Washington - The US will help China implement stricter emission standards for vehicles in a bid to help the world's biggest carbon emitter tackle rampant air pollution, the White House announced on Thursday.The announcement was one of several made at the conclusion of US Vice President Joe Biden's visit to China on Thursday, where he met with President Xi Jinping and other senior Chinese officials to discuss ways to strengthen economic ties between the countries in addition to the escalating geopolitical tensions in the East China Sea.Under the new agreement, the US pledged to give China technical assistance to implement a new round of vehicle emissions standards, known as China VI, which would require cars to have filters that capture particulate matter that contributes to heavy smog."These standards, when implemented, will have significant air quality and climate benefits and reduce vehicle fuel use," according to a White House fact sheet.Addressing climate change internationally through both multilateral and bilateral relationships is a pillar of US President Barack Obama's Climate Action Plan, a strategy released in June to tackle heat-trapping greenhouse gases.RestrictionsChina and the US are the world's No 1 and No 2 emitters of greenhouse gas emissions, respectively. Experts say joint action between the two provides the biggest hope for tackling global climate change.According to China's Ministry of Public Security, passenger car ownership in China reached 120 million by the end of 2012. At the current growth rate, passenger car ownership will top 200 million by 2020.But as some of China's major cities try to cope with choking air pollution, they have placed new restrictions on vehicle sales.China is currently in the process of implementing its fourth-stage emissions standards, or China IV diesel standards, which would cap the allowed sulphur content at 50 parts per million next year, down from current levels of 350 parts per million.China V standards for diesel and refined gasoline will be rolled out next; they will not take effect until 2017. They will lower the sulphur content limit to 10 parts per million.The city of Beijing, which has a population of over 20 million and will have as many as six million private cars by 2015, implemented the China V standard in February to tackle record pollution that surpassed hazardous levels early this year.By contrast, the US allows a sulphur content of 15 parts per million while the EU allows diesel fuel to have a sulphur content of 10 parts per million.StandardsOne US official said it is a significant development that the US is helping China jump-start the sixth stage of vehicle emissions standards while it is still working on its 2017 standards."The United States is interested in moving to China to six as soon as possible," the official said. "It is a clear signal that China wants to move forward in an accelerated way that will have far reaching impacts on air quality and public health."US Environmental Protection Agency and Energy Department officials will help Chinese counterparts with modelling, testing and other technical research required for developing those standards.The two countries also agreed to continue working together to phase down the consumption of hydrofluorocarbons, a highly potent greenhouse gas used in refrigeration.They also agreed to jointly study phasing out fossil fuel subsidies in both countries.