A Case Study of Urban Particle Acidity and Its Influence on Secondary Organic Aerosol
详细信息 查看全文
- 作者：Qi Zhang ; Jose L. Jimenez ; Douglas R. Worsnop ; Manjula Canagaratna
- 刊名：Environmental Science & Technology
- 出版年：2007
- 出版时间：May 1, 2007
- 年：2007
- 卷：41
- 期：9
- 页码：3213 - 3219
- 全文大小：501K
- 年卷期：v.41,no.9(May 1, 2007)
- ISSN：1520-5851
文摘
Size-resolved indicators of aerosol acidity, including H+ion concentrations (H+Aer) and the ratio of stoichiometricneutralization are evaluated in submicrometer aerosolsusing highly time-resolved aerosol mass spectrometer (AMS)data from Pittsburgh. The pH and ionic strength withinthe aqueous particle phase are also estimated using theAerosol Inorganics Model (AIM). Different mechanisms thatcontribute to the presence of acidic particles in Pittsburghare discussed. The largest H+Aer loadings and lowestlevels of stoichiometric neutralization were detected whenPM1 loadings were high and dominated by SO42-. Theaverage size distribution of H+Aer loading shows anaccumulation mode at Dva 
600 nm and an enhancedsmaller mode that centers at Dva 200 nm and tails intosmaller sizes. The acidity in the accumulation mode particlessuggests that there is generally not enough gas-phaseNH3 available on a regional scale to completely neutralizesulfate in Pittsburgh. The lack of stoichiometric neutralizationin the 200 nm mode particles is likely caused by the relativelyslow mixing of gas-phase NH3 into SO2-rich plumescontaining younger particles. We examined the influenceof particle acidity on secondary organic aerosol (SOA)formation by comparing the mass concentrations and sizedistributions of oxygenated organic aerosol (OOA-surrogate for SOA in Pittsburgh) during periods whenparticles are, on average, acidic to those when particlesare bulk neutralized. The average mass concentration of OOAduring the acidic periods (3.1 ± 1.7 
g m-3) is higherthan that during the neutralized periods (2.5 ± 1.3 g m-3).Possible reasons for this enhancement include increasedcondensation of SOA species, acid-catalyzed SOAformation, and/or differences in air mass transport andhistory. However, even if the entire enhancement (~0.6 gm-3) can be attributed to acid catalysis, the upper-bound increase of SOA mass in acidic particles is ~25%,an enhancement that is much more moderate than themultifold increases in SOA mass observed during some labstudies and inferred in SO2-rich industrial plumes. Inaddition, the mass spectra of OOA from these two periodsare almost identical with no discernible increase inrelative signal intensity at larger m/z's (>200 amu), suggestingthat the chemical nature of SOA is similar during acidicand neutralized periods and that there is no significantenhancement of SOA oligomer formation during acidic particleperiods in Pittsburgh.
600 nm and an enhancedsmaller mode that centers at Dva 200 nm and tails intosmaller sizes. The acidity in the accumulation mode particlessuggests that there is generally not enough gas-phaseNH3 available on a regional scale to completely neutralizesulfate in Pittsburgh. The lack of stoichiometric neutralizationin the 200 nm mode particles is likely caused by the relativelyslow mixing of gas-phase NH3 into SO2-rich plumescontaining younger particles. We examined the influenceof particle acidity on secondary organic aerosol (SOA)formation by comparing the mass concentrations and sizedistributions of oxygenated organic aerosol (OOA-surrogate for SOA in Pittsburgh) during periods whenparticles are, on average, acidic to those when particlesare bulk neutralized. The average mass concentration of OOAduring the acidic periods (3.1 ± 1.7 g m-3) is higherthan that during the neutralized periods (2.5 ± 1.3 g m-3).Possible reasons for this enhancement include increasedcondensation of SOA species, acid-catalyzed SOAformation, and/or differences in air mass transport andhistory. However, even if the entire enhancement (~0.6 gm-3) can be attributed to acid catalysis, the upper-bound increase of SOA mass in acidic particles is ~25%,an enhancement that is much more moderate than themultifold increases in SOA mass observed during some labstudies and inferred in SO2-rich industrial plumes. Inaddition, the mass spectra of OOA from these two periodsare almost identical with no discernible increase inrelative signal intensity at larger m/z's (>200 amu), suggestingthat the chemical nature of SOA is similar during acidicand neutralized periods and that there is no significantenhancement of SOA oligomer formation during acidic particleperiods in Pittsburgh.