Implications of the recent fluctuations in the growth rate of tropospheric methane

Global measurements show that the mixing ratio of tropospheric methane (CH4) increased by 1.1% (19.5 ± 1.7 ppbv) over the five-year period 1996-2000, with striking fluctuations in its annual growth rate. Whereas the global CH4 growth rate reached 15.9 ± 0.7 ppbv yr-1 in 1998, the growth rate was -2.1 ± 0.8 ppbv yr-1 in 2000. This is the first time in our 23-year global monitoring program that we have measured a negative annual CH4 growth rate. The CH4 growth rate fluctuates in an unpredictable fashion, and we reemphasize that global CH4 concentrations cannot be extrapolated into the future based on past trends. As a result, we suggest that the slowing of the CH4 growth rate during much of the 1980s and 1990s cannot be used to imply that CH4 will no longer be of concern in greenhouse gas studies during this century

Influence of biomass burning during recent fluctuations in the slow growth of global tropospheric methane

During the past 15 years the annual growth rate of tropospheric methane (CH4) has shown striking changes over 2-3 year periods, varying from + 1% yr-1 to slightly negative values (-0.2% yr-1). These fluctuations are superimposed on an overall slowdown of the CH4 growth rate since the 1980s. Here we use our complementary measurement of other compounds (ethane, tetrachloroethene) to confirm the influence of biomass burning on large global CH4 pulses in 1998 and 2002-2003. Methane growth rate fluctuations also track ENSO indices, most likely via the influence of ENSO activity on large-scale biomass burning. We also report the seventh year of near-zero growth of global CH4 levels (Dec. 1998-Dec. 2005). The global CH4 mixing ratio was 1772 ± 1 ppbv in 2005, and CH4 increases of 118-376 ppbv between 2000-2020 (ten scenarios in the 2001 IPCC report, to levels around 1900+ ppbv by 2020, now appear quite unlikely. Copyright 2006 by the American Geophysical Union

Acetone in the Atmosphere of Hong Kong, Abundance, Sources and Photochemical Precursors

Intensive field measurements were carried out at a mountain site and an urban site at the foot of the mountain from September to November 2010 in Hong Kong. Acetone was monitored using both canister air samples and 2,4-dinitrophenylhydrazine cartridges. The spatiotemporal patterns of acetone showed no difference between the two sites (p > 0.05), and the mean acetone mixing ratios on O3 episode days were higher than those on non-O3 episode days at both sites (p < 0.05). The source contributions to ambient acetone at both sites were estimated using a receptor model i.e. Positive Matrix Factorization (PMF). The PMF results showed that vehicular emission and secondary formation made the most important contribution to ambient acetone, followed by the solvent use at both sites. However, the contribution of biogenic emission at the mountain site was significantly higher than that at the urban site, whereas biomass burning made more remarkable contribution at the urban site than that at the mountain site. The mechanism of oxidation formation of acetone was investigated using a photochemical box model. The results indicated that i-butene was the main precursor of secondary acetone at the mountain site, while the oxidation of i-butane was the major source of secondary acetone at the urban site.Department of Civil and Environmental Engineerin

Dimethyl disulfide (DMDS) and dimethyl sulfide (DMS) emissions from biomass burning in Australia

We identify dimethyl disulfide (DMDS) as the major reduced sulfur-containing gas emitted from bushfires in Australia's Northern Territory. Like dimethyl sulfide (DMS), DMDS is oxidized in the atmosphere to sulfur dioxide (SO2) and methane sulfonic acid (MSA), which are intermediates in the formation of sulfuric acid (H2SO4). The mixing ratios of DMDS and DMS were the highest we have ever detected, with maximum values of 113 and 35 ppbv, respectively, whereas background values were below the detection limit (10 pptv). Molar emission ratios relative to carbon monoxide (CO) were [1.6 ± 0.1] × 10-5 and [6.2 ± 0.3] × 10-6, for DMDS and DMS respectively, while molar emission ratios relative to carbon dioxide (CO2) were [4.7 ± 0.4] × 10 6 and [1.4 ± 0.4] × 10 7, respectively. Assuming these observations are representative of biomass burning, we estimate that biomass burning could yield up to 175 Gg/yr of DMDS (119 Gg S/yr) and 13 Gg/yr of DMS

Observations of isoprene, methacrolein (MAC) and methyl vinyl ketone (MVK) at a mountain site in Hong Kong

A field campaign was carried out in September-November 2010 near the summit of Mt. Tai Mo Shan in Hong Kong. Isoprene, methyl vinyl ketone (MVK) and methacrolein (MAC) were measured. The average isoprene mixing ratio was 109 pptv, and the average MAC and MVK levels were 68 pptv and 164 pptv, respectively. The average daytime levels of isoprene (14920 pptv, average95% confidence interval, p<0.01), MAC (709 pptv, p<0.01) and MVK (16922 pptv, p<0.1) were significantly higher than the average nighttime values (205 pptv, 498 pptv and 13925 pptv, respectively). The relationship between MVK and MAC indicated that nearby isoprene oxidation dominated their daytime abundances, while NO3 chemistry and regional transport of anthropogenic sources from inland Pearl River Delta region could explain the higher MVK to MAC ratios at night. Correlation analysis of [MVK]/[isoprene] versus [MAC]/[isoprene] found that the isoprene photochemical ages were between 10 and 64min. Regression analysis of total O3 (O3+NO2) versus MVK resulted in an estimated contribution of isoprene oxidation to ozone production of 12.5%, consistent with the simulated contribution of 10-11% by an observation-based model. © 2012 American Geophysical Union. All Rights Reserved

Long-term decrease in the global atmospheric burden of tetrachloroethene (C2Cl4)

We present a 14-year record of tropospheric tetrachloroethene (C2Cl4) mixing ratios measured each season at remote surface locations throughout the Pacific Basin (71°N-47°S . Our calculated annual mean C2Cl4 mixing ratio for the extra-tropical northern hemisphere (NH) was 13.9 ± 0.5 pptv in 1989, but declined to less than half this value (to 6.5 ± 0.2 pptv) by 2002. Mid-latitude southern hemisphere (SH) mixing ratios, which were much smaller than NH values, showed interannual variations comparable to measured long-term changes, and the SH annual mean was slightly less in 2002 than in 1989. Driven by the relatively large decrease in NH values, the global C2Cl4, mixing ratio declined from 6.3 ± 0.6 pptv in 1989 to 3.5 ± 0.2 pptv in 2002. These values suggest that the global C2Cl4 burden decreased by roughly 205 Gg between 1989-2002, which is consistent with industrial estimates of declining NH emissions over a similar period. Copyright 2004 by the American Geophysical Union

Regional and local contributions to ambient non-methane volatile organic compounds at a polluted rural/coastal site in Pearl River Delta, China

Identification of major sources of airborne pollutants and their contribution to pollutant loadings are critical in developing effective pollution control and mitigation strategies. In this study, a comprehensive dataset of non-methane volatile organic compounds (NMVOCs) collected from August 2001 to December 2002 at a polluted rural/coastal site in the Pearl River Delta (PRD) is analyzed to assess the relative contributions of major pollution sources to ambient NMVOC mixing ratios. A unique approach based on emission ratios of individual chemical species was used to classify the bulk air samples in order to apportion regional and local source contributions to the measured mixing ratios. The collected air samples fell into four major groups, including air masses from the inner PRD region and Hong Kong (HK) urban area. To estimate the source apportionment of NMVOCs, a principal component analysis/absolute principal component scores receptor model was applied to the classified data points. The results indicate that the regional and local source contributions to ambient NMVOC levels at the site were significantly different due to the differences in local versus regional energy use and industrial activities. For air masses originating from HK, vehicular emissions accounted for approximately 39% of the total NMVOC levels, followed by industrial emissions (35%), gasoline evaporation (14%) and commercial/domestic liquefied petroleum gas/natural gas use (12%). By contrast, for air masses originating from the PRD the industrial emissions accounted for 43% of the total NMVOC burden, followed by vehicular emissions (32%) and biomass burning (25%). In particular, the higher regional contribution of biomass burning found in this study as compared to existing emission inventories suggests that further efforts are necessary to refine the emission inventories of NMVOCs in the PRD region. © 2006 Elsevier Ltd. All rights reserved

Strong evidence for negligible methyl chloroform (CH3CCl3) emissions from biomass burning

With the phase-out of industrial methyl chloroform (MCF) production, the atmospheric burden of this ozone-depleting gas has rapidly declined. Therefore any non-industrial sources are taking on greater significance in the MCF budget. The only natural MCF source that has been proposed, biomass burning, has been reported to emit up to 2-10 Gg MCF yr-1. We have re-examined MCF data for thousands of airborne and ground-based air samples collected by our group since 1990 that were directly impacted by major biomass burning sources. Without exception, we have found no positive evidence that MCF is released from biomass burning. Our results indicate that global biomass burning emissions of MCF have been significantly overestimated and are unlikely to exceed 0.014 Gg MCF yr-1. Lowering the uncertainty regarding the magnitude of the global MCF biomass burning source may extend its period of usefulness for determining global abundances and trends of the hydroxyl radical (OH). Copyright 2007 by the American Geophysical Union