Latitudinal, vertical, and seasonal variations of C1-C4 alkyl nitrates in the troposphere over the Pacific Ocean during PEM-Tropics A and B:: Oceanic and continental sources -: art. no. 8242

We present concentration distributions of C-1-C-4 alkyl nitrates observed during the NASA airborne campaigns Pacific Exploratory Mission (PEM)-Tropics A (September October 1996) and PEM-Tropics B (March - April 1999). The total geographic range for PEM-Tropics A was 45degreesN-72degreesS latitude and 153degreesE-75degreesW longitude, and for PEM-Tropics B was 40degreesN-36degreesS latitude and 149degreesE-75degreesW longitude. The maximum altitude for these missions was 12 km. These experiments provide the most extensive set of tropospheric measurements collected to date over the tropical Pacific Ocean. We observed high methyl nitrate (MeONO2, CH3ONO2) mixing ratios ( approximately 50 pptv) at low altitudes in a latitude band between 8degreesN to 13degreesS stretching across the equatorial Pacific, illustrating the oceanic source of MeONO2. This source may be associated with the high-nutrient, low-chlorophyll character of equatorial Pacific waters. We discuss MeONO2 and ethyl nitrate (EtONO2, C2H5ONO2), whose abundance is dominated by equatorial oceanic sources, 2-Propyl nitrate (2-PrONO2, 2-C3H7ONO2), which has significant oceanic and northern hemispheric (NH) sources associated with urban/industrial hydrocarbon emissions, and 2-butyl nitrate (2-BuONO2 2-C4H8ONO2), which has mostly NH sources. PEM-Tropics A and B resulted in remarkably similar equatorial mixing ratios. The excellent correlations between MeONO2 and the other alkyl nitrates in this region produced comparable correlation slopes between the two expeditions. By contrast, NH air masses influenced by urban/industrial emissions typically exhibited much lower MeONO2: EtONO2, MeONO2: 2-PrONO2, and MeONO2: 2-BuONO2 ratios. These relationships can be useful as a diagnostic of air mass origin. North of 10degreesN, the springtime PEM-Tropics B mixing ratios of C-2-C-4 alkyl nitrates were many-fold higher at low-mid altitudes than for late summer PEM-Tropics A, consistent with strong continental outflow of NMHC precursors during spring.