Roberto Sommariva

Reactive nitrogen transport and photochemistry in urban plumes over the North Atlantic Ocean

[1]xa0Photochemical and transport processes involving reactive nitrogen compounds were studied in plumes of urban pollutants using measurements obtained from the NOAA WP-3 aircraft during the ICARTT study in July and August 2004. Observations close to Boston and New York City were used to characterize urban emissions, and plume transport and transformation processes were studied in aged plumes located up to 1000 km downwind from the east coast of North America. Pollution was observed primarily below 1.5 km altitude in well-defined layers that were decoupled from the marine boundary layer. In aged plumes located over the North Atlantic Ocean, the nitric acid (HNO3) mixing ratios were large (up to 50 ppbv), and HNO3 accounted for the majority of reactive nitrogen. Plume CO and reactive nitrogen enhancement ratios were nearly equivalent in fresh and aged plumes. Efficient transport of HNO3 explained the observed trace gas ratios and abundances. Without substantial HNO3 loss, the ratio of HNO3 to NOx was between 13 and 42 in most highly aged plumes and sometimes exceeded calculated photochemical steady state values. Box model calculations that include nighttime reactions that convert NOx to HNO3 reproduce the observations. Photolysis and OH oxidation of over 10 ppbv of HNO3 that was in the troposphere for days resulted in reformation of hundreds of pptv of NOx, which is sufficient to maintain photochemical ozone production. Efficient transport of HNO3 over the North Atlantic Ocean for days carried both HNO3 and NOx far from their continental sources and increased their photochemical influence.

Measurements of PANs during the New England Air Quality Study 2002

[1]xa0Measurements of peroxycarboxylic nitric anhydrides (PANs) were made during the New England Air Quality Study 2002 cruise of the NOAA RV Ronald H Brown. The four compounds observed, PAN, peroxypropionic nitric anhydride (PPN), peroxymethacrylic nitric anhydride (MPAN), and peroxyisobutyric nitric anhydride (PiBN) were compared with results from other continental and Gulf of Maine sites. Systematic changes in PPN/PAN ratio, due to differential thermal decomposition rates, were related quantitatively to air mass aging. At least one early morning period was observed when O3 seemed to have been lost probably due to NO3 and N2O5 chemistry. The highest O3 episode was observed in the combined plume of isoprene sources and anthropogenic volatile organic compounds (VOCs) and NOx sources from the greater Boston area. A simple linear combination model showed that the organic precursors leading to elevated O3 were roughly half from the biogenic and half from anthropogenic VOC regimes. An explicit chemical box model confirmed that the chemistry in the Boston plume is well represented by the simple linear combination model. This degree of biogenic hydrocarbon involvement in the production of photochemical ozone has significant implications for air quality control strategies in this region.

Regional variation of the dimethyl sulfide oxidation mechanism in the summertime marine boundary layer in the Gulf of Maine

[1] Mixing ratios of dimethyl sulfide (DMS) and its nighttime oxidant, the nitrate radical (N0 3 ), were measured in the summertime marine boundary layer (MBL) of the Gulf of Maine during the New England Air Quality Study-International Transport and Chemical Transformation campaign in 2004. DMS fluxes from the ocean were derived from simultaneous measurements of the wind speed and DMS in seawater. Day and night DMS oxidation rates were determined from modeled OH and measured NO 3 concentrations. The average DMS lifetime with respect to oxidation by OH at noon was 13.5 ± 3.4 (1σ) h, while at night, DMS lifetimes with respect to N0 3 oxidation varied by sampling region from 11 min to 28 h. Oxidation by photochemically generated halogen species likely also played a role during the day, although the nature and extent of the halogen species is more difficult to predict due to lack of halogen measurements. Closure of the DMS budget in the MBL required a vertical entrainment velocity of ∼0.4 cm s ―1 . This study suggests that entrainment of DMS out of the MBL competes with daytime oxidation and that the presence of pollution in the form of NO x and 0 3 in near-coastal regions at night results in nearly complete DMS oxidation within the MBL via reaction with N0 3 , with a much smaller contribution from entrainment. One potential implication of near-complete DMS oxidation within the MBL is a reduction of the amount of sulfur available for aerosol formation and growth at higher altitudes in the atmosphere.

Observation of daytime N2O5in the marine boundary layer during New England Air Quality Study-Intercontinental Transport and Chemical Transformation 2004: OBSERVATION OF DAYTIME N2O5

[1]xa0The nitrate radical, NO3, and dinitrogen pentoxide, N2O5, are key reactive nocturnal nitrogen oxides in the troposphere. The daytime impact of NO3 and N2O5, however, is restricted by photochemical recycling of NO3 to NO2 and O3. In this paper, we report daytime measurements of N2O5 on board the NOAA research vessel Ronald H. Brown in the Gulf of Maine during the New England Air Quality Study–Intercontinental Transport and Chemical Transformation (NEAQS-ITCT) campaign in the summer of 2004. Daytime N2O5 mixing ratios of up to 4 pptv were observed, consistent with predictions from a steady state analysis. Predicted and observed NO3 mixing ratios were below the instrumental detection limit of ∼1 pptv; the average calculated concentration was 0.09 pptv. Important impacts of daytime NO3 and N2O5 in the marine boundary layer included increased rates of VOC oxidation (in particular dimethyl sulfide) and enhanced NOx to HNO3 conversion, both of which scaled with the available NOx. Smaller effects of daytime NO3 and N2O5 included chemical destruction of O3 and a shift of the NO2:NO ratio. Because the rates of heterogeneous conversion of N2O5 and NO3 to HNO3 scale with the surface area available for uptake, the importance of daytime fog is discussed.