.6 in Asian continental and maritime air masses with a core diameter of 200 nm, while in free tropospheric and Japanese air masses it was 1.3–1.4. On the basis of theoretical calculations using the size distribution and mixing state of BC aerosol, scattering and absorption properties of PM 1 aerosols were calculated under both dry and ambient conditions, considering the hygroscopic growth of aerosols. It was estimated that internal mixing enhanced the BC absorption by a factor of 1.5–1.6 compared to external mixing. The calculated absorption coefficient was 2–3 times higher in Asian continental air masses than in clean air. Coatings reduced the single‐scattering albedo (SSA) of PM 1 aerosol by 0.01–0.02, which indicates the importance of the mixing state of BC aerosol in evaluating its radiative influence. The SSA was sensitive to changes in air mass type, with a value of ∼0.98 in Asian continental air masses and ∼0.95 in Japanese and free tropospheric air masses under ambient conditions.

Vertical profiles in the lower troposphere of temperature, relative humidity, sulfur dioxide (SO 2 ), ozone (O 3 ), condensation nuclei (CN), and carbon monoxide (CO), and horizontal distributions of twenty gaseous and particulate species, are presented for five regions of southern Africa during the dry biomass burning season of 2000. The regions are the semiarid savannas of northeast South Africa and northern Botswana, the savanna‐forest mosaic of coastal Mozambique, the humid savanna of southern Zambia, and the desert of western Namibia. The highest average concentrations of carbon dioxide (CO 2 ), CO, methane (CH 4 ), O 3 , black particulate carbon, and total particulate carbon were in the Botswana and Zambia sectors (388 and 392 ppmv, 369 and 453 ppbv, 1753 and 1758 ppbv, 79 and 88 ppbv, 2.6 and 5.5 μg m −3 , and 13.2 and 14.3 μg m −3 ). This was due to intense biomass burning in Zambia and surrounding regions. The South Africa sector had the highest average concentrations of SO 2 , sulfate particles, and CN (5.1 ppbv, 8.3 μg m −3 , and 6400 cm −3 , respectively), which derived from biomass burning and electric generation plants and mining operations within this sector. Air quality in the Mozambique sector was similar to the neighboring South Africa sector. Over the arid Namibia sector there were polluted layers aloft, in which average SO 2 , O 3 , and CO mixing ratios (1.2 ppbv, 76 ppbv, and 310 ppbv, respectively) were similar to those measured over the other more polluted sectors. This was due to transport of biomass smoke from regions of widespread savanna burning in southern Angola. Average concentrations over all sectors of CO 2 (386 ± 8 ppmv), CO (261 ± 81 ppbv), SO 2 (2.5 ± 1.6 ppbv), O 3 (64 ± 13 ppbv), black particulate carbon (2.3 ± 1.9 μg m −3 ), organic particulate carbon (6.2 ± 5.2 μg m −3 ), total particle mass (26.0 ± 4.7 μg m −3 ), and potassium particles (0.4 ± 0.1 μg m −3 ) were comparable to those in polluted, urban air. Since the majority of the measurements in this study were obtained in locations well removed from industrial sources of pollution, the high average concentrations of pollutants reflect the effects of widespread biomass burning. On occasions, relatively thin (∼0.5 km) layers of remarkably clean air were located at ∼3 km above mean sea level, sandwiched between heavily polluted air. The data presented here can be used for inputs to and validation of regional and global atmospheric chemical models.

Two of the DC‐8 flights during the 1991–1992 second Airborne Arctic Stratospheric Expedition (AASE 2) were between California and Tahiti. Extremely abrupt changes in O 3 and NO y were observed on both flights as the aircraft crossed the subtropical jet. They indicate that the width of the transition from midlatitude to tropical air in the troposphere can be as short as 1 km. The NO y /O 3 ratio was remarkably stable across the transition. We discuss some of the dynamical features associated with the transitions and speculate on the reasons for their abruptness. They occurred south of the subtropical frontal zone and were accompanied by changes in humidity, NO x /NO y , and modest changes in CO, CH 4 , and CO 2 . In addition, a chemical model constrained by measurements of the long‐lived species is used to simulate the variation of NO x /NO y along the two flight tracks. Although this model is quite successful at simulating observed NO x /NO y in midlatitude air, it drastically overestimates NO x /NO y in tropical air. The rate at which the model converts NO x to HNO 3 via the NO 2 + OH reaction is very slow in the upper tropical troposphere because the low O 3 concentrations and cold temperatures force most of the NO x to be in the form of NO during the day. We argue that there is an important NO to NO 2 pathway in this region not presently included in models, that much of the NO y is in a stable (possibly aerosol) form that is not readily converted to NO x , or that there has been insufficient time since convection for NO x to be released from other more stable forms of NO y . It is important to resolve this discrepancy because present models which have the correct O 3 and NO y may overestimate O 3 production rates and OH concentrations in the upper tropical troposphere.

<strong class="journal-contentHeaderColor">Abstract.</strong> The NASA Atmospheric Tomography (ATom) mission built a photochemical climatology of air parcels based on in situ measurements with the NASA DC-8 aircraft along objectively planned profiling transects through the middle of the Pacific and Atlantic oceans. In this paper we present and analyze a data set of 10 s (2 km) merged and gap-filled observations of the key reactive species driving the chemical budgets of O<span class="inline-formula">₃</span> and CH<span class="inline-formula">₄</span> (O<span class="inline-formula">₃</span>, CH<span class="inline-formula">₄</span>, CO, H<span class="inline-formula">₂</span>O, HCHO, H<span class="inline-formula">₂</span>O<span class="inline-formula">₂</span>, CH<span class="inline-formula">₃</span>OOH, C<span class="inline-formula">₂</span>H<span class="inline-formula">₆</span>, higher alkanes, alkenes, aromatics, NO<span class="inline-formula">_<i>x</i></span>, HNO<span class="inline-formula">₃</span>, HNO<span class="inline-formula">₄</span>, peroxyacetyl nitrate, other organic nitrates), consisting of 146 494 distinct air parcels from ATom deployments 1 through 4. Six models calculated the O<span class="inline-formula">₃</span> and CH<span class="inline-formula">₄</span> photochemical tendencies from this modeling data stream for ATom 1. We find that 80 %–90 % of the total reactivity lies in the top 50 % of the parcels and 25 %–35 % in the top 10 %, supporting previous model-only studies that tropospheric chemistry is driven by a fraction of all the air. In other words, accurate simulation of the least reactive 50 % of the troposphere is unimportant for global budgets. Surprisingly, the probability densities of species and<span id="page13730"/> reactivities averaged on a model scale (100 km) differ only slightly from the 2 km ATom data, indicating that much of the heterogeneity in tropospheric chemistry can be captured with current global chemistry models. Comparing the ATom reactivities over the tropical oceans with climatological statistics from six global chemistry models, we find excellent agreement with the loss of O<span class="inline-formula">₃</span> and CH<span class="inline-formula">₄</span> but sharp disagreement with production of O<span class="inline-formula">₃</span>. The models sharply underestimate O<span class="inline-formula">₃</span> production below 4 km in both Pacific and Atlantic basins, and this can be traced to lower NO<span class="inline-formula">_<i>x</i></span> levels than observed. Attaching photochemical reactivities to measurements of chemical species allows for a richer, yet more constrained-to-what-matters, set of metrics for model evaluation.