Measurements of the photochemical production of aerosols in ambient air near a freeway for a range of SO2 concentrations

The formation and growth of photochemically produced aerosol particles were studied for initially particle-free ambient air collected downwind from a freeway in two 14 m³ transparent Teflon bags exposed to natural sunlight for SO₂ concentrations ranging from 0.006 to 0.067 ppm, by volume. Using a Thermo Systems Electrical Aerosol Analyzer, the concentration and size distribution of aerosol particles from 0.0056 to 0.36 μm dia. was measured as a function of time. The volumetric conversion rate of material from the vapor to the condensed phase was found to be a linear function of the initial SO₂ concentration and directly proportional to the initial non-methane hydrocarbon concentration in the bags. Filter samples of the photochemically produced aerosol particles were analyzed for sulfate, nitrate and nitrite. The sulfate and nitrate formation rates ranged from 0.72 to 2.6 μg m⁻³ h⁻¹ and 0.76 to 3.2 μg m⁻³ h⁻¹, respectively, while the nitrite formation rate ranged from 0.008 to 0.038 μg m⁻³ h⁻¹. The overall rate of SO₂ to sulfate conversion was estimated to range from 0.55 to 1.3% h⁻¹. X-ray photoelectron spectroscopy was used to determine the chemical states of sulfur, nitrogen and carbon on the surface of the filter samples. Sulfur was found in the form of SO²⁻₄, as well as chemisorbed SO₂ and SO₃. Nitrogen was present as NO⁻₃ and as two reduced forms identified as amines or amides, and carbon was present as either elemental carbon or as paraffinic hydrocarbon.