Reply on RC1

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> The UK proposes additional bioenergy plantations and afforestation as part of measures to meet net-zero greenhouse gas emissions, but species and locations are not yet decided. Different tree species emit varying amounts of isoprene and monoterpene volatile organic compounds that are precursors to ozone and secondary organic aerosol (SOA) formation, the latter of which is a component of PM_2.5. The forest canopy also acts as a depositional sink for air pollutants. All these processes are meteorologically influenced. We present here a first step at coupling information on tree species planting suitability and other planting constraints with data on UK-specific BVOC emission rates and tree canopy data to simulate via the WRF-EMEP4UK high spatial resolution atmospheric chemistry transport model the impact on UK air quality of four potential scenarios. Our &lsquo;maximum planting&rsquo; scenarios are based on planting areas where yields are predicted to be&nbsp; &ge;50 % of the maximum from the Ecological Site Classification Decision Support System (ESC-DSS) for <em>Eucalyptus gunnii</em>, hybrid aspen (<em>Populus tremula</em>), Italian alder (<em>Alnus cordata</em>) and Sitka spruce (<em>Picea sitchensis</em>). The additional areas of forest in our scenarios are 2.0 to 2.7 times current suggestions for new bioenergy and afforestation landcover in the UK. Our planting scenarios increase UK annual mean surface ozone concentrations by 1.0 ppb or 3 % relative to the baseline landcover for the highest BVOC emitting species (e.g., <em>E. gunni</em>). Increases in ozone reach 2 ppb in summer when BVOC emissions are greatest. In contrast, all the additional planting scenarios lead to reductions in UK annual mean PM_2.5 &ndash; ranging from -0.2 &micro;g m^-3 (-3 %) for Sitka spruce to -0.5 &micro;g m^-3 (-7 %) for aspen &ndash; revealing that PM_2.5 deposition to the additional forest canopy area more than offsets additional SOA formation. Relative decreases in annual mean PM_2.5 are greater than the relative increases in annual mean ozone. Reductions in PM_2.5 are least in summer, coinciding with the period of maximum monoterpene emissions. Although only a first step in evaluating the impact of increased forest plantation on UK air quality, our study demonstrates the need for locally relevant data on landcover suitability, emissions and meteorology in model simulations.