Impact of equatorial and continental airflow on primary greenhouse gases in the northern South China Sea

Abstract

© 2015 IOP Publishing Ltd. Four-year ground-level measurements of the two primary greenhouse gases (carbon dioxide (CO2) and methane (CH4)) were conducted at Dongsha Island (DSI), situated in the northern South China Sea (SCS), from March 2010 to February 2014. Their mean mixing ratios are calculated to be 396.3 ±5.4 ppm and 1863.6 ±50.5 ppb, with an annual growth rate of +2.19 ±0.5 ppm yr-1and +4.70 ±4.4 ppb yr-1for CO2and CH4, respectively, over the study period. Our results suggest that the Asian continental outflow driven by the winter northeast monsoon could have brought air pollutants into the northern SCS, as denoted by significantly elevated levels of 6.5 ppm for CO2and 59.6 ppb for CH4, which are greater than the marine boundary layer references at Cape Kumukahi (KUM) in the tropical northern Pacific in January. By contrast, the summertime CH4at DSI is shown to be lower than that at KUM by 19.7 ppb, whereas CO2is shown to have no differences (<0.42 ppm in July) during the same period. Positive biases of the Greenhouse Gases Observing Satellite (GOSAT) L4B data against the surface measurements are estimated to be 2.4 ±3.4 ppm for CO2and 43.2 ±36.8 ppb for CH4. The satellite products retrieved from the GOSAT showed the effects of anthropogenic emissions and vegetative sinks on land on a vertical profiling basis. The prevailing southeasterly winds originating from as far south as the equator or Southern Hemisphere pass through the lower troposphere in the northern SCS, forming a tunnel of relatively clean air masses as indicated by the low CH4mixing ratios observed on the DSI in summer.