Zhang Qi, Jimenez Jose L, Worsnop Douglas R, Canagaratna Manjula
Atmospheric Sciences Research Center (ASRC) and Department of Environmental Health Sciences, 251 Fuller Road, CESTM L110, University at Albany, State University of New York, Albany, New York 12203, USA.
Environ Sci Technol. 2007 May 1;41(9):3213-9. doi: 10.1021/es061812j.
Size-resolved indicators of aerosol acidity, including H+ ion concentrations (H+Aer) and the ratio of stoichiometric neutralization are evaluated in submicrometer aerosols using highly time-resolved aerosol mass spectrometer (AMS) data from Pittsburgh. The pH and ionic strength within the aqueous particle phase are also estimated using the Aerosol Inorganics Model (AIM). Different mechanisms that contribute to the presence of acidic particles in Pittsburgh are discussed. The largest H+Aer loadings and lowest levels of stoichiometric neutralization were detected when PM1 loadings were high and dominated by SO4(2-). The average size distribution of H+Aer loading shows an accumulation mode at Dva approximately 600 nm and an enhanced smaller mode that centers at Dva approximately 200 nm and tails into smaller sizes. The acidity in the accumulation mode particles suggests that there is generally not enough gas-phase NH3 available on a regional scale to completely neutralize sulfate in Pittsburgh. The lack of stoichiometric neutralization in the 200 nm mode particles is likely caused by the relatively slow mixing of gas-phase NH3 into SO2-rich plumes containing younger particles. We examined the influence of particle acidity on secondary organic aerosol (SOA) formation by comparing the mass concentrations and size distributions of oxygenated organic aerosol (00A--surrogate for SOA in Pittsburgh) during periods when particles are, on average, acidic to those when particles are bulk neutralized. The average mass concentration of ODA during the acidic periods (3.1 +/- 1.7 microg m(-3)) is higher than that during the neutralized periods (2.5 +/- 1.3 microg m(-3)). Possible reasons for this enhancement include increased condensation of SOA species, acid-catalyzed SOA formation, and/or differences in air mass transport and history. However, even if the entire enhancement (approximately 0.6 microg m(-3)) can be attributed to acid catalysis, the upperbound increase of SOA mass in acidic particles is approximately 25%, an enhancement that is much more moderate than the multifold increases in SOA mass observed during some lab studies and inferred in SO2-rich industrial plumes. In addition, the mass spectra of OOA from these two periods are almost identical with no discernible increase in relative signal intensity at larger m/z's (>200 amu), suggesting that the chemical nature of SOA is similar during acidic and neutralized periods and that there is no significant enhancement of SOA oligomer formation during acidic particle periods in Pittsburgh.
利用来自匹兹堡的高时间分辨率气溶胶质谱仪（AMS）数据，对亚微米气溶胶中气溶胶酸度的尺寸分辨指标进行了评估，包括氢离子浓度（H⁺Aer）和化学计量中和比。还使用气溶胶无机物模型（AIM）估算了水相颗粒相中的pH值和离子强度。讨论了导致匹兹堡酸性颗粒存在的不同机制。当PM1负荷较高且以SO₄²⁻为主时，检测到最大的H⁺Aer负荷和最低的化学计量中和水平。H⁺Aer负荷的平均尺寸分布在Dva约600 nm处呈现积聚模式，在Dva约200 nm处有一个增强的较小模式，并向更小尺寸延伸。积聚模式颗粒中的酸度表明，在区域尺度上，通常没有足够的气相NH₃来完全中和匹兹堡的硫酸盐。200 nm模式颗粒中缺乏化学计量中和可能是由于气相NH₃相对缓慢地混合到含有较年轻颗粒的富SO₂羽流中。通过比较颗粒平均呈酸性时期和颗粒整体中和时期含氧有机气溶胶（OOA，匹兹堡SOA的替代物）的质量浓度和尺寸分布，研究了颗粒酸度对二次有机气溶胶（SOA）形成的影响。酸性时期ODA的平均质量浓度（3.1±1.7 μg m⁻³）高于中和时期（2.5±1.3 μg m⁻³）。这种增强的可能原因包括SOA物种的冷凝增加、酸催化的SOA形成和/或气团传输和历史的差异。然而，即使整个增强（约0.6 μg m⁻³）可归因于酸催化，酸性颗粒中SOA质量的上限增加约为25%，这一增强比一些实验室研究中观察到的以及在富SO₂工业羽流中推断的SOA质量的多倍增加要温和得多。此外，这两个时期OOA的质谱几乎相同，在较大的m/z（>200 amu）处相对信号强度没有明显增加，这表明在酸性和中和时期SOA的化学性质相似，并且在匹兹堡酸性颗粒时期SOA低聚物形成没有显著增强。
