As part of the 2008 Campaign of Air Quality Research in Beijing and Surrounding Regions (CAREBeijing 2008), measurements of gaseous sulfuric acid (H<sub>2</sub>SO<sub>4</sub>) have been conducted at an urban site in Beijing, China from 7 July to 25 September 2008 using atmospheric pressure ion drift – chemical ionization mass spectrometry (AP-ID-CIMS). This represents the first gaseous H<sub>2</sub>SO<sub>4</sub> measurements in China. Diurnal profile of sulfuric acid is strongly dependent on the actinic flux, reaching a daily maximum around noontime and with an hourly average concentration of 5 × 10<sup>6</sup> molecules cm<sup>−3</sup>. Simulation of sulfuric acid on the basis of the measured sulfur dioxide concentration, photolysis rates of ozone and nitrogen dioxide, and aerosol surface areas captures the trend of the measured H<sub>2</sub>SO<sub>4</sub> diurnal variation within the uncertainties, indicating that photochemical production and condensation onto preexisting particle surface dominate the observed diurnal H<sub>2</sub>SO<sub>4</sub> profile. The frequency of the peak H<sub>2</sub>SO<sub>4</sub> concentration exceeding 5 × 10<sup>6</sup> molecules cm<sup>−3</sup> increases by 16 % during the period of the summer Olympic Games (8–24 August 2008), because of the implementation of air quality control regulations. Using a multivariate statistical method, the critical nucleus during nucleation events is inferred, containing two H<sub>2</sub>SO<sub>4</sub> molecules (<i>R</i><sup>2</sup> = 0.85). The calculated condensation rate of H<sub>2</sub>SO<sub>4</sub> can only account for 10–25 % of PM<sub>1</sub> sulfate formation, indicating that either much stronger sulfate production exists at the SO<sub>2</sub> source region or other sulfate production mechanisms are responsible for the sulfate production.