Examining the effects of anthropogenic emissions on isoprene-derived secondary organic aerosol formation during the 2013 Southern Oxidant and Aerosol Study (SOAS) at the Look Rock, Tennessee ground site

摘要

A suite of offline and real-time gas- and particle-phase measurements wasdeployed at Look Rock, Tennessee (TN), during the 2013 Southern Oxidant andAerosol Study (SOAS) to examine the effects of anthropogenic emissions onisoprene-derived secondary organic aerosol (SOA) formation. High- andlow-time-resolution PM samples were collected for analysis of knowntracer compounds in isoprene-derived SOA by gas chromatography/electronionization-mass spectrometry (GC/EI-MS) and ultra performance liquidchromatography/diode array detection-electrospray ionization-high-resolutionquadrupole time-of-flight mass spectrometry (UPLC/DAD-ESI-HR-QTOFMS). Sourceapportionment of the organic aerosol (OA) was determined by positive matrixfactorization (PMF) analysis of mass spectrometric data acquired on anAerodyne Aerosol Chemical Speciation Monitor (ACSM). Campaign average massconcentrations of the sum of quantified isoprene-derived SOA tracerscontributed to ~ 9 % (up to 28 %) of the total OA mass,with isoprene-epoxydiol (IEPOX) chemistry accounting for ~ 97 % of the quantified tracers. PMF analysis resolved a factor with aprofile similar to the IEPOX-OA factor resolved in an Atlanta study and wastherefore designated IEPOX-OA. This factor was strongly correlated( > 0.7) with 2-methyltetrols, C-alkene triols,IEPOX-derived organosulfates, and dimers of organosulfates, confirming therole of IEPOX chemistry as the source. On average, IEPOX-derived SOA tracermass was ~ 26 % (up to 49 %) of the IEPOX-OA factor mass,which accounted for 32 % of the total OA. A low-volatility oxygenatedorganic aerosol (LV-OOA) and an oxidized factor with a profile similar to91Fac observed in areas where emissions are biogenic-dominated were alsoresolved by PMF analysis, whereas no primary organic aerosol (POA) sourcescould be resolved. These findings were consistent with low levels of primarypollutants, such as nitric oxide (NO ~ 0.03 ppb), carbonmonoxide (CO ~ 116 ppb), and black carbon (BC ~ 0.2 μg m). Particle-phase sulfate is fairly correlated( ~ 0.3) with both methacrylic acid epoxide(MAE)/hydroxymethyl-methyl-α-lactone (HMML)- (henceforth calledmethacrolein (MACR)-derived SOA tracers) and IEPOX-derived SOA tracers, andmore strongly correlated ( ~ 0.6) with the IEPOX-OAfactor, in sum suggesting an important role of sulfate in isoprene SOAformation. Moderate correlation between the MACR-derived SOA tracer2-methylglyceric acid with sum of reactive and reservoir nitrogen oxides(NO; = 0.38) and nitrate ( = 0.45) indicates thepotential influence of anthropogenic emissions through long-range transport.Despite the lack of a clear association of IEPOX-OA with locally estimatedaerosol acidity and liquid water content (LWC), box model calculations ofIEPOX uptake using the simpleGAMMA model, accounting for the role of acidityand aerosol water, predicted the abundance of the IEPOX-derived SOA tracers2-methyltetrols and the corresponding sulfates with good accuracy (~ 0.5 and ~ 0.7, respectively). The modeling anddata combined suggest an anthropogenic influence on isoprene-derived SOAformation through acid-catalyzed heterogeneous chemistry of IEPOX in thesoutheastern US. However, it appears that this process was not limited byaerosol acidity or LWC at Look Rock during SOAS. Future studies shouldfurther explore the extent to which acidity and LWC as well as aerosolviscosity and morphology becomes a limiting factor of IEPOX-derived SOA, andtheir modulation by anthropogenic emissions.