Measurement of the Long-term trends of Methanol (CH3OH) and Carbonyl Sulfide (OCS)

• Both methyl chloride and carbonyl sulfide have strong infrared bands and have been detected by both space-based and ground-based infrared techniques

• The objective of this presentation is to describe the use those results to determine long-term atmospheric trends

Jungfraujoch Methane and CFC-12 Solar Spectra(Zander et al. Science of the Total Environment, 391 184-195 2008)

Lower tropospheric CH3OH over Beijing from TES Nadir Measurements

Objective of CH3OH Analysis

• Despite the large number of previous measurement obtained with a variety of techniques, no measurements of its long-trend trend exist

• The objective of this work is to analyze a 22 year time series of solar absorption measurements obtained in the infrared with the 1-m Fourier transform spectrometer located at the U.S. National Observatory on Kitt Peak in southern Arizona and their comparison with previous measurements over North America

Importance of Tropospheric CH3OH

• Methanol is the second most abundant organic molecule in the atmosphere after methane

• Methanol represents about half of the total global emission of oxygenates and nearly 20% of total global volatile organic compound (VOC) emissions

• Measurements include observations from surface stations during ship cruises, and aircraft campaigns

• Plant growth is the principal source and has been estimated to contribute 20-40% of the total emissions to the atmosphere

• Other sources include biomass burning decaying plant matter, atmospheric oxidation of methane and other VOC compounds, vehicles and industrial emissions including production via peroxy radical reactions

• It is a significant global source of tropospheric CO and formaldehyde• Despite numerous atmospheric measurements obtained with a wide range

of techniques, a factor of three uncertainty remains in the global atmospheric budget of methanol

• The large uncertainty in the methanol atmospheric budget exists despite extensive comparisons of observations with predictions of tropospheric chemistry obtained with chemical-transport models

CH3OH Global Distribution from ACE (Dufour et al. ACP, 7, 6119-6129, 2007)

Kitt Peak CH3OH Time Series

• Methanol has very weak absorption in the Kitt Peak solar spectra • Analysis limited to measurements with solar zenith angles of 80° to

85°• Average mixing ratios between 2.09-14 km reported (free

troposphere)• Total number of measurements=165• Number yielding valid measurement days=89• Time Span of 22 years• Weakly constrained a priori selected based on ACE measurements

over North America from spring 2004 to summer 2005 covering the 20°N-50°N latitude band

• ACE measurements indicate the upper tropospheric volume mixing ratio increasing progressively from less than 0.5 ppbv during northern winter to about 2.0 ppbv during summer [Dufour et al, 2006]

Kitt Peak simulation

Averaging Kernels Calculated for CH3OH with SFIT2

Daily Average Kitt Peak CH3OH Time Series and Fit

Kitt Peak CH3OH Free Troposphere Seasonal Cycle(JGR, in press, 2009)

Conclusions of Kitt Peak CH3OH time series analysis

• The first long-term measurements of free tropospheric methanol have been obtained from infrared solar absorption spectra

• No statistically significant long term trend in free tropospheric methanol detected over 22 years of measurements

• Trend of (0.007449±0.0079067) ppbv yr-1 (0.91±0.96)% yr-1) is obtained

• The maximum at the beginning of July measured is consistent with the analysis of ACE upper tropospheric measurements over North America and the key roll of plant growth in that region in determining its seasonal cycle

• As shown by summer free tropospheric measurements over North America obtained during the INTEX-A aircraft campaign (July 1-July 14, 2004),transport of emissions from Asian plumes sometimes impact North America during summer

OCS (Carbonyl Sulfide)JQSRT 109, 2679-2686, 2008

• Carbonyl sulfide (OCS) is important as it is the predominant sulfur-bearing molecule in the remote troposphere with a complex biogeochemical cycle, a globally-averaged lifetime of about 4 years and an average concentration in that region of ~500 parts per trillion (10-12 per unit volume)

• First reported measurements were derived from analysis of ambient surface air collected from several locations in 1975 with a condensed cryogenic procedure followed by infrared Fourier transform spectrometer (FTS) measurements of the sample in the region of the 3 band to determine sample mixing ratios [Hanst et al., J. Air Pollution Control. Assoc. 25, 1220-1226, 1975]

• Montzka et al. [JGR 112 D09302, doi: 10.1029/2006JD007665, 2007] reported the analysis of the budget and seasonality of OCS from surface and aircraft measurements

• There have been changes in the line intensities assumed in the HITRAN database by several percent assumed for the strong 3 band that is used in atmospheric retrievals

• Strong lines of OCS have been identified in 1951Jungfraujoch solar spectra recorded with a grating spectrometer

Kitt Peak JGR 2002

Updated Kitt Peak OCS time series

Brown et al. (Appl. Opt. 35, 3828- 4848, 1996)

• The update of the OCS parameters was much needed. There were sufficient experimental intensities that demonstrated that the ν3 band strength needed to be increased on the compilations by almost 9%. Atmospheric investigators should note this difference when comparing present and prior OCS field measurements

• A mean ratio of 1.0996 for the intensities (version 3 ATMOS intensities divided by those from the 1982 Kitt Peak retrievals has been derived

ATMOS-ACE OCS Trend Analysis

• The exponential model for the long-term trend mixing ratios used in the ATMOS-ACE trend study [GRL 23, 2349-2352, 2005] has been used in the analysis for the OCS lower stratospheric long-term trend. The trend was derived from average mixing ratios from each year which were fitted with the polynomial expression

V = a0 + a1(t-t0) + a2(t-t0)**2 (1) where V is the volume mixing ratio, t is time, and t0 is

the time of the measurements from the first ATMOS mission. The coefficients a0, a1, a2, and their statistical uncertainties were determined from a nonlinear least-squares fit to the measurement time series

• HITRAN 2004 parameters assumed

ATMOS-ACE OCS Time Series

Sunset over Kitt Peak, AZ