Spring and Summer Observations of Carbon Monoxide, Ozone, Non-Methane Hydrocarbons and Total Gaseous Mercury at Cheeka Peak, Washington During the PHOBEA II Campaign

During spring (March-May) and summer (June-August) of 2001 we made measurements of carbon monoxide (CO), ozone (O3), non-methane hydrocarbons (NMHC), and total gaseous mercury (TGM) as part of the ongoing Photochemical Ozone Budget of the Eastern North Pacific Atmosphere (PHOBEA II) project at Cheeka Peak Observatory on the northwestern tip of Washington State. The data were segregated by wind direction (SSE to WNW) to examine the mixing ratios for these compounds in the Pacific marine atmosphere. CO, O3 and TGM exhibited spring maxima, with CO and O3 having the most pronounced seasonal cycles. CO varied from 161 ppbv (April 7-17) to 76 ppbv (July 22 - August 1), O3 varied from 47 ppbv (April 26 - May 6) to 27 ppbv (July 23 - August 2), and TGM varied from 1.69 ngm3 (May 10-20) to 1.57 ngm3 (March 6-16 and August 1 -10). CO and ozone mixing ratios showed a positive correlation 13 out of 21 weeks during our study (R > 0.3). Anticorrelations occurred only during 2 out of 21 weeks. During one week in late April, CO-O3 correlation coefficient was 0.8. This is in stark contrast with the spring 1997 and 1998 data from PHOBEA I which showed few (1997) or no (1998) positive correlations (R > 0.3) between CO and O3. Diurnal variability was significant for TGM and O3, but not for CO. TGM had spring and summer maxima at 2200 and 0500 local time, respectively. Summer variation was much greater (7.5% around the mean) than spring (2% around the mean). O3 had spring and summer maxima at 0200 and 2200 local time, respectively, and also showed much greater variability in the summer (11%) compared to the spring (4%). During April 23rd through 29th we observed two air masses with distinctly different photochemical ages. 10-day back trajectories from the 23rd to the 28th show air originating from swirling low-pressure systems around 40oN, apparently uninfluenced by continents. At CPO we observed very low levels of CO, O3, TGM and ethane (18%, 43%, 6%, and 38% below April monthly averages). Back trajectories for April 29th show a transition to rapid trans-Pacific flow (crossing time=5-8 days), which corresponded to restored mixing ratios of all four species to near April monthly average values.