Timescales of surface-to-tropopause transport in the tropics, using Flexpart

The timescales of transport from the surface to the tropical tropopause layer (TTL) was studied using the Lagrangian transport model Flexpart. The model was driven by the ERA-Interim reanalysis from the European Center for Medium-Range Weather Forecasts (ECMWF). Trajectories were released each month in the period 1. June 2002 - 1. May 2013 at 15 km and 17 km over the whole tropics and simulated 90 days backward in time. The age of air at 15 km and 17 km relative to the last contact with the boundary layer (BL) was computed using a constant BL-height of 1 km above sea level. The aim of the study was to give a detailed description of the tropospheric age of air in the TTL, mainly motivated by the importance of transport timescales for the entry of short-lived compounds to the stratosphere. Several sensitivity studies were carried out. The most important of these were the sensitivity to the use of the convection scheme in Flexpart. In the run without the convection scheme, the median age at 15 km was 17 day longer, and at 17 km 25 days longer, than in the runs using the convection scheme. In particular, the fraction of the air at 17 km younger than 10 days decreased with an order of magnitude, from 11.1 % to 0.9 %. For 30˚S - 30˚N as a whole, the median age was 26 days at 15 km and 50 days at 17 km. A seasonal cycle in the age was found at both altitudes. The seasonal cycle was most pronounced at 17 km, where the median age varied by ∼14 days during the year, being highest in August and lowest in May. At both altitudes, the air was younger near the main convective areas in the tropics, such as the Intertropical Convergence Zone (ITCZ), with less young air approaching the subtropics. The air was particularly young above the tropical western Pacific; the median age there was only 16 days at 15 km and 30 days at 17 km. The air at both 15 km and 17 km was found to originate from the BL above the main convective regions in the tropics. In particular, the West and Central Pacific stood for 40-50 % of the BL-origins. The age decreased over the period, both at 15 km and 17 km. The decrease in the annual median age during 2003-2012 was 2.0 days per decade at 15 km and 9.7 days per decade at 17 km (for 30˚S - 30˚N). Much of this decrease appeared to have taken place around 2009. Interannual variability in the age above the tropical Pacific in December-February (DJF) was found to be related to the El Niño-Southern Oscillation (ENSO). The air was younger above the Pacific in El Niño and older in La Niña. A shift in the BL-origins above the Pacific ocean, eastward in El Niño and westward in La Niña, was also found

Quantification of temperature dependence of NOx emissions from road traffic in Norway using air quality modelling and monitoring data

Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)Emissions of nitrogen oxides (NOx) from road traffic are dependent on a range of factors including vehicle type, speed, driving patterns and engine temperature. Recently a number of studies have indicated that ambient air temperature plays an important role in vehicle NOx emissions, mainly due to various technical challenges of diesel vehicles that occur at low ambient temperatures. This study aims to derive a correction formula to account for this temperature dependence when calculating emissions from road traffic in Norway. Measured NOx concentrations in the period 2016–2019 at 46 sites dominated by road traffic sources are compared to the NOx concentrations calculated with the chemistry-transport modelling system EMEP/uEMEP at the same sites. The model has good road traffic volume input data, but no temperature dependence in the emission factors. A clear temperature dependence in the observed-to-modelled ratio of NOx concentration is found. The ratio increases from 1.09 at high temperatures to 2.9 at low temperatures. The increase occurs gradually in the temperature range from -13 °C to +14 °C. Assuming this temperature dependence in the bias is due to the road traffic emissions, a correction formula for these emissions is derived. The correction factor is 1 at high temperatures and 3.28 at low temperatures, with a linear increase in the range from +12.4 °C to -12.9 °C. Thus, our results suggest that road traffic emissions should be 3.3 times higher at temperatures below -13 °C than at high temperatures, and 2.7 times higher at -7 °C. The temperature range and magnitude of this temperature dependence are consistent with the existing literature on emission measurement experiments performed on various models of diesel vehicles. The derived temperature dependence can be used to correct road traffic emissions. However, the parameter values in the correction are dependent on the vehicle fleet composition and are applicable only for the current Norwegian vehicle fleet.publishedVersio

Description of the uEMEP_v5 downscaling approach for the EMEP MSC-W chemistry transport model

A description of the new air quality downscaling model – the urban EMEP (uEMEP) and its combination with the EMEP MSC-W model (European Monitoring and Evaluation Programme Meteorological Synthesising Centre West) – is presented. uEMEP is based on well-known Gaussian modelling principles. The uniqueness of the system is in its combination with the EMEP MSC-W model and the “local fraction” calculation contained within it. This allows the uEMEP model to be imbedded in the EMEP MSC-W model and downscaling can be carried out anywhere within the EMEP model domain, without any double counting of emissions, if appropriate proxy data are available that describe the spatial distribution of the emissions. This makes the model suitable for high-resolution calculations, down to 50 m, over entire countries. An example application, the Norwegian air quality forecasting and assessment system, is described where the entire country is modelled at a resolution of between 250 and 50 m. The model is validated against all available monitoring data, including traffic sites, in Norway. The results of the validation show good results for NO2, which has the best known emissions, and moderately good for PM10 and PM2.5. In Norway, the largest contributor to PM, even in cities, is long-range transport followed by road dust and domestic heating emissions. These contributors to PM are more difficult to quantify than NOx exhaust emission from traffic, which is the major contributor to NO2 concentrations. In addition to the validation results, a number of verification and sensitivity results are summarised. One verification showed that single annual mean calculations with a rotationally symmetric dispersion kernel give very similar results to the average of an entire year of hourly calculations, reducing the runtime for annual means by 4 orders of magnitude. The uEMEP model, in combination with EMEP MSC-W model, provides a new tool for assessing local-scale concentrations and exposure over large regions in a consistent and homogenous way and is suitable for large-scale policy applications