Street-scale air quality modelling for Beijing during a winter 2016 measurement campaign

• Main Authors: M. Biggart, J. Stocker, R. M. Doherty, O. Wild, M. Hollaway, D. Carruthers, J. Li, Q. Zhang, R. Wu, S. Kotthaus, S. Grimmond, F. A. Squires, J. Lee, Z. Shi
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-03-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/20/2755/2020/acp-20-2755-2020.pdf

<p><span id="page2756"/>We examine the street-scale variation of <span class="inline-formula">NO_<i>x</i></span>, <span class="inline-formula">NO₂</span>, <span class="inline-formula">O₃</span> and PM<span class="inline-formula">_2.5</span> concentrations in Beijing during the Atmospheric Pollution and Human Health in a Chinese Megacity (APHH-China) winter measurement campaign in November–December 2016. Simulations are performed using the urban air pollution dispersion and chemistry model ADMS-Urban and an explicit network of road source emissions. Two versions of the gridded Multi-resolution Emission Inventory for China (MEIC v1.3) are used: the standard MEIC v1.3 emissions and an optimised version, both at 3&thinsp;km resolution. We construct a new traffic emissions inventory by apportioning the transport sector onto a detailed spatial road map. Agreement between mean simulated and measured pollutant concentrations from Beijing's air quality monitoring network and the Institute of Atmospheric Physics (IAP) field site is improved when using the optimised emissions inventory. The inclusion of fast <span class="inline-formula">NO_<i>x</i></span>–<span class="inline-formula">O₃</span> chemistry and explicit traffic emissions enables the sharp concentration gradients adjacent to major roads to be resolved with the model. However, <span class="inline-formula">NO₂</span> concentrations are overestimated close to roads, likely due to the assumption of uniform traffic activity across the study domain. Differences between measured and simulated diurnal <span class="inline-formula">NO₂</span> cycles suggest that an additional evening <span class="inline-formula">NO_<i>x</i></span> emission source, likely related to heavy-duty diesel trucks, is not fully accounted for in the emissions inventory. Overestimates in simulated early evening <span class="inline-formula">NO₂</span> are reduced by delaying the formation of stable boundary layer conditions in the model to replicate Beijing's urban heat island. The simulated campaign period mean PM<span class="inline-formula">_2.5</span> concentration range across the monitoring network (<span class="inline-formula">∼15</span>&thinsp;<span class="inline-formula">µg m⁻³</span>) is much lower than the measured range (<span class="inline-formula">∼40</span>&thinsp;<span class="inline-formula">µg m⁻³</span>). This is likely a consequence of insufficient PM<span class="inline-formula">_2.5</span> emissions and spatial variability, neglect of explicit point sources, and assumption of a homogeneous background PM<span class="inline-formula">_2.5</span> level. Sensitivity studies highlight that the use of explicit road source emissions, modified diurnal emission profiles, and inclusion of urban heat island effects permit closer agreement between simulated and measured <span class="inline-formula">NO₂</span> concentrations. This work lays the foundations for future studies of human exposure to ambient air pollution across complex urban areas, with the APHH-China campaign measurements providing a valuable means of evaluating the impact of key processes on street-scale air quality.</p>