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Mass accommodation of water: bridging the gap between molecular dynamics simulations and kinetic condensation models.

ABSTRACT: The condensational growth of submicrometer aerosol particles to climate relevant sizes is sensitive to their ability to accommodate vapor molecules, which is described by the mass accommodation coefficient. However, the underlying processes are not yet fully understood. We have simulated the mass accommodation and evaporation processes of water using molecular dynamics, and the results are compared to the condensation equations derived from the kinetic gas theory to shed light on the compatibility of the two. Molecular dynamics simulations were performed for a planar TIP4P-Ew water surface at four temperatures in the range 268-300 K as well as two droplets, with radii of 1.92 and 4.14 nm at T = 273.15 K. The evaporation flux from molecular dynamics was found to be in good qualitative agreement with that predicted by the simple kinetic condensation equations. Water droplet growth was also modeled with the kinetic multilayer model KM-GAP of Shiraiwa et al. [Atmos. Chem. Phys. 2012, 12, 2777]. It was found that, due to the fast transport across the interface, the growth of a pure water droplet is controlled by gas phase diffusion. These facts indicate that the simple kinetic treatment is sufficient in describing pure water condensation and evaporation. The droplet size was found to have minimal effect on the value of the mass accommodation coefficient. The mass accommodation coefficient was found to be unity (within 0.004) for all studied surfaces, which is in agreement with previous simulation work. Additionally, the simulated evaporation fluxes imply that the evaporation coefficient is also unity. Comparing the evaporation rates of the mass accommodation and evaporation simulations indicated that the high collision flux, corresponding to high supersaturation, present in typical molecular dynamics mass accommodation simulations can under certain conditions lead to an increase in the evaporation rate. Consequently, in such situations the mass accommodation coefficient can be overestimated, but in the present cases the corrected values were still close to unity with the lowest value at ≈0.99.

SUBMITTER: Julin J

PROVIDER: S-EPMC3600785 | biostudies-other | 2013 Jan

REPOSITORIES: biostudies-other

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Project description:INTRODUCTION:Significant gaps persist in providing HIV treatment to all who are in need. Restricting care delivery to healthcare facilities will continue to perpetuate this gap in limited resource settings. We assessed a large-scale community-based programme for effectiveness in identifying people living with HIV and linking them to antiretroviral treatment. METHODS:A retrospective secular trend study of 14 high burden local government areas of Nigeria was conducted in which two models of community antiretroviral treatment delivery were implemented: Model A (on-site initiation) and Model B (immediate referral) clusters. Model A cluster offered services within communities, from HIV diagnosis to immediate antiretroviral therapy initiation and some follow-up. Model B cluster offered services for HIV diagnosis up to baseline evaluation and provided referral for antiretroviral therapy initiation to nearest health facility providing HIV services. For controls, we selected and cluster-matched 34 local government areas where community antiretroviral treatment delivery was not implemented. Outcomes of interest were: the number of people identified as HIV positive and the number of HIV-positive individuals started on antiretroviral treatment; from June 2014 to May 2016. We used interrupted time-series analysis to estimate outcome levels and trends across the pre-and post-intervention periods. RESULTS:Before community antiretrovial treatment introduction, Model A cluster identified, per 100,000 catchment population, 500 HIV-positives (95% CI: 399.66 to 601.41) and initiated 216 HIV-positives on antiretroviral treatment (95% CI: 152.72 to 280.10). Model B cluster identified 32 HIV-positives (95% CI: 25.00 to 40.51) and initiated 8 HIV-positives on antiretroviral treatment (95% CI: 5.54 to 10.33). After commART introduction, Model A cluster showed an immediate significant increase in 744 HIV-positive persons (p = 0.00, 95% CI: 360.35 to 1127.77) and 560 HIV-positives initiated on treatment (p = 0.00, 95% CI: 260.56 to 859.64). Model B cluster showed an immediate significant increase in 30 HIV-positive persons identified (p = 0.01, 95% CI: 8.38 to 51.93) but not in the number of HIV-positives initiated on treatment. Model B cluster showed increased month-on-month trends of both outcomes of interest (3.4, p = 0.02, 95% CI: 0.44 to 6.38). CONCLUSION:Both community-models had similar population-level effectiveness for rapidly identifying people living with HIV but differed in effectively transitioning them to treatment. Comprehensiveness, integration and attention to barriers to care are important in the design of community antiretroviral treatment delivery.

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Mass accommodation of water: bridging the gap between molecular dynamics simulations and kinetic condensation models.

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