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Abstract. Isoprene has the largest global non-methane hydrocarbon emission, and the oxidation of isoprene plays a crucial role in the formation of secondary organic aerosol (SOA). Two primary processes are known to contribute to SOA formation from isoprene oxidation: (1) the reactive uptake of isoprene-derived epoxides on acidic or aqueous particle surfaces and (2) the absorptive gas–particle partitioning of low-volatility oxidation products. In this study, we developed a new multiphase condensed isoprene oxidation mechanism that includes these processes with key molecular intermediates and products. The new mechanism was applied to simulate isoprene gas-phase oxidation products and SOA formation from previously published chamber experiments under a variety of conditions and atmospheric observations during the Southern Oxidant and Aerosol Studies (SOAS) field campaign. Our results show that SOA formation from most of the chamber experiments is reasonably reproduced using our mechanism, except when the concentration ratios of initial nitric oxide to isoprene exceed ∼ 2, the formed SOA is significantly underpredicted. The SOAS simulations also reasonably agree with the measurements regarding the diurnal pattern and concentrations of different product categories, while the total isoprene SOA remains underestimated. The molecular compositions of the modeled SOA indicate that multifunctional low-volatility products contribute to isoprene SOA more significantly than previously thought, with a median mass contribution of ∼ 57 % to the total modeled isoprene SOA. However, this contribution is intricately intertwined with IEPOX-derived SOA (IEPOX: isoprene-derived epoxydiols), posing challenges for their differentiation using bulk aerosol composition analysis (e.g., the aerosol mass spectrometer with positive matrix factorization). Furthermore, the SOA from these pathways may vary greatly, mainly dependent on the volatility estimation and treatment of particle-phase processes (i.e., photolysis and hydrolysis). Our findings emphasize that the various pathways to produce these low-volatility species should be considered in models to more accurately predict isoprene SOA formation. The new condensed isoprene chemical mechanism can be further incorporated into regional-scale air quality models, such as the Community Multiscale Air Quality Modelling System (CMAQ), to assess isoprene SOA formation on a larger scale. 

Abstract. A new inlet for studying the aerosol particles andhydrometeor residuals that compose mixed-phase clouds – the phaSeseParation Inlet for Droplets icE residuals and inteRstitial aerosolparticles (SPIDER) – is described here. SPIDER combines a large pumpedcounterflow virtual impactor (L-PCVI), a flow tube evaporation chamber, anda pumped counterflow virtual impactor (PCVI) to separate droplets, icecrystals (∼3–25 µm), and interstitial aerosolparticles for simultaneous sampling. Laboratory verification tests of eachindividual component and the composite SPIDER system were conducted.Transmission efficiency, evaporation, and ice crystals' survival weredetermined to show the capability of the system. The experiments show theSPIDER system can separate distinct cloud elements and interstitial aerosolparticles for subsequent analysis. As a field instrument, SPIDER will helpexplore the properties of different cloud elements and interstitial aerosolparticles in mixed-phase clouds. 

Abstract. The aerosol–planetary boundary layer (PBL) interaction wasproposed as an important mechanism to stabilize the atmosphere andexacerbate surface air pollution. Despite the tremendous progress made inunderstanding this process, its magnitude and significance still have largeuncertainties and vary largely with aerosol distribution and meteorologicalconditions. In this study, we focus on the role of aerosol verticaldistribution in thermodynamic stability and PBL development by jointly usingmicropulse lidar, sun photometer, and radiosonde measurements taken inBeijing. Despite the complexity of aerosol vertical distributions,cloud-free aerosol structures can be largely classified into three types:well-mixed, decreasing with height, and inverse structures. The aerosol–PBLrelationship and diurnal cycles of the PBL height and PM2.5 associated with these different aerosol vertical structures showdistinct characteristics. The vertical distribution of aerosol radiativeforcing differs drastically among the three types, with strong heating in thelower, middle, and upper PBL, respectively. Such a discrepancy in the heatingrate affects the atmospheric buoyancy and stability differently in the threedistinct aerosol structures. Absorbing aerosols have a weaker effect ofstabilizing the lower atmosphere under the decreasing structure than underthe inverse structure. As a result, the aerosol–PBL interaction can bestrengthened by the inverse aerosol structure and can be potentiallyneutralized by the decreasing structure. Moreover, aerosols can both enhanceand suppress PBL stability, leading to both positive and negativefeedback loops. This study attempts to improve our understanding of theaerosol–PBL interaction, showing the importance of the observationalconstraint of aerosol vertical distribution for simulating this interactionand consequent feedbacks.