On 4/21/16, the AirNow Data Management Center rolled out two new changes which effect how ozone is reported.
Ozone NowCast
The ozone NowCast replaces the ozone surrogate (slope/intercept method) for real time reporting. The ozone NowCast calculation is similar to the PM2.5 and PM10 NowCast calculations described in this presentation, but does include some slight differences (indicated by the bold text below). The process for calculating the ozone NowCast is as follows:

Compute the concentrations range (maxmin) over the last 8 hours.

Divide the range by the maximum concentration in the 8 hour period to obtain the scaled rate of change.

Compute the weight factor by subtracting the scaled rate from 1. There is no minimum weight factor (PM2.5 and PM10 have a minimum weight factor of 0.5). There is a maximum weight factor of 1.

Multiply each hourly concentrations by the weight factor raised to the power of how many hours ago the concentration was measured (for the current hour, the factor is raised to the zero power).

Compute the NowCast by summing these products and dividing by the sum of the weight factors raised to the power of how many hours ago the concentration was measured.
Missing data is handled the same way as the PM2.5 and PM10 NowCast: 2 of the last 3 hours of data must be valid for a NowCast calculation. An example of a NowCast calculation is below, with a missing hour included.
Example Calculation
 Range: 8022=58
 Scaled rate of change: 58/80= .725
 Weight Factor: 1.725= .275
 63*(.275)^{0}+69*(.275)^{1}+80*(.275)^{2}+74*(.275)^{3}+60*(.275)^{4}+
Missing*(.275)^{5}+35*(.275)^{6}+22*(.275)^{7}= 89.925  89.925/ (.275)^{0}+(.275)^{1}+(.275)^{2}+(.275)^{3}+(.275)^{4}+
(.275)^{5}+(.275)^{6}+(.275)^{7}=65.2 ppb = 84 AQI
Daily maximum 8hour average ozone
In accordance with the 2016 National Ambient Air Quality Standard (NAAQS), AirNowTech has updated the hours considered in a daily maximum 8hour average ozone calculation. The change is as follows:
"The daily maximum 8hour average ozone concentration for a given day is the highest of the 17 consecutive 8hour averages beginning with the 8hour period from 7:00 a.m. to 3:00 p.m. Local Standard Time (LST) and ending with the 8hour period from 11:00 p.m. to 7:00 a.m. LST the following day (i.e., the 8hour averages for 7:00 a.m. to 11:00 p.m.LST).
A daily maximum 8hour average Ozone concentration shall be considered valid if valid 8hour averages are available for at least 13 of the 17 consecutive 8hour periods starting from 7:00 a.m. to 11:00 p.m. LST. In addition, in the event that fewer than 13/17 valid 8hour averages are available, a daily maximum 8hour average ozone concentration shall also be considered valid if it is greater than the level of the NAAQS"
The purpose of this change was to avoid double counting overnight exceedances from a single ‘event’ (e.g. 73 ppb 8hour average at 11:00 p.m. and 12:00 a.m. counting as two exceedances). Please note that, as before, 6/8 hours are needed for a valid 8hour average (75%).
Below are possible changes and scenarios that you can expect to see:

Changes in daily maximum 8hour average if the maximum 8hour average occured between 00:00 and 06:00

A day that did not meet 75% completeness criteria before (<18/24 8hour averages in a day) is now valid (>=13/17 8hour averages from 07:00 to 23:00)

A day that did meet 75% completeness criteria before (>=18/24 8hour averages in a day) is now invalid (<13/17 8hour averages from 07:00 to 23:00)
An example of each of these cases is below:
Morning Max
Below are hourly 8hour averages for a given day.
Old daily maximum 8hour average: 44 ppb
New daily maximum 8hour average: 42 ppb
Invalid to Valid
Below are hourly 8hour averages for a given day. The blue outline indicates insufficient data/invalid 8hour averages Old # of valid 8hour averages: 17/24 (<75%) Old daily maximum 8hour average: Insufficient dataNew # of valid 8hour averages: 14/17 (>=75%)
New daily maximum 8hour average: 68 ppb
Valid to Invalid
Below are hourly 8hour averages for a given day. The blue outline indicates insufficient data/invalid 8hour averagesOld # of valid 8hour averages: 18/24 (>=75%)
Old daily maximum 8hour average: 51 ppb
New # of valid 8hour averages: 11/17 (<75%)
New daily maximum 8hour average: Insufficient data