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Aircraft observations of the lower troposphere above a megacity: Alkyl nitrate and ozone chemistry
Abstract 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 (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 O 3, NO, NO 2, NO 3, N 2 O 5, HNO 3, peroxy nitrates (ΣPNs), ΣANs, OH, and HO 2. 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 NO x and HO x can have different terminations forming ΣPNs, ΣANs, HNO 3 or peroxides (H2O2, ROOH) altering the O 3 production. In the observations reported here, we found that a strong ozone titration (ΔO 3 = −16 ppb), due to a rapid increase of NO x (ΔNO x = 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 O x (O 3 + NO 2), suggesting that in the London plume, at least during these observations, the formation of ΣANs effectively removes active NO x 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 ΣANs production is given by Alkanes. Observations and analysis reported here suggest that in the London plume the high NO x emissions and the chemistry of some VOCs (mainly Alkanes) produce high concentrations of ΣANs competing against the local ozone production.
Highlights Daytime chemical and aerosol composition have been measured around London with an aircraft. Unexpected high production of alkyl nitrate comparable to that of ozone. The low O 3 production put the chemistry above London quite different compared with other megacities.
Aircraft observations of the lower troposphere above a megacity: Alkyl nitrate and ozone chemistry
Abstract 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 (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 O 3, NO, NO 2, NO 3, N 2 O 5, HNO 3, peroxy nitrates (ΣPNs), ΣANs, OH, and HO 2. 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 NO x and HO x can have different terminations forming ΣPNs, ΣANs, HNO 3 or peroxides (H2O2, ROOH) altering the O 3 production. In the observations reported here, we found that a strong ozone titration (ΔO 3 = −16 ppb), due to a rapid increase of NO x (ΔNO x = 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 O x (O 3 + NO 2), suggesting that in the London plume, at least during these observations, the formation of ΣANs effectively removes active NO x 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 ΣANs production is given by Alkanes. Observations and analysis reported here suggest that in the London plume the high NO x emissions and the chemistry of some VOCs (mainly Alkanes) produce high concentrations of ΣANs competing against the local ozone production.
Highlights Daytime chemical and aerosol composition have been measured around London with an aircraft. Unexpected high production of alkyl nitrate comparable to that of ozone. The low O 3 production put the chemistry above London quite different compared with other megacities.
Aircraft observations of the lower troposphere above a megacity: Alkyl nitrate and ozone chemistry
Aruffo, Eleonora (author) / Di Carlo, Piero (author) / Dari-Salisburgo, Cesare (author) / Biancofiore, Fabio (author) / Giammaria, Franco (author) / Busilacchio, Marcella (author) / Lee, James (author) / Moller, Sarah (author) / Hopkins, James (author) / Punjabi, Shalini (author)
Atmospheric Environment ; 94 ; 479-488
2014-05-14
10 pages
Article (Journal)
Electronic Resource
English
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