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

Abstract

Measurements of nitryl chloride (ClNO2) and its precursors (O3, NO2, particulate chloride) were made in 2014–2016 at three contrasting locations in the United Kingdom: Leicester, Penlee Point and Weybourne. ClNO2 was observed at all sites and in every season, with the highest concentrations between 00:00 and 04:00 GMT. The median nocturnal concentration of ClNO2 ranged between the detection limit (4.2 ppt) and 139 ppt. A clear seasonal cycle, with maxima in spring and winter, and significant differences between locations in the same season were observed. The main source of particulate chloride was sea salt aerosol (including at Leicester, ∼200 km from the coast). In general, ClNO2 levels were controlled by the concentrations of O3 and NO2, rather than by the uptake and reaction of N2O5 with particulate chloride. Under these conditions, the seasonality and geographical distribution of ClNO2 can be explained in terms of O3‐limited and NO2‐limited regimes affecting the formation of the N2O5 precursor. A global version of the GEOS‐Chem model at medium resolution (2° × 2.5°) was not able to fully capture the observed seasonality of ClNO2, mostly because the model overestimated the concentrations of the precursors, particularly of nocturnal O3. A higher‐resolution (0.25° × 0.3125°) version of GEOS‐Chem showed better agreement with the observations, although it still overestimated ClNO2 concentrations during summer.

Shipborne measurements of ClNO<sub>2</sub> in the Mediterranean Sea and around the Arabian Peninsula during summer

“…The AMS concentrations (PM 1 ) were on average ≈ 1 % of those reported by the MARGA (TSP) with the correlation between them indicating that the AMS chloride (PM 1 ) is mainly due to sea salt rather than NH 4 Cl. If we assume that ≈ 10 % of the sea salt mass (TSP) is associated with the fine mode (PM 1 ) as previously derived (Sommariva et al, 2018), we can use the MARGA (TSP) [Cl − ] to estimate that the true PM 1 [Cl − ] would be about an order of magnitude higher than measured by the AMS. Under the assumption that this is true, f is > 0.67 in all seven regions, implying that a lack of Cl − is not the reason for low values of ε, as may be expected for a marine environment.…”

“…the Los Angeles Basin in California (Riedel et al, 2012a;Wagner et al, 2012;Young et al, 2012). Other studies included coastal sites in Canada (Osthoff et al, 2018) and coastal/urban sites in the United Kingdom (Bannan et al, 2015(Bannan et al, , 2017Priestley et al, 2018;Sommariva et al, 2018). Whereas ClNO 2 was initially believed to play a significant role only in areas with marine influence (Behnke et al, 1997;Keene et al, 1999), midcontinental measurements in the US (Thornton et al, 2010;Riedel et al, 2013;Faxon et al, 2015) revealed the im-portance of anthropogenic sources (e.g.…”

Impact of halogen chemistry on summertime air quality in coastal and continental Europe: application of the CMAQ model and implications for regulation

“…The discrepancy among previous studies and between those works and the present one is difficult to deduce and requires further investigation. Several possible causes could lead to different simulated levels of halogens and their impact on oxidants, including the different mechanisms of producing and recycling halogen species (Sarwar et al, 2019), spatial resolution (Sommariva et al, 2018), emission inventory (Wang Q. Li et al: Halogen impact on air quality in Europe Figure 3. Monthly average OH and HO 2 concentration in the BASE simulation and changes due to chlorine (CL) and full halogen chemistry (HAL).…”

“…A number of modeling studies have been conducted to investigate the impact of individual halogen species on air quality. The chemistry of chlorine, mainly that of nitryl chloride (ClNO 2 ), has been reported to increase the oxidation capacity and the formation of O 3 in recent studies (Sarwar et al, 2012(Sarwar et al, , 2014Li et al, 2016;Sherwen et al, 2017;Sommariva et al, 2018). Bromine (Fernandez et al, 2014) and iodine (Saiz-Lopez et al, 2014) chemistry are reported to decrease the concentration of O 3 over oceanic and terrestrial regions.…”

“…Because the influence of halogens on air quality is uncertain and potentially has an impact on air quality management decisions, we have conducted regional simulations using the latest version of the CMAQ model implemented with comprehensive halogen sources and chemistry (Sarwar et al, 2019) to examine the overall effect of halogen species on air pollution over Europe. Considering that the grid size has a noticeable impact on air quality model predictions (Sommariva et al, 2018), we used a CMAQ model domain with 12 km horizontal resolution (higher than the previous studies on the halogen impact covering Europe) to simulate the levels of halogen species over Europe, examine the effect on the oxidation capacity and the concentration of air pollutants, and explore the potential implications for air quality policy related to NO 2 and O 3 .…”