Measurements of gaseous H 2 SO 4 by AP - ID - CIMS during CAREBeijing 2008 Campaign

As part of the 2008 Campaign of Air Quality Research in Beijing and Surrounding Regions (CAREBei-jing 2008), measurements of gaseous sulfuric acid (H 2 SO 4) 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 2 SO 4 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 6 molecules cm −3. Simulation of sulfuric acid on the basis of the measured sulfur dioxide concentration, pho-tolysis rates of ozone and nitrogen dioxide, and aerosol surface areas captures the trend of the measured H 2 SO 4 diur-nal variation within the uncertainties, indicating that photo-chemical production and condensation onto preexisting particle surface dominate the observed diurnal H 2 SO 4 profile. The frequency of the peak H 2 SO 4 concentration exceeding 5 × 10 6 molecules cm −3 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 2 SO 4 molecules (R 2 = 0.85). The calculated condensation rate of H 2 SO 4 can only account for 10–25 % of PM 1 sul-fate formation, indicating that either much stronger sulfate production exists at the SO 2 source region or other sulfate production mechanisms are responsible for the sulfate production .