Abstract
We present a 16-month record of ozone (O3), carbon monoxide (CO), total reactive nitrogen (NO y ), sulphur dioxide (SO2), methane (CH4), C2 – C8 non-methane hydrocarbons (NMHCs), C1 – C2 halocarbons, and dimethyl sulfide (DMS) measured at a southern China coastal site. The study aimed to establish/update seasonal profiles of chemically active trace gases and pollution tracers in subtropical Asia and to characterize the composition of the `background' atmosphere over the South China Sea (SCS) and of pollution outflow from the industrialized Pearl River Delta (PRD) region and southern China. Most of the measured trace gases of anthropogenic origin exhibited a winter maximum and a summer minimum, while O3 showed a maximum in autumn which is in contrast to the seasonal behavior of O3 in rural eastern China and in many mid-latitude remote locations in the western Pacific. The data were segregated into two groups representing the SCS background air and the outflow of regional continental pollution (PRD plus southern China), based on CO mixing ratios and meteorological conditions. NMHCs and halocarbon data were further analyzed to examine the relationships between their variability and atmospheric lifetime and to elucidate the extent of atmospheric processing in the sampled air parcels. The trace gas variability (S) versus lifetime (τ) relationship, defined by the power law, S lnx = Aτ− b, (where X is the trace gas mixing ratio) gives a fit parameter A of 1.39 and exponent b of 0.42 for SCS air, and A of 2.86 and b of 0.31 for the regional continental air masses. An examination of ln[n-butane]/ln[ethane] versus ln[propane]/ln[ethane] indicates that their relative abundance was dominated by mixing as opposed to photochemistry in both SCS and regional outflow air masses. The very low ratios of ethyne/CO, propane/ethane and toluene/benzene suggest that the SCS air mass has undergone intense atmospheric processing since these gases were released into the atmosphere. Compared to the results from other polluted rural sites and from urban areas, the large values of these species in the outflow of PRD/southern China suggest source(s) emitting higher levels of ethyne, benzene, and toluene, relative to light alkanes. These chemical characteristics could be unique indicators of anthropogenic emissions from southern China.
Highlights
The impacts of continental air pollution on the remote maritime atmosphere have been a focus of global tropospheric chemistry and climate change research
The seasonal profiles of SO2, NOy, non-methane hydrocarbons (NMHCs) and halocarbons are reported for the first time for the non-urban atmosphere in southern China, So and Wang (2004) presented the C3–C12 NMHC profiles based on limited samples (n = 20) from Hok Tsui
By examining the chemical data and the surface winds at Tai O, we found that air samples with carbon monoxide (CO) mixing ratios ≤125 ppbv were indicative of large-scale inflow of ‘pristine’ background air from the South China Sea (SCS) and the tropics
Summary
The impacts of continental air pollution on the remote maritime atmosphere have been a focus of global tropospheric chemistry and climate change research. Most of the ground-based measurements were made at Hok Tsui, a coastal site situated in the southeast part of Hong Kong and upwind of the PRD region under prevailing northeasterly wind conditions (Figure 1) These studies provided valuable information on the long-range transport of trace gases and aerosols to southern China from central and eastern regions of China, the island of Taiwan, and northeast Asia. These measurements were made in a non-urban area situated over the Pearl River Estuary roughly along the north-south centerline of the PRD, and provide unique data for characterizing the outflow of pollution plumes from the PRD including Hong Kong under northerly and northeasterly winds, and for determining SCS background mixing ratios when southerly winds prevail. The results of this study are expected to be useful for assessing the atmospheric oxidizing capacity, elucidating the transport and chemistry of air pollution, and validating chemical transport models
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