The effect of relative humidity (RH) on the chemical composition of secondary organic aerosol (SOA) formed from low-NO_x toluene oxidation in the absence of seed particles was investigated. SOA samples were prepared in an aerosol smog chamber at < 2 % RH and 75 % RH, collected on Teflon filters, and analyzed with nanospray desorption electrospray ionization high-resolution mass spectrometry (nano-DESI–HRMS). Measurements revealed a significant reduction in the fraction of oligomers present in the SOA generated at 75 % RH compared to SOA generated under dry conditions. In a separate set of experiments, the particle mass concentrations were measured with a scanning mobility particle sizer (SMPS) at RHs ranging from < 2 to 90 %. It was found that the particle mass loading decreased by nearly an order of magnitude when RH increased from < 2 to 75–90 % for low-NO_x toluene SOA. The volatility distributions of the SOA compounds, estimated from the distribution of molecular formulas using the molecular corridor approach, confirmed that low-NO_x toluene SOA became more volatile on average under high-RH conditions. In contrast, the effect of RH on SOA mass loading was found to be much smaller for high-NO_x toluene SOA. The observed increase in the oligomer fraction and particle mass loading under dry conditions were attributed to the enhancement of condensation reactions, which produce water and oligomers from smaller compounds in low-NO_x toluene SOA. The reduction in the fraction of oligomeric compounds under humid conditions is predicted to partly counteract the previously observed enhancement in the toluene SOA yield driven by the aerosol liquid water chemistry in deliquesced inorganic seed particles.

This paper presents a comprehensive description and benchmark evaluation of the tropospheric gas-phase chemistry component of the Multiscale Online Nonhydrostatic AtmospheRe CHemistry model (NMMB-MONARCH), formerly known as NMMB/BSC-CTM, that can be run on both regional and global domains. Here, we provide an extensive evaluation of a global annual cycle simulation using a variety of background surface stations (EMEP, WDCGG and CASTNET), ozonesondes (WOUDC, CMD and SHADOZ), aircraft data (MOZAIC and several campaigns), and satellite observations (SCIAMACHY and MOPITT). We also include an extensive discussion of our results in comparison to other state-of-the-art models. We note that in this study, we omitted aerosol processes and some natural emissions (lightning and volcano emissions). The model shows a realistic oxidative capacity across the globe. The seasonal cycle for CO is fairly well represented at different locations (correlations around 0.3–0.7 in surface concentrations), although concentrations are underestimated in spring and winter in the Northern Hemisphere, and are overestimated throughout the year at 800 and 500 hPa in the Southern Hemisphere. Nitrogen species are well represented in almost all locations, particularly NO₂ in Europe (root mean square error – RMSE – below 5 ppb). The modeled vertical distributions of NO_x and HNO₃ are in excellent agreement with the observed values and the spatial and seasonal trends of tropospheric NO₂ columns correspond well to observations from SCIAMACHY, capturing the highly polluted areas and the biomass burning cycle throughout the year. Over Asia, the model underestimates NO_x from March to August, probably due to an underestimation of NO_x emissions in the region. Overall, the comparison of the modeled CO and NO₂ with MOPITT and SCIAMACHY observations emphasizes the need for more accurate emission rates from anthropogenic and biomass burning sources (i.e., specification of temporal variability). The resulting ozone (O₃) burden (348 Tg) lies within the range of other state-of-the-art global atmospheric chemistry models. The model generally captures the spatial and seasonal trends of background surface O₃ and its vertical distribution. However, the model tends to overestimate O₃ throughout the troposphere in several stations. This may be attributed to an overestimation of CO concentration over the Southern Hemisphere leading to an excessive production of O₃ or to the lack of specific chemistry (e.g., halogen chemistry, aerosol chemistry). Overall, O₃ correlations range between 0.6 and 0.8 for daily mean values. The overall performance of the NMMB-MONARCH is comparable to that of other state-of-the-art global chemistry models.

While there is increasing evidence for unique chemical reactions at interfaces, there are fewer data on photochemistry at liquid–vapor junctions. This paper reports a comparison of the photolysis of molybdenum hexacarbonyl, Mo(CO)₆, in 1-decene either as liquid droplets or in bulk-liquid solutions. Mo(CO)₆ photolysis is faster by at least three orders of magnitude in the aerosols than in bulk-liquids. Two possible sources of this enhancement are considered: (1) increased light intensity due to either Morphology-Dependent Resonances (MDRs) in the spherical aerosol particles and/or to increased pathlengths for light inside the droplet due to refraction, which are termed physical effects in this paper; and (2) interface effects such as an incomplete solvent-cage at the gas–liquid boundary and/or enhanced interfacial concentrations of Mo(CO)₆, which are termed chemical effects. Quantitative calculations of the first possibility were carried out in which the light intensity distribution in the droplets averaged over 215–360 nm was obtained for 1-decene droplets. Calculations show that the average increase in light intensity over the entire droplet is 106%, with an average increase of 51% at the interface. These increases are much smaller than the observed increase in the apparent photolysis rate of droplets compared to the bulk. Thus, chemical effects, i.e., a decreased solvent-cage effect at the interface and/or enhancement in the surface concentration of Mo(CO)₆, are most likely responsible for the dramatic increase in the photolysis rate. Similar calculations were also carried out for broadband (290–600 nm) solar irradiation of water droplets, relevant to atmospheric conditions. These calculations show that, in agreement with previous calculations by Mayer and Madronich [B. Mayer and S. Madronich, Atmos. Chem. Phys., 2004, 4, 2241] MDRs produce only a moderate average intensity enhancement relative to the corresponding bulk-liquid slabs when averaged over a range of wavelengths characteristic of solar radiation at the Earth’s surface. However, as in the case of Mo(CO)₆ in 1-decene, chemical effects may play a role in enhanced photochemistry at the aerosol–air interface for airborne particles.

Laboratory data suggest that atmospheric models are missing a source of hydroxyl, which plays a central role in tropospheric ozone production.