Effects of halogens on European air-quality

Abstract

Halogens (Cl, Br) have a profound influence on stratospheric ozone (O). They (Cl, Br and I) have recently also been shown to impact the troposphere, notably by reducing the mixing ratios of O and OH. Their potential for impacting regional air-quality is less well understood. We explore the impact of halogens on regional pollutants (focussing on O) with the European grid of the GEOS-Chem model (0.25° × 0.3125°). It has recently been updated to include a representation of halogen chemistry. We focus on the summer of 2015 during the ICOZA campaign at the Weybourne Atmospheric Observatory on the North Sea coast of the UK. Comparisons between these observations together with those from the UK air-quality network show that the model has some skill in representing the mixing ratios/concentration of pollutants during this period. Although the model has some success in simulating the Weybourne ClNO observations, it significantly underestimates ClNO observations reported at inland locations. It also underestimates mixing ratios of IO, OIO, I and BrO, but this may reflect the coastal nature of these observations. Model simulations, with and without halogens, highlight the processes by which halogens can impact O. Throughout the domain O mixing ratios are reduced by halogens. In northern Europe this is due to a change in the background O advected into the region, whereas in southern Europe this is due to local chemistry driven by Mediterranean emissions. The proportion of hourly O above 50 nmol mol in Europe is reduced from 46% to 18% by halogens. ClNO from NO uptake onto sea-salt leads to increases in O mixing ratio, but these are smaller than the decreases caused by the bromine and iodine. 12% of ethane and 16% of acetone within the boundary layer is oxidised by Cl. Aerosol response to halogens is complex with small (∼10%) reductions in PM in most locations. A lack of observational constraints coupled to large uncertainties in emissions and chemical processing of halogens make these conclusions tentative at best. However, the results here point to the potential for halogen chemistry to influence air quality policy in Europe and other parts of the world.

BrO and inferred Br<sub><i>y</i></sub> profiles over the western Pacific: relevance of inorganic bromine sources and a Br<sub><i>y</i></sub> minimum in the aged tropical tropopause layer

“…Recent first simultaneous measurements of BrO and IO over the entire height of the tropical troposphere suggest that bromine and iodine together account regionally for 34 % of column-integrated ozone loss over the tEPO . Globally, the impact of bromine and other halogens reduces the ozone burden between 10 and 18 %, which is comparable to the ozone sink from the HO 2 + O 3 reaction Sherwen et al, 2016Sherwen et al, , 2017b. Bromine oxidation of NO x is an important NO x sink, and contributes to ozone reduction by reducing the photochemical formation of ozone .…”

“…Stratospheric concentrations (monthly and diurnally varying) of Br y are taken from a GEOS-CCM (chemistry climate model) simulation (Liang et al, 2010). Halogen chemistry in the model from Parrella et al (2012) as referenced in Wang et al (2015) has been updated Sherwen et al, 2016Sherwen et al, , 2017a) and now includes more extensive heterogeneous chemistry including reactions on ice crystals and cross-halogen coupling. These changes have led to a reduction in modeled HBr / Br y as well as Br y washout, leading to an increase in BrO x / Br y .…”

Seasonal and geographical variability of nitryl chloride and its precursors in Northern Europe

“…The model includes HO x ‐NO x ‐VOC‐O 3 ‐BrO x chemistry and has been recently updated to include Cl, Br, I chemistry (Schmidt et al, ; Sherwen et al, ; ), as described in detail by Sherwen et al . (). The chlorine chemical mechanism in the model includes further reactions of chlorine and bromine with organics compounds, ClNO 2 formation following N 2 O 5 uptake on sea salt, and heterogeneous iodine cycling to produce ICl, IBr.…”

“…The model showed good agreement with the measurements during spring and winter but overestimated ClNO 2 during summer and autumn (Figure , top). However, the model showed good agreement with the summertime measurements of ClNO 2 at Weybourne (Figure , bottom), as previously reported by (Sherwen et al, ). The high‐resolution version of GEOS‐Chem (0.25° × 0.3125°) calculated roughly the same concentrations of ClNO 2 as the medium‐resolution version for spring in Leicester and summer in Weybourne, thereby agreeing reasonably well with the observations.…”

“…The GEOS‐Chem results complement the work by (Sherwen et al, ), which focused on the observations at Weybourne in summer 2015 and explored the regional effects of halogen chemistry. The expanded set of model results and measurements discussed here, covering a larger temporal and geographical range, highlight the challenges and possible future directions for chemical transport models.…”