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

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 45°N–72°S latitude and 153°E–75°W longitude, and for PEM‐Tropics B was 40°N–36°S latitude and 149°E–75°W 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 (MeONO 2 , CH 3 ONO 2 ) mixing ratios (approximately 50 pptv) at low altitudes in a latitude band between 8°N to 13°S stretching across the equatorial Pacific, illustrating the oceanic source of MeONO 2 . This source may be associated with the high‐nutrient, low‐chlorophyll character of equatorial Pacific waters. We discuss MeONO 2 and ethyl nitrate (EtONO 2 , C 2 H 5 ONO 2 ), whose abundance is dominated by equatorial oceanic sources, 2‐Propyl nitrate (2‐PrONO 2 , 2‐C 3 H 7 ONO 2 ), which has significant oceanic and northern hemispheric (NH) sources associated with urban/industrial hydrocarbon emissions, and 2‐butyl nitrate (2‐BuONO 2 2‐C 4 H 8 ONO 2 ), which has mostly NH sources. PEM‐Tropics A and B resulted in remarkably similar equatorial mixing ratios. The excellent correlations between MeONO 2 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 MeONO 2 :EtONO 2 , MeONO 2 :2‐PrONO 2 , and MeONO 2 :2‐BuONO 2 ratios. These relationships can be useful as a diagnostic of air mass origin. North of 10°N, 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.