Aircraft observations of the lower troposphere above a megacity: Alkyl nitrate and ozone chemistry

Within the framework of the RONOCO (ROle of Nighttime chemistry in controlling the Oxidising Capacity of the atmOsphere) campaign a daytime flight over the metropolitan area of London were carried out to study the nitrogen oxide chemistry and its role in the production and loss of ozone (O3) and alkyl and multifunctional nitrate (ΣANs). The FAAM BAe-146 aircraft, used for these observations, was equipped with instruments to measure the most relevant compounds that control the lower troposphere chemistry, including O3, NO, NO2, NO3, N2O5, HNO3, peroxy nitrates (ΣPNs), ΣANs, OH, and HO2. In the Londonʹs flight a strong ozone titration process was observed when flying above Reading (downwind of London) and when intercepting the London plume. The coupled cycles of NOx and HOx can have different terminations forming ΣPNs, ΣANs, HNO3 or peroxides (H2O2, ROOH) altering the O3 production. In the observations reported here, we found that a strong ozone titration (ΔO3 = −16 ppb), due to a rapid increase of NOx (ΔNOx = 27 ppb), corresponds also to a high increase of ΣANs concentrations (ΔΣANs = 3 ppb), and quite stable concentrations of HNO3 and ΣPNs. Unexpectedly, compared with other megacities, the production of ΣANs is similar to that of Ox (O3 + NO2), suggesting that in the London plume, at least during these observations, the formation of ΣANs effectively removes active NOx and hence reduces the amount of O3 production. In fact, we found that the ratio between the ozone production and the alkyl nitrates production (observed) approximate the unity; on the contrary the calculated ratio is 7. In order to explain this discrepancy, we made sensitivity tests changing the alkyl nitrates branching ratio for some VOCs and we investigated the impact of the unmeasured VOCs during the flight, founding that the calculated ratio decreases from 7 to 2 and that, in this condition, the major contribution to the ΣANs production is given by Alkanes. Observations and analysis reported here suggest that in the London plume the high NOx emissions and the chemistry of some VOCs (mainly Alkanes) produce high concentrations of ΣANs competing against the local ozone production.