Abstract
Abstract. As part of the 2008 Campaign of Air Quality Research in Beijing and Surrounding Regions (CAREBeijing 2008), measurements of gaseous sulfuric acid (H2SO4) 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 H2SO4 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 × 106 molecules cm−3. 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 H2SO4 diurnal variation within the uncertainties, indicating that photochemical production and condensation onto preexisting particle surface dominate the observed diurnal H2SO4 profile. The frequency of the peak H2SO4 concentration exceeding 5 × 106 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 H2SO4 molecules (R2 = 0.85). The calculated condensation rate of H2SO4 can only account for 10–25 % of PM1 sulfate formation, indicating that either much stronger sulfate production exists at the SO2 source region or other sulfate production mechanisms are responsible for the sulfate production.
Highlights
Gaseous sulfuric acid (GSA) is of critical atmospheric interest for its important role in various atmospheric processes, such as new particle formation (Seinfeld and Pandis, 1998; Finlayson-Pitts and Pitts, 1999), modification of optical properties and hygroscopicity of preexisting aerosols (Khalizov et al, 2009a, b), and formation of polar stratospheric clouds (Zhang et al, 1993a, b, 1996)
Since the instrument was located about 5 m below the observatory rooftop and the inlet can only be positioned toward the east, these physical conditions may cause some bias for the GSA measurements to represent the real ambient level of GSA, especially under certain wind direction and a high wind speed
A normalized sequential difference (NSD) analysis of the GSA data is performed to evaluate the performance of the AP-ID-CIMS instrument and measurement results
Summary
Gaseous sulfuric acid (GSA) is of critical atmospheric interest for its important role in various atmospheric processes, such as new particle formation (Seinfeld and Pandis, 1998; Finlayson-Pitts and Pitts, 1999), modification of optical properties and hygroscopicity of preexisting aerosols (Khalizov et al, 2009a, b), and formation of polar stratospheric clouds (Zhang et al, 1993a, b, 1996). H2SO4 forms molecular complexes with inorganic and organic species (Zhang et al, 2004) The size of these complexes is usually less than a few nanometers, below the detection limit of conventional instrumentation for nanoparticles. Sulfuric acid can efficiently catalyze aqueous reactions of organic compounds, formed from photo-oxidation of volatile organic compounds (Suh et al, 2001; Zhang et al, 2002b), contributing to the formation of secondary organic aerosols (Zhao et al, 2005, 2006). Rapid soot aging has been reported in the Beijing area, when the single scattering albedo of the aerosols increases to values above 0.9 within several hours due to secondary aerosol formation and condensation growth (Cheng et al, 2009).
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