| Measurement of halocarbons in the atmosphere has been of great concern for their effects on depleting stratospheric ozone-layer and for their global warming potential. More than 30 global background stations have been established since 1970s to study the changes of these ozone depletion substances (ODS) after reduced production and consumption, and to investigate the response of stratospheric ozone to the changes of the world's use of these gases. With the rapid industrialization and economic development, China has played a role of increasing importance in the production and consumption of chlorofluorocarbons (CFCs), therefore attentions have been paid to the emissions and phase-out of ODS in China. A large portion of these halocarbons in China might be consumed in the Pearl River Delta (PRD), one of the most industrialized regions in China. In the Pearl River Delta region, observations of seven trace gases, namely CFC-12 (CCl2F2), CFC-11 (CCl3F), CFC-113 (CCl2FCClF2), CFC-114 (CClF2CClF2), HCFC-22 (CHClF2), carbon tetrachloride (CCl4) and methyl chloride (CH3Cl), have been conducted by canister sampling and pre-concentration-GC-MSD analysis in recent years. The study focused on the background levels and spatial and temporal variations of these halocarbons, as well as a preliminary estimation of their annual trends. The purpose is to elucidate the situation of production, comsumption, emiision and atmospheric levels of these ODSunder the influence of Montreal Protocol and its amendments in China.Mixing ratios of most halocarbons (except CH3C1) were highest in the densly populated metropolitans (such as Shenzhen, Macau, Hong Kong and Guangzhou) and lowest in remote area (such as Dinghu Mountain). Mixing ratios for CH3CI, however, were higher in Dinghu Mountain than in Guangzhou and other cities. Compared to global background values, the mean mixing ratios for CFC-12, CFC-11, HCFC-22 and CH3CI in the Pearl River Delta were much higher. Especially for HCFC-22 and CH3CI, the levels in PRD presented a striking contrast to those in the global background monitoring stations. The average concentrations of HCFC-22 in Guangzhou and Dinghu Mountain are 324.9pptv and 136.3pptv higher than that in Cape Grim, respectively. Mean mixing ratios of HCFC-22 in fact were 2-3 times of that in Cape Grim. CH3CI in Guangzhou and Dinghu Mountain are 291.1pptv and 481.0pptv higher than that in Cape Grim, respectively, and were about 1.6-2.0 times of the CH3CI levels in Cape Grim. This large contrast of concentrations reveals strong emission sources of these two gases in PRD. Mean mixing ratios for both CFC-113 and CCI4 are comparable with the global background concentrations, indicating very limited local emission of CFC-113 and CCl4in PRD.Significant seasonal variations for four CFCs and HCFC-22 were observed, and peak levels in Guangzhou were found in summer and autumn with higher air temperature. The monthly mean concentrations were positively correlated with the air temperature. The highest monthly mean mixing ratio of CFCs and HCFC-22 occurred in May. They were 341.76±77.42 pptv for CFC-11, 780.05±287.06 pptv for CFC-12, 101.42±20.38 pptv for CFC-113,18.18±6.68 pptv for CFC-114 and 860.8±829.1 pptv for HCFC-22. The lowest levels were found in winter time, which were 226.67±37.66 pptv for CFC-11, 538.76±67.41 pptv for CFC-12, 68.44±13.34 pptv for CFC-113, 12.79±1.05 pptv for CFC-114 and 149.8±24.2 pptv for HCFC-22. Their higher monthly mean concentrations in summer and autumn are mainly due to the use of CFCs and HCFC-22 as refrigerants and spraying agents. Air-conditioners, chillers, cold storage refrigerators and sprayers are largely used in hot seasons, thus larger emission ocuured. The average concentrations of 7 halocarbons were high in summerand autumn and low in winter and spring in Dinghu Mountain, as is similar to other air pollutants mainly controlled and influenced by diffusion conditions and monsoons in Pearl River Delta.The diurnal variation for mixing ratios of CFCs was within limited scales in Guangzhou, though the concentrations for the four CFCs were higher during 8:00-9:00, 12:00-13:00 and 18:00-19:00, which are traffic rush hours when the air-conditioner in motor vehicles may release these CFCs. HCFC-22 and CH3CI in Guangzhou, however, showed large diurnal variation without obvious diurnal pattern. In Dinghu Mountain, the diurnal variations for all gases but CH3CI is even smaller than in Guangzhou, but larger diurnal variation of CH3CI mixing ratios is observed.Priliminary estimation shows that in PRD atmospheric levels of CFC-12, CFC-11, CFC-114, CCl4and CH3C1 peaked around 2001 and decreased slowly afterwards, whereas levels of CFC-113 monotonously decreased in 1997 to 2005. Although this decrease lagged behind about 7-10 years compared to the global trends, it reveals that actions for the phase-out of ODS are taking effects in China under the control of Montreal Protocol and its amendments. HCFC-22 in PRD, however, presented much higher mixing ratios and continually rapid growth rates more than four times of the global background, and the mixing ratios for HCFC-22 are expected to rise continually in the following years in PRD.
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