Assessment of carbonaceous aerosols at Mukteshwar: A high-altitude (~2200 m amsl) background site in the foothills of the Central Himalayas.

The present study examined the equivalent black carbon (eBC) mass concentrations measured over 10.5 years (September 2005-March 2016) using a 7-wavelength Aethalometer (AE-31) at Mukteshwar, a high-altitude and regional background site in the foothills of Indian central Himalayas. The total spectral absorption coefficient (babs) was divided into three categories: black carbon (BC) and brown carbon (BrC); fossil fuels (FF) and wood/biomass burning (WB/BB); and primary and secondary sources. At the wavelength of 370 nm, a significant BrC contribution (25 %) to the total babs is identified, characterized by a pronounced seasonal variation with winter (December-January-February) maxima (31 %) and post-monsoon (October and November) minima (20 %); whereas, at 660 nm, the contribution of BrC is dramatically less (9 %). Climatologically, the estimated BCFF at 880 nm ranges from 0.25 ± 0.19 µg m-3 in July to 1.17 ± 0.80 µg m-3 in May with the annual average of 0.67 ± 0.63 µg m-3, accounting for 79 % of the BC mass. The maximum BCFF/BC fraction reaches its peak value during the monsoon (July and August, 85 %), indicating the dominance of local traffic emissions due to tourism activities. Further, the highest BCWB concentration observed during pre-monsoon (March-May) suggests the influence of local forest fires along with long-range transported aerosols from the low-altitude plains. The increased contribution of BrC (26 % at 370 nm) and WB absorption (61 % at 370 nm) to the total absorption at the shorter wavelengths suggests that wood burning is one of the major sources of BrC emissions. Secondary BrC absorption accounts for 24 % [91 %] of the total absorption [BrC absorption] at 370 nm, implying the dominance of secondary sources in BrC formation. A trend analysis for the measured BC concentration shows a statistically significant increasing trend with a slope of 0.02 µgm-3/year with a total increase of about 22 % over the study period. A back trajectory-based receptor model, potential source contribution function (PSCF), was used to identify the potential regional source region of BC. The main source regions of BC are the northwest states of India in the IGP region and the northeast Pakistan region.

Atmospheric aerosols at a regional background Himalayan site--Mukteshwar, India.

Continuous aerosol measurements were made at a regional background station (Mukteshwar) located in a rural Himalayan mountain terrain from December 2005 to December 2008 for a period of 3 years. The average concentrations of particulate matter less than or equal to 10 µm (PM10), particulate matter less than or equal to 2.5 µm (PM2.5) and black carbon (BC) are 46.0, 26.6 and 0.85 µg/m(3) during the study period. Majority of the PM10 values lie below 100 µg/m(3) while majority of the PM2.5 values lie below 30 µg/m(3). It is further seen that during the monsoon months, especially July and August, the average values are comparatively low. It is also noted that the PM2.5/PM10 ratios between 0.50 and 0.75 have the maximum frequency distribution in the data set. Furthermore, the monthly mean ratio of BC to PM2.5 mass lies between 3.0 and 7.5 % during the study period. Though the average PM10 and PM2.5 concentrations during the study period are less than the respective Indian ambient air quality standards, however, they are still above the WHO guidelines and would have adverse health impacts. This shows that even in rural/background regions that are far away from major pollution sources or urban areas, the aerosol concentrations are significant and require long-term monitoring, source quantification and aerosol model simulations.

A computational fluid dynamics approach to nucleation in the water-sulfuric acid system.

This study presents a computational fluid dynamics modeling approach to investigate the nucleation in the water-sulfuric acid system in a flow tube. On the basis of an existing experimental setup (Brus, D.; Hyvärinen, A.-P.; Viisanen, Y.; Kulmala, M.; Lihavainen, H. Atmos. Chem. Phys. 2010, 10, 2631-2641), we first establish the effect of convection on the flow profile. We then proceed to simulate nucleation for relative humidities of 10, 30, and 50% and for sulfuric acid concentration between 10(9) to 3 x 10(10) cm(-3). We describe the nucleation zone in detail and determine how flow rate and relative humidity affect its characteristics. Experimental nucleation rates are compared to rates gained from classical binary and kinetic nucleation theory as well as cluster activation theory. For low RH values, kinetic theory yields the best agreement with experimental results while binary nucleation best reproduces the experimental nucleation behavior at 50% relative humidity. Particle growth is modeled for an example case at 50% relative humidity. The final simulated diameter is very close to the experimental result.

Re-evaluation of the pressure effect for nucleation in laminar flow diffusion chamber experiments with fluent and the fine particle model.

This study is an investigation of the effect of total pressure on homogeneous nucleation rates of n-butanol in helium and n-pentanol in helium and argon in a laminar flow diffusion chamber (LFDC). To verify earlier findings, experimental data was re-evaluated using the computational fluid dynamics (CFD) software FLUENT in combination with the fine particle model (FPM) for aerosol dynamics calculations. This approach has been introduced in an earlier paper [Herrmann, E.; Lihavainen, H.; Hyvarinen, A.-P.; Riipinen, I.; Wilck, M.; Stratmann, F.; Kulmala, M. J. Phys. Chem. A 2006, 110, 12448]. As a result of our evaluation, a flaw in the femtube2 code was found which had been used in the original data analysis [Hyvarinen, A.-P.; Brus, D.; Zdimal, V.; Smolik, J.; Kulmala, M.; Viisanen, Y.; Lihavainen, H. J. Chem. Phys. 2006, 124, 224304]. The FLUENT analysis yielded a weak positive pressure effect for the nucleation of n-butanol in helium at low nucleation temperatures (265-270 K). n-Pentanol in helium showed a positive pressure effect at all temperatures (265-290 K), while the effect for the nucleation of n-pentanol in argon was negative at high temperatures (280 and 285 K) and positive at lower nucleation temperatures (265 K). These findings support results gained with the corrected femtube2 model. In this study, we also carried out a detailed comparison of FLUENT and femtube2 modeling results, especially focusing on the calculation of temperature and saturation ratio at nucleation rate maximum (T(nuc) and S(nuc), respectively) in both models.

The homogeneous nucleation of 1-pentanol in a laminar flow diffusion chamber: the effect of pressure and kind of carrier gas.

The influence of total pressure and kind of carrier gas on homogeneous nucleation rates of 1-pentanol was investigated using experimental method of laminar flow diffusion chamber in this study. Two different carrier gases (helium and argon) were used in the total pressure range from 50 to 400 kPa. Nucleation temperatures ranged from 265 to 290 K for 1-pentanol-helium and from 265 to 285 K for 1-pentanol-argon. Nucleation rates varied between 10(1) and 10(6) cm(-3) s(-1) for 1-pentanol-helium and between 10(2) and 10(5) cm(-3) s(-1) for 1-pentanol-argon. Both positive and slight negative pressure effects were observed depending on temperature and carrier gas. The trend of pressure effect was found similar for both carrier gases. Error analysis on thermodynamic properties was conducted, and the lowering of surface tension due to adsorption of argon on nucleated droplets was estimated. A quantitative overview of pressure effect is provided.