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 (O-3) and alkyl and multifunctional nitrate (Sigma ANs). 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 O-3, NO, NO2, NO3, N2O5, HNO3, peroxy nitrates (Sigma PNs), Sigma ANs, OH, and HO2. In the London's 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 Sigma PNs, Sigma ANs, HNO3 or peroxides (H2O2, ROOH) altering the 03 production. In the observations reported here, we found that a strong ozone titration (Delta O-3 = -16 ppb), due to a rapid increase of NOx (Delta NOx = 27 ppb), corresponds also to a high increase of Sigma ANs concentrations (Delta Sigma ANs = 3 ppb), and quite stable concentrations of HNO3 and Sigma PNs. Unexpectedly, compared with other megacities, the production of Sigma ANs is similar to that of O-x (O-3 + NO2), suggesting that in the London plume, at least during these observations, the formation of Sigma ANs effectively removes active NO and hence reduces the amount of O-3 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 Sigma ANs 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 Sigma ANs competing against the local ozone production.