Ambient measurements of HONO and HNO3, using a highly sensitive coil scrubbing/HPLC/visible detection technique, were made at a rural site in southwestern New York State from 26 June to 14 July 1998, along with concurrent measurements of NOx, NOy, O3, and various meteorological parameters. The mean (and median) half-hour concentrations of HONO and HNO3 during this period were 63 (and 56) pptv and 418 (and 339) pptv, respectively. On average, there were two HONO concentration peaks, the first around 0200--0300 LT and the second around 0700--0800 LT, and a minimum at about 2000 LT. The sum of NOx, HONO, and HNO3 (ΣNOyi) was highly correlated with the measured NOy concentration (r2 = 0.64). The average HONO/NOx ratio was 0.07, while the average ΣNOyi/NOy ratio was 0.66. During the early morning hours, the photolysis of HONO appeared to be a dominant source of HOx radicals in boundary layer near the ground surface. The average daily radical production from HONO photolysis was 2.3 ppbv, accounting for 24% of the total production from photolyses of HONO, O3, and HCHO at the measurement height of 4 m above the ground. Diurnal patterns of HONO and relative humidity suggest that the ground and vegetation surfaces were sinks for HONO in the boundary layer when dew droplets were formed at night and that the subsequent release of the trapped nitrous acid/nitrite from the surfaces acted as a strong HONO source in the morning as the dew droplets evaporated. Our data also suggest that, in order to maintain the observed daytime HONO concentration of ~60 pptv, there should be a strong daytime source of 220 pptv hr-1, which was much greater than the nighttime source of 13 pptv hr-1 and the estimated production of ~ 40 pptv hr-1 from the gas-phase NO-OH reaction. Photolysis of HNO3, which deposits and accumulates on the ground and vegetation surfaces, may contribute significantly to the missing daytime HONO sources.