Characterization of the size-segregated water-soluble inorganic ions at eight Canadian rural sites

[摘要] Size-segregated water-soluble inorganic ions, includingparticulate sulphate (SO₄^2-), nitrate (NO₃^-), ammonium(NH₄⁺), chloride (Cl^-), and base cations (K⁺, Na⁺,Mg²⁺, Ca²⁺), were measured using a Micro-Orifice Uniform DepositImpactor (MOUDI) during fourteen short-term field campaigns at eightlocations in both polluted and remote regions of eastern and central Canada.The size distributions of SO₄^2- and NH₄⁺ were unimodal,peaking at 0.3–0.6 µm in diameter, during most of the campaigns,although a bimodal distribution was found during one campaign and a trimodaldistribution was found during another campaign made at a coastal site.SO₄^2- peaked at slightly larger sizes in the cold seasons (0.5–0.6 µm)compared to the hot seasons (0.3–0.4 µm) due to the higherrelative humidity in the cold seasons. The size distributions ofNO₃^- were unimodal, peaking at 4.0–7.0 µm during thewarm-season campaigns, and bimodal, with one peak at 0.3–0.6 µm andanother at 4–7 µm during the cold-season campaigns. A unimodal sizedistribution, peaking at 4–6 µm, was found for Cl^-, Na⁺,Mg²⁺, and Ca²⁺ during approximately half of the campaigns and abimodal distribution, with one peak at 2 µm and the other at 6 µm,was found during the rest of the campaigns. For K⁺, a bimodaldistribution, with one peak at 0.3 µm and the other at 4 µm, wasobserved during most of the campaigns. Seasonal contrasts in thesize-distribution profiles suggest that emission sources and air massorigins were the major factors controlling the size distributions of theprimary aerosols while meteorological conditions were more important for thesecondary aerosols.

The dependence of the particle acidity on the particle size from thenucleation mode to the accumulation mode was not consistent from site tosite or from season to season. Particles in the accumulation mode were moreacidic than those in the nucleation mode when submicron particles were inthe state of strong acidity; however, when submicron particles were neutralor weakly acidic, particles in the nucleation mode could sometimes be moreacidic. The inconsistency of the dependence of the particle acidity on theparticle size should have been caused by the different emission sources ofall the related species and the different meteorological conditions duringthe different campaigns. The results presented here at least partiallyexplain the controversial phenomenon found in previous studies on thistopic.

[学科分类] 大气科学