A three‐dimensional global model study of atmospheric methyl chloride budget and distributions

Abstract

[1] Global simulations of atmospheric methyl chloride (CH 3 Cl) are conducted using the GEOS-CHEM model in order to understand better its sources and sinks. Observations from 7 surface sites and 9 aircraft field experiments are used to evaluate the model simulations with assimilated meteorology fields for 7 years. The model simulates CH 3 Cl observations at northern mid and high latitudes reasonably well. The seasonal variation of CH 3 Cl at southern mid and high latitudes is severely overestimated, however. Simulated vertical profiles of CH 3 Cl are in general agreement with the observations in most regions; the disagreement occurs in the vicinities of major sources, principally reflecting the uncertainties in the estimated distributions of our added pseudobiogenic and the biomass burning sources. Our estimate of known sources (1.5 Tg yr À1 ) from ocean, biomass burning, incineration/industry, salt marshes, and wetlands accounts for only 34% of the total source (4.4 Tg yr À1 ). We hypothesize that the missing source of 2.9 Tg yr À1 is likely of biogenic origin. On the basis of the observed CH 3 Cl seasonality at northern mid and high latitudes, we find that this pseudobiogenic source is located at 30°N-30°S, not at mid and high latitudes. If so, the observed CH 3 Cl latitudinal distribution indicates that the annual hemispheric mean OH ratio is within the range of 0.8-1.3. The net uptake regions by ocean are located at high latitudes. A relatively small loss of 150 Gg yr À1 over these regions is critical for the model to reproduce the observed annual mean latitudinal gradient of CH 3 Cl in the southern hemisphere. The large overestimate of the seasonal variation of CH 3 Cl at southern mid and high latitudes likely implies that the seasonality of simulated oceanic uptake is incorrect as a result of defects in the parameterization of this loss in the model.

New insight into the atmospheric chloromethane budget gained using stable carbon isotope ratios

“…However, in scenarios B and C sources of 1759 or 2469 Gg yr −1 with respective signatures of −140.2±15.9 per mil and −122.8±11.4 per mil fit well with both the range of CH 3 Cl emissions by senescent plants and leaf litter in the tropics and sub-tropics and the δ 13 C signature of these emissions (δ 13 C=−135±12 per mil; Keppler et al, 2004). Furthermore the source strengths of CH 3 Cl used for scenario C are in general agreement with experimental observations by Scheeren et al (2003) and a three dimensional global model study of atmospheric CH 3 Cl published recently by Yoshida et al (2004). On the basis of their measurements Scheeren and co-workers suggest a total global emission of 2000 Gg yr −1 from tropical forests whilst the Yoshida group hypothesise that a missing terrestrial source of 2900 Gg yr −1 is located between 30 • N and 30 • S. Our calculations also lend credence to the proposal that CH 3 Cl degradation by soil microorganisms is a much bigger sink for atmospheric CH 3 Cl (>1000 Gg yr −1 ) than that postulated by Montzka and Fraser (2003).…”

“…Chloromethane (CH 3 Cl) is the most abundant halocarbon in the atmosphere with a reported average mixing ratio in the range of 530 to 560 pptv (Montzka and Fraser, 2003;Simmonds et al, 2004;Trudinger et al, 2004;Aydin et al, 2004;Yoshida et al, 2004), corresponding to a total atmospheric burden of around 4000 to 5000 Gg (giga gram=10 9 gram). Although largely of natural origin CH 3 Cl is responsible for around 16% of chlorine-catalysed ozone destruction in the stratosphere (Montzka and Fraser, 2003).…”

Evaluation of model-simulated source contributions to tropospheric ozone with aircraft observations in the factor-projected space

“…This factor contains significant CO variability, which can imply the biomass burning influence. However, the very small factor correlations with C 2 H 6 /C 3 H 8 ratio and negative correlation with latitude (Table 1) may support the large biogenic CH 3 Cl emissions from the tropics (e.g., Yoshida et al, 2004Yoshida et al, , 2006 rather than biomass burning.…”

“…For model simulated CH 3 Cl, we used the GEOS-Chem results by Yoshida et al (2004). Contributions from the six sources (pseudo-biogenic, oceanic, biomass burning, incineration/industrial, salt mash and wet land) are considered.…”

“…CO is for combustion influence primarily from fossil fuel, biofuel, and biomass and C 3 H 8 is a good liquefied petroleum gas (LPG) tracer. CH 3 Cl has its major sources from terrestrial biosphere and biomass burning (Yoshida et al, 2004(Yoshida et al, , 2006. 7 Be is produced mainly by cosmic rays in the stratosphere and upper troposphere and is generally used as a tracer for stratospheric air mass (Dibb et al, 2003).…”