A14C-01
Trace Gases and Aerosol in the Boundary Layer of the Northern Asia: TROICA Experiments
The TROICA experiment (Transcontinental Observations Into the Chemistry of the Atmosphere) started in 1995. A mobile railroad laboratory is being used for measurements of atmospheric gases, aerosol, solar radiation and meteorological parameters. The laboratory wagon is directly coupled to the locomotive of a passenger train traveling along electrified railroads of Russia. Eleven expeditions have been conducted to the moment of which nine were performed along the Trans-Siberian railroad from Moscow to Vladivostok (around 9300 km). One expedition was North-South between Murmansk and Kislovodsk, and one was around the mega-city of Moscow. The huge coverage of the continental regions and the repetition of the expeditions provide unique information on processes controlling variability of the key trace gases (O3, NOx, CO, CO2, CH4, some VOCs) and aerosols with high temporal and spatial resolution over different scales from continental to local (hundreds meters). Multiple crossings of settlements allowed determining typical variations of surface gases and aerosol concentrations within cities and their plumes. 222Rn concentration data were used for estimates of CO, CH4 and CO2 nocturnal fluxes from the soil and vegetation. Impacts of different factors, like Western Siberian gas and oil industry, forest fires, transboundary air pollution transport and some other can be evaluated based on the measurement data by comparing them with results of model output and hence can be used for model validation. Emissions of the atmospheric CO and CH4 were studied in several expeditions using isotopes analysis.
A14C-02
Assessing our Understanding of Recent Trends in Atmospheric Methane
Direct and indirect components to anthropogenic radiative forcing by atmospheric CH4 are estimated to be 0.7 W m-2, or about 1/2 the contribution of CO2. Through its chemistry, methane also affects the abundances of tropospheric ozone, a strong oxidant and greenhouse gas that impacts human health and agricultural crop yields, and OH, a radical whose concentration determines the lifetimes of many greenhouse gases. Methane has been identified as a greenhouse gas to target for short-term stabilization in radiative forcing because it has a relatively short life time and because reductions in its emissions from many sources are cost effective. But large uncertainties in the CH4 budget still exist, so it is not possible to predict the future atmospheric burden of CH4 and it potential impact on climate or atmospheric chemistry. NOAA has been monitoring atmospheric CH4 since 1983. At the start of our program, the rate of increase in atmospheric methane was ~15 ppb yr-1, but since 1999, the growth rate has been near zero. Through 1990, the monotonic decrease in global growth rate was consistent with a system approaching steady state with constant global emissions and a lifetime of ~10 yr. Had this trend toward steady state continued, the rate of increase would have slowly approached zero. Interannual variability in the growth rate makes it difficult to say whether the atmospheric burden is currently increasing, stable, or decreasing, but we have observed net decreases in globally averaged CH4 in 4 of the last 7 years. It would be surprising if atmospheric methane were decreasing, because CH4 emissions are not regulated and scenarios of emissions such as those used by IPCC suggest that improved living standards in the developing world and increased energy demand should result in increasing emissions, which would result in an increase in the atmospheric CH4 burden. We examine current ideas on why atmospheric CH4 has stabilized in light of the NOAA ESRL CH4 observations. We conclude that significantly more in situ measurements and improved modeling capability are needed before a detailed understanding of the global CH4 budget will be achieved.
A14C-03
Sensing methane emissions from space - An improved retrieval version from SCIAMACHY onboard ENVISAT
Methane is the second most important anthropogenic greenhouse gas and, although the global budget is relatively well constrained, partitioning among sources remains highly uncertain. SCIAMACHY from its vantage point in space offers the unique opportunity to sense methane globally with high sensitivity towards the surface. Recent retrievals of methane column averaged mixing ratios using short wave infrared nadir spectra obtained by SCIAMACHY are presented. An improved retrieval version using ECMWF pressure and temperature profiles as prior input is presented. Furthermore, laboratory measurements of methane have been used to improve spectroscopic parameters in the short wave infrared, thereby minimizing previously existing systematic biases in the SCIAMACHY retrievals. We present retrieval results from 2003 through 2005, focussing on global long-term and seasonal averages as well as timeseries over specific regions of interest. Large scale methane enhancements due to man-made (e.g. rice agriculture) as well as natural (e.g. wetlands) emissions can be clearly identified and their temporal evolution be followed. Further, we present a comparison of the satellite retrievals with atmospheric models optimized for ground based methane measurements. Especially in tropical regions, which are not well constrained by the ground based network, large discrepancies still exits, pointing to high tropical methane emissions. As an example of the potential of the global methane dataset, some first results of emission inversions using a four- dimensional variational (4D-Var) data assimilation system are presented.
A14C-04
Carbon 13 Enrichment in Atmospheric Methane at Mace Head, Ireland: are Boreal Forest Fires the Source?
A bi-weekly record of δ13C of CH4 was maintained for ambient air collected at the Mace Head Atlantic background site over the period 1995-2004. These data have been used in conjunction with AGAGE and NOAA CH4 and CO mixing ratio records for the site and records of forest fires in Canada to assess the influence of biomass burning on the North Atlantic CH4 record. Enrichments in 13C of CH4 and anomalies in CH4 and CO mixing ratios in the Mace Head record during the latter parts of 1998 and 2002 can be correlated with extensive summer boreal forest burning in the Northern Hemisphere. Prior to October in these years there is a significant 13C-depleted wetland CH4 emission, in addition to relatively 13C-enriched CH4 from biomass burning, suppressing the isotopic anomaly. The enrichments were identified using δ13C of CH4 both crudely using monthly excess 13C for air from the Atlantic sector over a 10-year period and by comparing results for individual trajectories from Canada with expected background conditions for that period. Both techniques suggest a source with δ13C in the range -30 to -25‰. Recent analysis of samples collected downwind of controlled burns in Canada confirms that the excess CH4 emitted by burning has δ13C around - 28 to -27‰. Continued enrichment of the δ13C signal at Mace Head throughout 2003 and early 2004 is consistent with the circulation at these latitudes of CH4 from the very large boreal fires that persisted in Siberia for much of 2003. The data also suggest that the burning of boreal forests contributed significantly to the high CH4 growth rate in mid- to high- northern latitudes during 1998.
A14C-05
Towards an Understanding of Atmospheric Methanol
Methanol, the most abundant non-methane organic gas in the atmosphere, is an important global source of tropospheric CO and formaldehyde, and plays a significant role in the tropical HOx and ozone budgets. The atmospheric methanol budget is highly uncertain, with estimates of the global source ranging from 75 to 490 Tg/yr. New measurements from recent field experiments (INTEX-B, MILAGRO, TEXAQS-II, INTEX-A, and ICARTT) provide quantitative constraints on methanol sources and sinks. Here we use a 3D model of atmospheric chemistry (GEOS-Chem) to interpret these datasets and their implications for the global methanol budget. We find that emissions from terrestrial plants (thought to be the main source) are overestimated by 40-50%; the discrepancy appears specific to certain plant functional types (broadleaf trees and crops). Recent measurements in the surface ocean imply a large in situ biotic source, so that methanol emissions from the ocean biosphere are comparable in magnitude to those from terrestrial ecosystems. The oceans are also a large gross sink for atmospheric methanol (similar to oxidation by OH). Even with the plant growth source decreased by 40-50% according to these new constraints, we find that methanol emissions from the terrestrial biosphere still dominate over those from urban and industrial sources, in contrast to other recent studies.
A14C-06
Observational Constraints on the Global Budget of Ethanol
Ethanol, an oxygenated volatile organic compound (OVOC), is used extensively as a motor fuel and fuel additive to promote clean combustion. Ethanol can affect the oxidizing capacity and the ozone-forming potential of the atmosphere. Limited available atmospheric observations suggest a global background atmospheric ethanol mixing ratio of about 20 pptv, with values up to 3 ppbv near source regions; however, the atmospheric distribution and budget of ethanol remain poorly understood. Here, we use the global three-dimensional chemical transport model MOZART-4 to investigate the global ethanol distribution and budget, and place constraints on the budget by evaluating the model with atmospheric observations. We implement a global ethanol source of 14.7 Tg yr-1 in the model consisting of biogenic emissions (9.2 Tg yr-1), industrial/anthropogenic emissions (3.2 Tg yr-1), emissions from biofuels (1.8 Tg yr-1), biomass burning emissions (0.5 Tg yr-1), and a secondary source from atmospheric production (0.056 Tg yr-1). Gas-phase oxidation by the hydroxyl radical accounts for 66% of the global sink of ethanol in the model, dry deposition 9%, and wet scavenging 25%. The simulation yields a global mean ethanol burden of 0.11 Tg and an atmospheric lifetime of 3 days. The simulated boundary layer mean ethanol concentrations underestimate observations from field campaigns over the United States by 50%, downwind of Asia by 76% and over the remote Pacific Ocean by 86%. Because of the short lifetime of ethanol, the model discrepancy over remote tropical regions cannot be attributed to an underestimate of surface emissions over continents. In these regions, the dominant model source is secondary atmospheric production, from the reaction of the ethyl peroxy radical (C2H5O2) either with itself or with the methyl peroxy radical (CH3O2). A ~500-fold increase in this diffuse source (to ~30 Tg yr-1) distributed uniformly throughout the troposphere would largely correct the observation-model mismatch, resulting in a best estimate of the global ethanol source of 44 Tg yr-1. This finding could indicate omission of other chemical species in the model that can provide additional sources of C2H5O2. Candidate OVOCs, such as propionaldehyde, and peroxypropionic nitric anhydride (PPN) that are precursors to C2H5O2, have been measured in the remote troposphere. This hypothesis, however, needs testing by direct measurements of C2H5O2 in the remote tropical troposphere.
A14C-07
GEM-AQ global simulation of HCN and comparison with ACE satellite observations
Biomass burning is a significant source of trace gases and particulate matter in the troposphere and has an important impact on the global climate system. It is therefore important to understand and quantify its effect on the chemistry of the atmosphere. Critical issues relate to estimating emissions and their spatial and temporal distribution. In this study we investigate these issues through numerical simulations and comparison with observations from the ACE-FTS an instrument on SCISAT-I, a Canadian small satellite. Since hydrogen cyanide (HCN) is considered to be a sensitive tracer of biomass burning, we focus on this species. To perform the simulations, we used the Global Environmental Multiscale Air Quality model (GEM-AQ). GEM-AQ is based on the 3-D global variable-resolution multiscale operational weather forecast model (GEM) developed by the Meteorological Service of Canada and incorporates on-line air quality modules including gas phase chemistry and size-resolved multi-component aerosols. For these simulations GEM-AQ was run for the period 2004--2006 on a 4° × 4° global grid with 28 hybrid vertical levels from the surface up to 10 hPa. Objective analysis data were used to update the meteorological fields every 24 hours. Fire emission fluxes of gas species were generated by using year-specific inventories of carbon emissions with 8-day temporal resolution from the Global Fire Emission Database version 2. The results from model simulations are compared with troposphere/lower stratosphere HCN profiles measured by the ACE-FTS. Several experiments have been made varying injection heights which indicate that generally better agreement with measurements is obtained when the injection is not solely into the PBL.