Nicola J. Blake

Hydrocarbon and halocarbon measurements as photochemical and dynamical indicators of atmospheric hydroxyl, atomic chlorine, and vertical mixing obtained during Lagrangian flights

Nonmethane hydrocarbons and halocarbons were measured during two Lagrangian experiments conducted in the lower troposphere of the North Atlantic as part of the June 1992, Atlantic Stratosphere Transition Experiment/Marine Aerosol and Gas Exchange (ASTEX/MAGE) expedition. The first experiment was performed in very clean marine air. Meteorological observations indicate that the height of the marine boundary layer rose rapidly, entraining free tropospheric air. However, the free tropospheric and marine boundary layer halocarbon concentrations were too similar to allow this entrainment to be quantified by these measurements. The second Lagrangian experiment took place along the concentration gradient of an aged continental air mass advecting from Europe. The trace gas measurements confirm that the National Center for Atmospheric Research (NCAR) Electra aircraft successfully intercepted the same air mass on consecutive days. Two layers, a surface layer and a mixed layer with chemically distinct compositions, were present within the marine boundary layer. The composition of the free troposphere was very different from that of the mixed layer, making entrainment from the free troposphere evident. Concentrations of the nonmethane hydrocarbons in the Lagrangian surface layer were observed to become depleted relative to the longer-lived tetrachloroethene. A best fit to the observations was calculated using various combinations of the three parameters, loss by reaction with hydroxyl, loss by reaction with chlorine, and/or dilution from the mixed layer. These calculations provided estimated average concentrations in the surface layer for a 5-hour period from dawn to 11 UT of 0.3 ± 0.5 × 106 molecules cm−3 for HO, and 3.3 ± 1.1 × 104 molecules cm−3 for Cl. Noontime concentration estimates were 2.6 ± 0.7 × 106 molecules cm−3 for HO and 6.5 ± 1.4 × 104 molecules cm−3 for Cl.

Distribution and fate of selected oxygenated organic species in the troposphere and lower stratosphere over the Atlantic

A large number of oxygenated organic chemicals (peroxyacyl nitrates, alkyl nitrates, acetone, formaldehyde, methanol, methylhydroperoxide, acetic acid and formic acid) were measured during the 1997 Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) airborne field campaign over the Atlantic. In this paper, we present a first picture of the distribution of these oxygenated organic chemicals (Ox-organic) in the troposphere and the lower stratosphere, and assess their source and sink relationships. In both the troposphere and the lower stratosphere, the total atmospheric abundance of these oxygenated species (ΣOx-organic) nearly equals that of total nonmethane hydrocarbons (ΣNMHC), which have been traditionally measured. A sizable fraction of the reactive nitrogen (10–30%) is present in its oxygenated organic form. The organic reactive nitrogen fraction is dominated by peroxyacetyl nitrate (PAN), with alkyl nitrates and peroxypropionyl nitrate (PPN) accounting for <5% of total NOy. Comparison of observations with the predictions of the Harvard three-dimensional global model suggests that in many key areas (e.g., formaldehyde and peroxides) substantial differences between measurements and theory are present and must be resolved. In the case of CH3OH, there appears to be a large mismatch between atmospheric concentrations and estimated sources, indicating the presence of major unknown removal processes. Instrument intercomparisons as well as disagreements between observations and model predictions are used to identify needed improvements in key areas. The atmospheric chemistry and sources of this group of chemicals is poorly understood even though their fate is intricately linked with upper tropospheric NOx and HOx cycles.

Biomass burning emissions and vertical distribution of atmospheric methyl halides and other reduced carbon gases in the South Atlantic region

The NASA TRACE A experiment (September – October 1992) investigated effects of dry season biomass burning emissions from both South America and southern Africa on the tropical South Atlantic troposphere. Whole air canister samples were collected aboard the NASA DC-8 aircraft and analyzed for a wide range of nonmethane hydrocarbons (NMHCs) and halocarbons. Fast response in situ quantification of CH4, CO, and CO2 were also performed on the DC-8. Sampling took place over Brazilian agricultural areas and southern African savanna where there was active biomass burning. The vertical distribution of the measured gases revealed that the concentrations of most hydrocarbons, methyl halides, CH4, CO, and CO2, were enhanced in the boundary layer of these regions principally as a result of biomass fires. Brazilian and African biomass burning emission ratios were calculated for CH3Br, CH3Cl, CH3I, and NMHCs relative to CO and CO2. Although both fire regions were dominated by efficient (flaming) combustion (CO/CO2 ratios <0.1), the Brazilian samples exhibited the lower (more flaming) CO/CO2 ratio of 0.037 compared with the African savanna value of 0.062. This difference in combustion efficiency was reflected in lower ratios versus CO2 for all correlated gases. However, the gases more closely associated with smoldering combustion (e.g., C3H8 and CH3Cl) exhibited significantly higher ratios relative to CO for the Brazilian fires, while the African samples exhibited higher values versus CO for compounds associated more closely with flaming combustion (e.g., C2H2). This variation in the trace gas ratios versus CO is most likely caused by different fuel characteristics. On the basis of the emission ratios obtained, the total biomass burning emission rates for savannas and worldwide were calculated for the hydrocarbons and methyl halides. From these it was estimated that roughly 25% and 20% of global CH3Cl and CH3Br emissions, respectively, derive from biomass burning but that the contribution of biomass burning to total CH3I emissions was not significant.

Aerosols from biomass burning over the tropical South Atlantic region: Distributions and impacts

The NASA Global Tropospheric Experiment (GTE) Transport and Atmospheric Chemistry Near the Equator-Atlantic (TRACE A) expedition was conducted September 21 through October 26, 1992, to investigate factors responsible for creating the seasonal South Atlantic tropospheric ozone maximum. During these flights, fine aerosol (0.1–3.0 μm) number densities were observed to be enhanced roughly tenfold over remote regions of the tropical South Atlantic and greater over adjacent continental areas, relative to northern hemisphere observations and to measurements recorded in the same area during the wet season. Chemical and meteorological analyses as well as visual observations indicate that the primary source of these enhancements was biomass burning occurring within grassland regions of north central Brazil and southeastern Africa. These fires exhibited fine aerosol (N) emission ratios relative to CO (dN/dCO) of 22.5 ± 9.7 and 23.6 ± 15.1 cm−3 parts per billion by volume (ppbv)−1 over Brazil and Africa, respectively. Convection coupled with counterclockwise flow around the South Atlantic subtropical anticyclone subsequently distributed these aerosols throughout the remote South Atlantic troposphere. We calculate that dilute smoke from biomass burning produced an average tenfold enhancement in optical depth over the continental regions as well as a 50% increase in this parameter over the middle South Atlantic Ocean; these changes correspond to an estimated net cooling of up to 25 W m−2 and 2.4 W m−2 during clear-sky conditions over savannas and ocean respectively. Over the ocean our analyses suggest that modification of CCN concentrations within the persistent eastern Atlantic marine stratocumulus clouds by entrainment of subsiding haze layers could significantly increase cloud albedo resulting in an additional surface radiative cooling potentially greater in magnitude than that caused by direct extinction of solar radiation by the aerosol particles themselves.

Three-dimensional distribution of nonmenthane hydrocarbons and halocarbons over the northwestern Pacific during the 1991 Pacific Exploratory Mission (PEM-West A)

A total of 1667 whole air samples were collected onboard the NASA DC-8 aircraft during the 6-week Pacific Exploratory Mission over the western Pacific (PEM-West A) in September and October 1991. The samples were assayed for 15 C2-C7 hydrocarbons and six halocarbons. Latitudinal (0.5°S to 59.5°N) and longitudinal (114°E to 122°W) profiles were obtained from samples collected between ground level and 12.7 km. Thirteen of the 18 missions exhibited at least one vertical profile where the hydrocarbon mixing ratios increased with altitude. Longitude-latitude color patch plots at three altitude levels and three-dimensional color latitude-altitude and longitude-altitude contour plots exhibit a significant number of middle-upper tropospheric pollution events. These and several lower tropospheric pollution plumes were characterized by comparison with urban data from Tokyo and Hong Kong, as well as with natural gas and the products from incomplete combustion. Elevated levels of nonmethane hydrocarbons (NMHC) and other trace gases in the upper-middle free troposphere were attributed to deep convection over the Asian continent and to typhoon-driven convection near the western Pacific coast of Asia. In addition, NMHCs and CH3CCl3 were found to be useful tracers with which to distinguish hydrocarbon and halocarbon augmented plumes emitted from coastal Asian cities into the northwestern Pacific.

Distribution and seasonality of selected hydrocarbons and halocarbons over the western Pacific basin during PEM‐West A and PEM‐West B

Nonmethane hydrocarbons (NMHCs) and halocarbons were measured in the troposphere over the northwestern Pacific as part of the airborne component of NASAs Pacific Exploratory Mission-West Phase B (PEM-West B). This study took place in late winter of 1994, a period characterized by maximum outflow from the Asian continent. The results are compared to those from Pacific Exploratory Mission-West Phase A (PEM-West A), which was flown in the same region during late summer of 1991, when flow from the subtropical western Pacific dominated the lower troposphere. Mixing ratios of NMHCs, tetrachloroethene (C2Cl4), and methyl bromide (CH3Br) were significantly higher during PEM-West B than during PEM-West A, particularly at latitudes north of 25°N and altitudes lower than 6 km. The primary reasons for these higher ambient concentrations were the seasonal increase in the atmospheric lifetimes of trace gases controlled by HO radical reactions, and the more frequent input of continental air masses. During PEM-West B, air masses of continental origin observed north of 25°N latitude were augmented with urban signature gases such as C2Cl4. By contrast, more southerly continental outflow had characteristics associated with combustion sources such as biomass burning, including wood fuel burning. During the summer PEM-West A period, the spatial distribution of methyl iodide (CH3I) was consistent with effective oceanic sources at all latitudes, being especially strong in tropical and subtropical regions. At low latitudes, PEM-West B CH3I mixing ratios in the lower troposphere were similar to PEM-West A, but at latitudes greater than about 25°N PEM-West B concentrations were significantly reduced. Equatorial regions exhibited enhanced CH3I mixing ratios extending into the upper tropical troposphere, consistent with fast vertical transport of air from the tropical marine boundary layer.

Photosynthetic control of atmospheric carbonyl sulfide during the growing season.

Climate models incorporate photosynthesis-climate feedbacks, yet we lack robust tools for large-scale assessments of these processes. Recent work suggests that carbonyl sulfide (COS), a trace gas consumed by plants, could provide a valuable constraint on photosynthesis. Here we analyze airborne observations of COS and carbon dioxide concentrations during the growing season over North America with a three-dimensional atmospheric transport model. We successfully modeled the persistent vertical drawdown of atmospheric COS using the quantitative relation between COS and photosynthesis that has been measured in plant chamber experiments. Furthermore, this drawdown is driven by plant uptake rather than other continental and oceanic fluxes in the model. These results provide quantitative evidence that COS gradients in the continental growing season may have broad use as a measurement-based photosynthesis tracer.

Long-term decline of global atmospheric ethane concentrations and implications for methane.

After methane, ethane is the most abundant hydrocarbon in the remote atmosphere. It is a precursor to tropospheric ozone and it influences the atmosphere’s oxidative capacity through its reaction with the hydroxyl radical, ethane’s primary atmospheric sink. Here we present the longest continuous record of global atmospheric ethane levels. We show that global ethane emission rates decreased from 14.3 to 11.3 teragrams per year, or by 21 per cent, from 1984 to 2010. We attribute this to decreasing fugitive emissions from ethane’s fossil fuel source—most probably decreased venting and flaring of natural gas in oil fields—rather than a decline in its other major sources, biofuel use and biomass burning. Ethane’s major emission sources are shared with methane, and recent studies have disagreed on whether reduced fossil fuel or microbial emissions have caused methane’s atmospheric growth rate to slow. Our findings suggest that reduced fugitive fossil fuel emissions account for at least 10–21 teragrams per year (30–70 per cent) of the decrease in methane’s global emissions, significantly contributing to methane’s slowing atmospheric growth rate since the mid-1980s.

Tropospheric hydroxyl and atomic chlorine concentrations, and mixing timescales determined from hydrocarbon and halocarbon measurements made over the Southern Ocean

Author(s): Wingenter, OW; Blake, DR; Blake, NJ; Sive, BC; Rowland, FS; Atlas, E; Flocke, F | Abstract: During the First Aerosol Characterization Experiment (ACE 1) field campaign, 1419 whole air samples were collected over the Southern Ocean, of which approximately 700 samples were collected in the marine boundary layer (MBL), 300 samples were taken in the free troposphere (FT), and the remainder were collected in the buffer layer (BuL), the layer between the MBL and FT. Concentrations of tetrachloroethene, ethane, ethyne, and propane decayed over the 24 day duration of the intensive portion of the field campaign, which began November 18, 1995. This decline was consistent with what is known about seasonal increase of HO and the seasonal decrease in biomass burning. Using a simple empirical model, the best fit to the observations was obtained when the average [HO] was 6.1 ± 0.3 × 105 HO cm-3, and an average [Cl] of 720 ± 100 Cl cm-3. The corresponding exchange times were 14 ± 2 days between the MBL and FT, and 49 +40/-13 days between the MBL in the intensive campaign region and the MBL region to the north (nMBL). Copyright 1999 by the American Geophysical Union.

Impacts of snowpack emissions on deduced levels of OH and peroxy radicals at Summit, Greenland

Abstract Levels of OH and peroxy radicals in the atmospheric boundary layer at Summit, Greenland, a location surrounded by snow from which HOx radical precursors are known to be emitted, were deduced using steady-state analyses applied to (OH+HO2+CH3O2), (OH+HO2), and OH–HO2 cycling. The results indicate that HOx levels at Summit are significantly increased over those that would result from O3 photolysis alone, as a result of elevated concentrations of HONO, HCHO, H2O2, and other compounds. Estimated midday levels of (HO2+CH3O2) reached 30– 40 pptv during two summer seasons. Calculated OH concentrations averaged between 05:00 and 20:00 (or 21:00) exceeded 4×106 molecules cm−3, comparable to (or higher than) levels expected in the tropical marine boundary layer. These findings imply rapid photochemical cycling within the boundary layer at Summit, as well as in the upper pore spaces of the surface snowpack. The photolysis rate constants and OH levels calculated here imply that gas-phase photochemistry plays a significant role in the budgets of NOx, HCHO, H2O2, HONO, and O3, compounds that are also directly affected by processes within the snowpack.