Effect of humidity on the composition of isoprene photooxidation secondary organic aerosol
The effect of relative humidity (RH) on the composition and concentrations\nof gas-phase products and secondary organic aerosol (SOA) generated\nfrom the photooxidation of isoprene under high-NOx conditions was\ninvestigated. Experiments were performed with hydrogen peroxide as\nthe OH precursor and in the absence of seed aerosol. The relative\nyields of most gas-phase products were the same regardless of initial\nwater vapor concentration with exception of hydroxyacetone and glycolaldehyde,\nwhich were considerably affected by RH. A significant change was\nobserved in the SOA composition, with many unique condensed-phase\nproducts formed under humid (90 % RH) vs. dry (<2 % RH) conditions,\nwithout any detectable effect on the rate and extent of the SOA mass\ngrowth. There is a 40 % reduction in the number and relative abundance\nof distinct particle-phase nitrogen-containing organic compounds\n(NOC) detected by high resolution mass spectrometry. The suppression\nof condensation reactions, which produce water as a product, is the\nmost important chemical effect of the increased RH. For example,\nthe total signal from oligomeric esters of 2-methylglyceric acid\nwas reduced by about 60 % under humid conditions and the maximum\noligomer chain lengths were reduced by 7–11 carbons. Oligomers formed\nby addition mechanisms, without direct involvement of water, also\ndecreased at elevated RH but to a much smaller extent. The observed\nreduction in the extent of condensation-type oligomerization at high\nRH may have substantial impact on the phase characteristics and hygroscopicity\nof the isoprene aerosol. The reduction in the amount of organic nitrates\nin the particle phase has implications for understanding the budget\nof NOC compounds.