Reactive intermediates revealed in secondary organic aerosol formation from isoprene.

Isoprene is a significant source of atmospheric organic aerosol; however, the oxidation pathways that lead to secondary organic aerosol (SOA) have remained elusive. Here, we identify the role of two key reactive intermediates, epoxydiols of isoprene (IEPOX = beta-IEPOX + delta-IEPOX) and methacryloylperoxynitrate (MPAN), which are formed during isoprene oxidation under low- and high-NO(x) conditions, respectively. Isoprene low-NO(x) SOA is enhanced in the presence of acidified sulfate seed aerosol (mass yield 28.6%) over that in the presence of neutral aerosol (mass yield 1.3%). Increased uptake of IEPOX by acid-catalyzed particle-phase reactions is shown to explain this enhancement. Under high-NO(x) conditions, isoprene SOA formation occurs through oxidation of its second-generation product, MPAN. The similarity of the composition of SOA formed from the photooxidation of MPAN to that formed from isoprene and methacrolein demonstrates the role of MPAN in the formation of isoprene high-NO(x) SOA. Reactions of IEPOX and MPAN in the presence of anthropogenic pollutants (i.e., acidic aerosol produced from the oxidation of SO(2) and NO(2), respectively) could be a substantial source of "missing urban SOA" not included in current atmospheric models.

Photooxidation of 2-methyl-3-Buten-2-ol (MBO) as a potential source of secondary organic aerosol.

2-Methyl-3-buten-2-ol (MBO) is an important biogenic hydrocarbon emitted in large quantities by pine forests. Atmospheric photooxidation of MBO is known to lead to oxygenated compounds, such as glycolaldehyde, which is the precursor to glyoxal. Recent studies have shown that the reactive uptake of glyoxal onto aqueous particles can lead to formation of secondary organic aerosol (SOA). In this work, MBO photooxidation under high- and low-NO(x) conditions was performed in dual laboratory chambers to quantify the yield of glyoxal and investigate the potential for SOA formation. The yields of glycolaldehyde and 2-hydroxy-2-methylpropanal (HMPR), fragmentation products of MBO photooxidation, were observed to be lower at lower NO(x) concentrations. Overall, the glyoxal yield from MBO photooxidation was 25% under high-NO(x) and 4% under low-NO(x) conditions. In the presence of wet ammonium sulfate seed and under high-NO(x) conditions, glyoxal uptake and SOA formation were not observed conclusively, due to relatively low (< 30 ppb) glyoxal concentrations. Slight aerosol formation was observed under low-NO(x) and dry conditions, with aerosol mass yields on the order of 0.1%. The small amount of SOA was not related to glyoxal uptake, but is likely a result of reactions similar to those that generate isoprene SOA under low-NO(x) conditions. The difference in aerosol yields between MBO and isoprene photooxidation under low-NO(x) conditions is consistent with the difference in vapor pressures between triols (from MBO) and tetrols (from isoprene). Despite its structural similarity to isoprene, photooxidation of MBO is not expected to make a significant contribution to SOA formation.

Direct measurements of the convective recycling of the upper troposphere.

We present a statistical representation of the aggregate effects of deep convection on the chemistry and dynamics of the upper troposphere (UT) based on direct aircraft observations of the chemical composition of the UT over the eastern United States and Canada during summer. These measurements provide unique observational constraints on the chemistry occurring downwind of convection and the rate at which air in the UT is recycled. These results provide quantitative measures that can be used to evaluate global climate and chemistry models.

Measurement of gas-phase hydroperoxides by chemical ionization mass spectrometry.

A new method for the detection of gas-phase hydroperoxides is described. The clustering chemistry of CF3O- is exploited to produce speciated measurements of several hydroperoxides with high sensitivity and fast time response. Correspondence of airborne observations made with this technique and the established HPLC method is illustrated. CF3O- appears to be a highly versatile reagent ion for measurements of both weak and strong acids in the atmosphere.